Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > > > But we still have to think about similar cases (index entries pointing > > > inside CITC chains), unless we plan to disallow adding indexes to > > > tables. > > > > CREATE INDEX has to undo any chains where the new indexed columns change > > in the chain, and add index entries to remove the chain. > > Yes, that would be the most straightforward solution. > > It could be better in some cases, if we could avoid adding entries to > other indexes. Maybe we can just reset some flags, so that some SITC ops > like finding tuples by the CITC index pointer still work while adding > new entries wont. > > But it will be tricky to make this work for bitmap index scans. > > So yes, index build is a slop operation anyway, so making it even a > little slower is probably not a big problem. And most CITC chains will > have only one visible row at a time, this will probably not be a big > issue. Consider that index scans coming from different indexes have to span the same SITC. I see no clean way to avoid making all the indexes consistent, and as you said, CREATE INDEX isn't a frequent operation. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, T, 2006-06-27 kell 12:16, kirjutas Bruce Momjian: > Hannu Krosing wrote: > > ?hel kenal p?eval, T, 2006-06-27 kell 10:38, kirjutas Hannu Krosing: > > > ?hel kenal p?eval, E, 2006-06-26 kell 23:08, kirjutas Bruce Momjian: > > > > Jim C. Nasby wrote: > > > > > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > > > > > > > > > > > It is certainly possible to do what you are suggesting, that is > > > > > > have two > > > > > > index entries point to same chain head, and have the index access > > > > > > routines figure out if the index qualifications still hold, but that > > > > > > seems like a lot of overhead. > > > > > > I think Jim meant not 2 pointing to the same head, but 2 pointing into > > > the same chain. Say we have table with (id serial, ts timestamp) where > > > ts changes at each update and id does not. > > > > > > So after 3 updates on one page we have one CITC/ITPC head with pointers > > > from both indexes and two follow-up tuples with pointers from only ts > > > index. > > > > > > The problem with this setup is, that we can't reuse any of those > > > follow-up tuples without index cleanup. > > > > But we still have to think about similar cases (index entries pointing > > inside CITC chains), unless we plan to disallow adding indexes to > > tables. > > CREATE INDEX has to undo any chains where the new indexed columns change > in the chain, and add index entries to remove the chain. Yes, that would be the most straightforward solution. It could be better in some cases, if we could avoid adding entries to other indexes. Maybe we can just reset some flags, so that some SITC ops like finding tuples by the CITC index pointer still work while adding new entries wont. But it will be tricky to make this work for bitmap index scans. So yes, index build is a slop operation anyway, so making it even a little slower is probably not a big problem. And most CITC chains will have only one visible row at a time, this will probably not be a big issue. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Greg Stark wrote: > > Bruce Momjian <[EMAIL PROTECTED]> writes: > > > PFC wrote: > > > > > > > My idea is that if an UPDATE places the new tuple on the same page as > > > > the old tuple, it will not create new index entries for any indexes > > > > where the key doesn't change. > > > > > > Basically the idea behind preventing index bloat by updates is to have > > > one index tuple point to several actual tuples having the same value. > > > > > > > The idea is not to avoid index bloat, but to allow heap reuse, and having > > one index entry for multiple versions of an UPDATEd row is merely an > > implementation detail. > > It sort of sounds like you're describing a whole new index type that stores > only the page, not the precise record of any tuple it indexes. If your table Background, indexes point to page item pointers, not to actual offsets in the page. This is how vacuum can move around tuples without modifying the indexes. The index points to a page item pointer that is a chain of tuples with the same indexed columns. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
Bruce Momjian <[EMAIL PROTECTED]> writes: > PFC wrote: > > > > > My idea is that if an UPDATE places the new tuple on the same page as > > > the old tuple, it will not create new index entries for any indexes > > > where the key doesn't change. > > > > Basically the idea behind preventing index bloat by updates is to have > > one index tuple point to several actual tuples having the same value. > > > > The idea is not to avoid index bloat, but to allow heap reuse, and having > one index entry for multiple versions of an UPDATEd row is merely an > implementation detail. It sort of sounds like you're describing a whole new index type that stores only the page, not the precise record of any tuple it indexes. If your table has only such indexes then you never need to worry about updating indexes if your new tuple version goes on the same page as the old one. It's an interesting thought experiment. It might trade off a lot of work in index maintenance as well as saving space in the index for a lot of additional work performing index scans. There can easily be enough tuples on a page to make scanning the entire page pretty costly. -- greg ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
On Tue, Jun 27, 2006 at 10:42:54AM +0200, PFC wrote: > Also, I insist (again) that there is a lot to gain by using a bit of > compression on the data pages, even if it's very simple compression like > storing the new version of a row as a difference from the previous version > (ie. only store the columns that changed). > I think DB2 stores the latest version entirely, and stores the > previous versions as a delta. This is more efficient. This would only help on tables that: have many columns[1] are frequently updated the updates normally touch few columns [1] I'm assuming that un-changed toasted fields keep the same pointer I'm doubtful that that case is common enough to warrant the amount of work that would be involved in doing this. I think it might be useful to consider ways to make vertical partitioning easier, since that's the common means to reduce the impact of these scenarios. > In the case of tables containing TEXT values, these could also get > TOASTed. When an update does not modify the TOASTed columns, it would be > nice to simply be able to keep the reference to the TOASTed data instead > of decompressing it and recompressing it. Or is it already the case ? Hopefully it is, but I'm not sure... something that would be good is a means to force fields to be toasted sooner than when the tuple is bigger than 2k, because that'd be a very easy way to get gains from vertical partitioning. -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.comwork: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Jim C. Nasby wrote: > > > Perhaps my point got lost... in the case where no index keys change > > > during an update, SITC seems superior in every way to my proposal. My > > > idea (let's call it Index Tuple Page Consolidation, ITPC) would be > > > beneficial to UPDATEs that modify one or more index keys but still put > > > the tuple on the same page. Where SITC would be most useful for tables > > > that have a very heavy update rate and very few indexes, ITPC would > > > benefit tables that have more indexes on them; where presumably it's > > > much more likely for UPDATEs to change at least one index key (which > > > means SITC goes out the window, if I understand it correctly). If I'm > > > missing something and SITC can in fact deal with some index keys > > > changing during an UPDATE, then I see no reason for ITPC. > > > > I understood what you had said. The question is whether we want to get > > that complex with this feature, and if there are enough use cases > > (UPDATE with index keys changing) to warrant it. > > Ideas on how to test a table to see how many tuples would fit this > criteria? > > Or we could just shelve ITPC as a possibility in the future, after we > see how much the limitations of SITC hurt. Probably. I am not sure even SITC is a win given the complexity it will add, but I think it is worth trying. Getting into more complex cases where chains change indexed values seems like something we could try later if we have to. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
Martijn van Oosterhout wrote: -- Start of PGP signed section. > On Mon, Jun 26, 2006 at 11:29:27AM -0400, Bruce Momjian wrote: > > > Yes, and for index_getmulti (which doesn't visit the heap at all) we'll > > > have to change all the users of that (which aren't many, I suppose). > > > It's probably worth making a utility function to expand them. > > > > > > I'm still confused where bitmap index scan fit into all of this. Is > > > preserving the sequential scan aspect of these a goal with this new > > > setup? > > > > No. I was just pointing out that if you get to the tuple via an index, > > you get handed the head of the SITC via the index tuple, but if you are > > doing a sequential scan, you don't get it, so you have to find it, or > > any other non-visible SITC header. > > Ok, but it remains true that you can only have one SITC per tuple. So > if you have 5 indexes on a table, any SITC will only join tuples that > didn't change any values in any of the indexed columns. That's probably > not a big deal though; indexes columns arn't likely to be the ones > changing much. Right. > So, for the bitmap scan you have to make sure that within a single > transaction, scanning multiple indexes will have to provide the same > SITC for each set of tuples, even in the face of concurrent updates. > Otherwise the BitmapAnd will incorrectly throw them out. The index points to the item id on the page, and that never changes, even if the head tuple changes later. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
On Mon, Jun 26, 2006 at 11:08:24PM -0400, Bruce Momjian wrote: > Jim C. Nasby wrote: > > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > > > > > It is certainly possible to do what you are suggesting, that is have two > > > index entries point to same chain head, and have the index access > > > routines figure out if the index qualifications still hold, but that > > > seems like a lot of overhead. > > > > > > Also, once there is only one visible row in the chain, removing old > > > index entries seems quite complex because you have to have vacuum keep > > > the qualifications of each row to figure out which index tuple is the > > > valid one (seems messy). > > > > Perhaps my point got lost... in the case where no index keys change > > during an update, SITC seems superior in every way to my proposal. My > > idea (let's call it Index Tuple Page Consolidation, ITPC) would be > > beneficial to UPDATEs that modify one or more index keys but still put > > the tuple on the same page. Where SITC would be most useful for tables > > that have a very heavy update rate and very few indexes, ITPC would > > benefit tables that have more indexes on them; where presumably it's > > much more likely for UPDATEs to change at least one index key (which > > means SITC goes out the window, if I understand it correctly). If I'm > > missing something and SITC can in fact deal with some index keys > > changing during an UPDATE, then I see no reason for ITPC. > > I understood what you had said. The question is whether we want to get > that complex with this feature, and if there are enough use cases > (UPDATE with index keys changing) to warrant it. Ideas on how to test a table to see how many tuples would fit this criteria? Or we could just shelve ITPC as a possibility in the future, after we see how much the limitations of SITC hurt. -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.comwork: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
PFC wrote: > > > My idea is that if an UPDATE places the new tuple on the same page as > > the old tuple, it will not create new index entries for any indexes > > where the key doesn't change. > > Basically the idea behind preventing index bloat by updates is to have > one index tuple point to several actual tuples having the same value. > The idea is not to avoid index bloat, but to allow heap reuse, and having one index entry for multiple versions of an UPDATEd row is merely an implementation detail. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > ?hel kenal p?eval, T, 2006-06-27 kell 10:38, kirjutas Hannu Krosing: > > ?hel kenal p?eval, E, 2006-06-26 kell 23:08, kirjutas Bruce Momjian: > > > Jim C. Nasby wrote: > > > > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > > > > > > > > > It is certainly possible to do what you are suggesting, that is have > > > > > two > > > > > index entries point to same chain head, and have the index access > > > > > routines figure out if the index qualifications still hold, but that > > > > > seems like a lot of overhead. > > > > I think Jim meant not 2 pointing to the same head, but 2 pointing into > > the same chain. Say we have table with (id serial, ts timestamp) where > > ts changes at each update and id does not. > > > > So after 3 updates on one page we have one CITC/ITPC head with pointers > > from both indexes and two follow-up tuples with pointers from only ts > > index. > > > > The problem with this setup is, that we can't reuse any of those > > follow-up tuples without index cleanup. > > But we still have to think about similar cases (index entries pointing > inside CITC chains), unless we plan to disallow adding indexes to > tables. CREATE INDEX has to undo any chains where the new indexed columns change in the chain, and add index entries to remove the chain. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
On Mon, Jun 26, 2006 at 11:29:27AM -0400, Bruce Momjian wrote: > > Yes, and for index_getmulti (which doesn't visit the heap at all) we'll > > have to change all the users of that (which aren't many, I suppose). > > It's probably worth making a utility function to expand them. > > > > I'm still confused where bitmap index scan fit into all of this. Is > > preserving the sequential scan aspect of these a goal with this new > > setup? > > No. I was just pointing out that if you get to the tuple via an index, > you get handed the head of the SITC via the index tuple, but if you are > doing a sequential scan, you don't get it, so you have to find it, or > any other non-visible SITC header. Ok, but it remains true that you can only have one SITC per tuple. So if you have 5 indexes on a table, any SITC will only join tuples that didn't change any values in any of the indexed columns. That's probably not a big deal though; indexes columns arn't likely to be the ones changing much. So, for the bitmap scan you have to make sure that within a single transaction, scanning multiple indexes will have to provide the same SITC for each set of tuples, even in the face of concurrent updates. Otherwise the BitmapAnd will incorrectly throw them out. That should be doable, if you only change the head of the SITC on VACUUM. I'm not sure if that's what's being suggested right now. Have a nice day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
> On Fri, Jun 23, 2006 at 06:37:01AM -0400, Mark Woodward wrote: >> While we all know session data is, at best, ephemeral, people still want >> some sort of persistence, thus, you need a database. For mcache I have a >> couple plugins that have a wide range of opitions, from read/write at >> startup and shut down, to full write through cache to a database. >> >> In general, my clients don't want this, they want the database to store >> their data. When you try to explain to them that a database may not be >> the >> right place to store this data, they ask why, sadly they have little >> hope >> of understanding the nuances and remain unconvinced. > > Have you done any benchmarking between a site using mcache and one not? > I'll bet there's a huge difference, which translates into hardware $$. > That's something managers can understand. > Last benchmark I did was on a pure data level, a couple years ago, PostgreSQL could handle about 800 session transactions a second, but degraded over time, MCache was up about 7500 session transactions a second and held steady. I should dig up that code and make it available on my site. I have a couple users that tell me that their sites couldn't work without it, not even with MySQL. ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
> Ãhel kenal päeval, E, 2006-06-26 kell 09:10, kirjutas Mark Woodward: >> > ÃÅhel kenal päeval, R, 2006-06-23 kell 17:27, kirjutas Bruce >> Momjian: >> >> Jonah H. Harris wrote: >> >> > On 6/23/06, Tom Lane <[EMAIL PROTECTED]> wrote: >> >> > > What I see in this discussion is a huge amount of "the grass must >> be >> >> > > greener on the other side" syndrome, and hardly any recognition >> that >> >> > > every technique has its downsides and complications. >> >> > >> >> > I'm being totally objective. I don't think we should abandon >> >> > PostgreSQL's overall design at all, because we do perform INSERTs >> and >> >> > DELETEs much better than most systems. However, I've looked at >> many >> >> > systems and how they implement UPDATE so that it is a scalable >> >> > operation. Sure, there are costs and benefits to each >> implementation, >> >> > but I think we have some pretty brilliant people in this community >> and >> >> > can come up with an elegant design for scalable UPDATEs. >> >> >> >> I think the UPDATE case is similar to the bitmap index scan or >> perhaps >> >> bitmap indexes on disk --- there are cases we know can not be handled >> >> well by our existing code, so we have added (or might add) these >> >> features to try to address those difficult cases. >> > >> > Not really. Bitmap index scan and bitmap index are both new additions >> > working well with existing framework. >> > >> > While the problem of slowdown on frequent updates is real, the >> suggested >> > fix is just plain wrong, as it is based on someones faulty assumption >> on >> > how index lookup works, and very much simplified view of how different >> > parts of the system work to implement MVCC. >> >> Yes, the suggestion was based on MVCC concepts, not a particular >> implementation. > > On the contrary - afaik, it was loosely based on how Oracle does it with > its rollback segments, only assuming that rollback segments are kept in > heap and that indexes point only to the oldest row version :p Well, give me a little more credit than that. Yes, Oracle did play small part in my thinking, but only in as much as "they can't do it, why can't we?" The problem was how to get the most recent tuple to be more efficient and not have tuples that will never be used impact performance without excessive locking or moving data around. It was a just a quick idea. Bruce's solution, you have to admit, is somewhat similar. > >> > The original fix he "suggests" was to that imagined behaviour and thus >> > ignored all the real problems of such change. >> >> The original suggestion, was nothing more than a hypothetical for the >> purpose of discussion. >> >> The problem was the steady degradation of performance on frequent >> updates. >> That was the point of discussion. I brought up "one possible way" to >> start a "brain storm." The discussion then morphed into critisizing the >> example and not addressing the problem. > > The problem is heatedly discussed every 3-4 months. And yet, here we are again. > >> Anyway, I think some decent discussion about the problem did happen, and >> that is good. > > Agreed. > > Maybe this _was_ the best way to bring up the discussion again. I have a way, for better or worse, I guess, of stirring up the pot. :-) Cry as we may about MySQL, but I have a sneaking suspicion that this is one of the issues that puts PostgreSQL at a serious disadvantage. While heavily updated rows are a single type of problem, these days I think *most* database deployments are as back-ends for web sites. This problem is *very* critical to that type of application, consequently probably why PostgreSQL has difficulty in that space. If PostgreSQL can be made *not* to suffer performance degradation on heavily updated rows, then that is realy the last issue in the way of it being a completely creadible medium to large enterprise back end. This combined with its amazing pragramability, should make it unstoppable. ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, E, 2006-06-26 kell 11:31, kirjutas Bruce Momjian: > Hannu Krosing wrote: > > > > pass 3: clean heap based on ctid from pass 1 > > > > > > > > If yo do it this way, you dont need to invent new data structures to > > > > pass extra info about CITC internals to passes 2 and 3 > > > > > > > > On more thing - when should free space map be notified about free space > > > > in pages with CITC chains ? > > > > > > Uh, well, I am thinking we only free CITC space when we are going to use > > > it for an UPDATE, rather than free things while doing an operation. It > > > is good to keep the cleanup overhead out of the main path as much as > > > possible. > > > > So vacuum should only remove dead CITC chains and leave the ones with > > live tuples to CITC internal use ? > > Yes, it has to. What else would it do? Add index entries? No, clean out the dead part. But this would probably add the page to FSM - do we want that. Also, this cleaning should probably be done at pass1, so we dont have to carry the ctids of tuples which have no index entries around to passes 2 and 3 . This has the downside of possibly writing the heap page twice, so maybe we dont want it. > > That would also suggest that pages having live CITC chains and less than > > N% of free space should mot be reported to FSM. > > Parts of the CITC that are not visible can be used for free space by > vacuum, but the visible part is left alone. > -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, T, 2006-06-27 kell 10:38, kirjutas Hannu Krosing: > Ühel kenal päeval, E, 2006-06-26 kell 23:08, kirjutas Bruce Momjian: > > Jim C. Nasby wrote: > > > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > > > > > > > It is certainly possible to do what you are suggesting, that is have two > > > > index entries point to same chain head, and have the index access > > > > routines figure out if the index qualifications still hold, but that > > > > seems like a lot of overhead. > > I think Jim meant not 2 pointing to the same head, but 2 pointing into > the same chain. Say we have table with (id serial, ts timestamp) where > ts changes at each update and id does not. > > So after 3 updates on one page we have one CITC/ITPC head with pointers > from both indexes and two follow-up tuples with pointers from only ts > index. > > The problem with this setup is, that we can't reuse any of those > follow-up tuples without index cleanup. But we still have to think about similar cases (index entries pointing inside CITC chains), unless we plan to disallow adding indexes to tables. Perhaps that case has to simply disable heap tuple reuse until some event. what would that event be? Or maybe we should have some bitmap of dirty tuple ids inside each page, that is tuple ids that have index pointers to them. and then avoid using these ? -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
My idea is that if an UPDATE places the new tuple on the same page as the old tuple, it will not create new index entries for any indexes where the key doesn't change. Basically the idea behind preventing index bloat by updates is to have one index tuple point to several actual tuples having the same value. So : Index entry -> list of tuples having the same value -> actual tuples (-> represents an indirection) I proposed to put the list of tuples in the index ; you propose to put it in data pages. I think both solutions have pros and cons : * List of tuples in the index : + reduces index size, makes cacheability in RAM more likely + speeds up index scans - complexity - slows down modifications to the index (a bit) * List of tuples in the page + simpler to implement + reduces index size, but less so than previous solution - useless if UPDATE puts the new tuple on a different page I guess the best solution would be a mix of both. Also, I insist (again) that there is a lot to gain by using a bit of compression on the data pages, even if it's very simple compression like storing the new version of a row as a difference from the previous version (ie. only store the columns that changed). I think DB2 stores the latest version entirely, and stores the previous versions as a delta. This is more efficient. In the case of tables containing TEXT values, these could also get TOASTed. When an update does not modify the TOASTed columns, it would be nice to simply be able to keep the reference to the TOASTed data instead of decompressing it and recompressing it. Or is it already the case ? ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, E, 2006-06-26 kell 23:08, kirjutas Bruce Momjian: > Jim C. Nasby wrote: > > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > > > > > It is certainly possible to do what you are suggesting, that is have two > > > index entries point to same chain head, and have the index access > > > routines figure out if the index qualifications still hold, but that > > > seems like a lot of overhead. I think Jim meant not 2 pointing to the same head, but 2 pointing into the same chain. Say we have table with (id serial, ts timestamp) where ts changes at each update and id does not. So after 3 updates on one page we have one CITC/ITPC head with pointers from both indexes and two follow-up tuples with pointers from only ts index. The problem with this setup is, that we can't reuse any of those follow-up tuples without index cleanup. > > > Also, once there is only one visible row in the chain, removing old > > > index entries seems quite complex because you have to have vacuum keep > > > the qualifications of each row to figure out which index tuple is the > > > valid one (seems messy). > > > > Perhaps my point got lost... in the case where no index keys change > > during an update, SITC seems superior in every way to my proposal. My > > idea (let's call it Index Tuple Page Consolidation, ITPC) would be > > beneficial to UPDATEs that modify one or more index keys but still put > > the tuple on the same page. Where SITC would be most useful for tables > > that have a very heavy update rate and very few indexes, ITPC would > > benefit tables that have more indexes on them; where presumably it's > > much more likely for UPDATEs to change at least one index key (which > > means SITC goes out the window, if I understand it correctly). If I'm > > missing something and SITC can in fact deal with some index keys > > changing during an UPDATE, then I see no reason for ITPC. > > I understood what you had said. The question is whether we want to get > that complex with this feature, and if there are enough use cases > (UPDATE with index keys changing) to warrant it. I'd like to think that most heavily-updated tables avoid that, but there may be still cases where this is needed. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Jim C. Nasby wrote: > On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > > > It is certainly possible to do what you are suggesting, that is have two > > index entries point to same chain head, and have the index access > > routines figure out if the index qualifications still hold, but that > > seems like a lot of overhead. > > > > Also, once there is only one visible row in the chain, removing old > > index entries seems quite complex because you have to have vacuum keep > > the qualifications of each row to figure out which index tuple is the > > valid one (seems messy). > > Perhaps my point got lost... in the case where no index keys change > during an update, SITC seems superior in every way to my proposal. My > idea (let's call it Index Tuple Page Consolidation, ITPC) would be > beneficial to UPDATEs that modify one or more index keys but still put > the tuple on the same page. Where SITC would be most useful for tables > that have a very heavy update rate and very few indexes, ITPC would > benefit tables that have more indexes on them; where presumably it's > much more likely for UPDATEs to change at least one index key (which > means SITC goes out the window, if I understand it correctly). If I'm > missing something and SITC can in fact deal with some index keys > changing during an UPDATE, then I see no reason for ITPC. I understood what you had said. The question is whether we want to get that complex with this feature, and if there are enough use cases (UPDATE with index keys changing) to warrant it. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On Mon, Jun 26, 2006 at 02:32:59PM -0400, Bruce Momjian wrote: > > It is certainly possible to do what you are suggesting, that is have two > index entries point to same chain head, and have the index access > routines figure out if the index qualifications still hold, but that > seems like a lot of overhead. > > Also, once there is only one visible row in the chain, removing old > index entries seems quite complex because you have to have vacuum keep > the qualifications of each row to figure out which index tuple is the > valid one (seems messy). Perhaps my point got lost... in the case where no index keys change during an update, SITC seems superior in every way to my proposal. My idea (let's call it Index Tuple Page Consolidation, ITPC) would be beneficial to UPDATEs that modify one or more index keys but still put the tuple on the same page. Where SITC would be most useful for tables that have a very heavy update rate and very few indexes, ITPC would benefit tables that have more indexes on them; where presumably it's much more likely for UPDATEs to change at least one index key (which means SITC goes out the window, if I understand it correctly). If I'm missing something and SITC can in fact deal with some index keys changing during an UPDATE, then I see no reason for ITPC. > --- > > Jim C. Nasby wrote: > > On Sun, Jun 25, 2006 at 09:13:48PM +0300, Heikki Linnakangas wrote: > > > >If you can't expire the old row because one of the indexed columns was > > > >modified, I see no reason to try to reduce the additional index entries. > > > > > > It won't enable early expiration, but it means less work to do on update. > > > If there's a lot of indexes, not having to add so many index tuples can > > > be > > > a significant saving. > > > > While catching up on this thread, the following idea came to me that I > > think would allow for not updating an index on an UPDATE if it's key > > doesn't change. If I understand Bruce's SITC proposal correctly, this > > would differ in that SITC requires that no index keys change. > > > > My idea is that if an UPDATE places the new tuple on the same page as > > the old tuple, it will not create new index entries for any indexes > > where the key doesn't change. This means that when fetching tuples from > > the index, ctid would have to be followed until you found the version > > you wanted OR you found the first ctid that pointed to a different page > > (because that tuple will have it's own index entry) OR you found a tuple > > with a different value for the key of the index you're using (because > > it'd be invalid, and there'd be a different index entry for it). I > > believe that the behavior of the index hint bits would also have to > > change somewhat, as each index entry would now essentially be pointing > > at all the tuples in the ctid chain that exist on a page, not just > > single tuple. > > > > In the case of an UPDATE that needs to put the new tuple on a different > > page, our current behavior would be used. This means that the hint bits > > would still be useful in limiting the number of heap pages you hit. I > > also believe this means that we wouldn't suffer any additional overhead > > from our current code when there isn't much free space on pages. > > > > Since SITC allows for in-page space reuse without vacuuming only when no > > index keys change, it's most useful for very heavily updated tables such > > as session handlers or queue tables, because those tables typically have > > very few indexes, so it's pretty unlikely that an index key will change. > > For more general-purpose tables that have more indexes but still see a > > fair number of updates to a subset of rows, not having to update every > > index would likely be a win. I also don't see any reason why both > > options couldn't be used together. > > -- > > Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] > > Pervasive Software http://pervasive.comwork: 512-231-6117 > > vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 > > > > -- > Bruce Momjian [EMAIL PROTECTED] > EnterpriseDBhttp://www.enterprisedb.com > > + If your life is a hard drive, Christ can be your backup. + > > ---(end of broadcast)--- > TIP 5: don't forget to increase your free space map settings > -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.comwork: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Zeugswetter Andreas DCP SD wrote: > > > > head of the chain yet. With an index scan, finding the head is > easy, > > > but for a sequential scan, it seems more difficult, and we don't > have > > > any free space in the tail of the chain to maintain a pointer to the > head. > > > > Thinking some more, there will need to be a bit to uniquely > > identify the head of a CITC. > > I don't think so. It would probably be sufficient to impose an order on > the CITC. > e.g. the oldest tuple version in the CITC is the head. > (An idea just in case we can't spare a bit :-) Well, if we need to scan the page quickly, having the bit, or a bit combination that can only be the head, is helpful. What we usually do is to combine a SITC bit with another bit that would never be set for SITC, and that is the head, and you use macros to properly do tests. We do this already in a number of cases. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On Fri, Jun 23, 2006 at 06:37:01AM -0400, Mark Woodward wrote: > While we all know session data is, at best, ephemeral, people still want > some sort of persistence, thus, you need a database. For mcache I have a > couple plugins that have a wide range of opitions, from read/write at > startup and shut down, to full write through cache to a database. > > In general, my clients don't want this, they want the database to store > their data. When you try to explain to them that a database may not be the > right place to store this data, they ask why, sadly they have little hope > of understanding the nuances and remain unconvinced. Have you done any benchmarking between a site using mcache and one not? I'll bet there's a huge difference, which translates into hardware $$. That's something managers can understand. -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.comwork: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
It is certainly possible to do what you are suggesting, that is have two index entries point to same chain head, and have the index access routines figure out if the index qualifications still hold, but that seems like a lot of overhead. Also, once there is only one visible row in the chain, removing old index entries seems quite complex because you have to have vacuum keep the qualifications of each row to figure out which index tuple is the valid one (seems messy). --- Jim C. Nasby wrote: > On Sun, Jun 25, 2006 at 09:13:48PM +0300, Heikki Linnakangas wrote: > > >If you can't expire the old row because one of the indexed columns was > > >modified, I see no reason to try to reduce the additional index entries. > > > > It won't enable early expiration, but it means less work to do on update. > > If there's a lot of indexes, not having to add so many index tuples can be > > a significant saving. > > While catching up on this thread, the following idea came to me that I > think would allow for not updating an index on an UPDATE if it's key > doesn't change. If I understand Bruce's SITC proposal correctly, this > would differ in that SITC requires that no index keys change. > > My idea is that if an UPDATE places the new tuple on the same page as > the old tuple, it will not create new index entries for any indexes > where the key doesn't change. This means that when fetching tuples from > the index, ctid would have to be followed until you found the version > you wanted OR you found the first ctid that pointed to a different page > (because that tuple will have it's own index entry) OR you found a tuple > with a different value for the key of the index you're using (because > it'd be invalid, and there'd be a different index entry for it). I > believe that the behavior of the index hint bits would also have to > change somewhat, as each index entry would now essentially be pointing > at all the tuples in the ctid chain that exist on a page, not just > single tuple. > > In the case of an UPDATE that needs to put the new tuple on a different > page, our current behavior would be used. This means that the hint bits > would still be useful in limiting the number of heap pages you hit. I > also believe this means that we wouldn't suffer any additional overhead > from our current code when there isn't much free space on pages. > > Since SITC allows for in-page space reuse without vacuuming only when no > index keys change, it's most useful for very heavily updated tables such > as session handlers or queue tables, because those tables typically have > very few indexes, so it's pretty unlikely that an index key will change. > For more general-purpose tables that have more indexes but still see a > fair number of updates to a subset of rows, not having to update every > index would likely be a win. I also don't see any reason why both > options couldn't be used together. > -- > Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] > Pervasive Software http://pervasive.comwork: 512-231-6117 > vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 > -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
On Sun, Jun 25, 2006 at 09:13:48PM +0300, Heikki Linnakangas wrote: > >If you can't expire the old row because one of the indexed columns was > >modified, I see no reason to try to reduce the additional index entries. > > It won't enable early expiration, but it means less work to do on update. > If there's a lot of indexes, not having to add so many index tuples can be > a significant saving. While catching up on this thread, the following idea came to me that I think would allow for not updating an index on an UPDATE if it's key doesn't change. If I understand Bruce's SITC proposal correctly, this would differ in that SITC requires that no index keys change. My idea is that if an UPDATE places the new tuple on the same page as the old tuple, it will not create new index entries for any indexes where the key doesn't change. This means that when fetching tuples from the index, ctid would have to be followed until you found the version you wanted OR you found the first ctid that pointed to a different page (because that tuple will have it's own index entry) OR you found a tuple with a different value for the key of the index you're using (because it'd be invalid, and there'd be a different index entry for it). I believe that the behavior of the index hint bits would also have to change somewhat, as each index entry would now essentially be pointing at all the tuples in the ctid chain that exist on a page, not just single tuple. In the case of an UPDATE that needs to put the new tuple on a different page, our current behavior would be used. This means that the hint bits would still be useful in limiting the number of heap pages you hit. I also believe this means that we wouldn't suffer any additional overhead from our current code when there isn't much free space on pages. Since SITC allows for in-page space reuse without vacuuming only when no index keys change, it's most useful for very heavily updated tables such as session handlers or queue tables, because those tables typically have very few indexes, so it's pretty unlikely that an index key will change. For more general-purpose tables that have more indexes but still see a fair number of updates to a subset of rows, not having to update every index would likely be a win. I also don't see any reason why both options couldn't be used together. -- Jim C. Nasby, Sr. Engineering Consultant [EMAIL PROTECTED] Pervasive Software http://pervasive.comwork: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
> > head of the chain yet. With an index scan, finding the head is easy, > > but for a sequential scan, it seems more difficult, and we don't have > > any free space in the tail of the chain to maintain a pointer to the head. > > Thinking some more, there will need to be a bit to uniquely > identify the head of a CITC. I don't think so. It would probably be sufficient to impose an order on the CITC. e.g. the oldest tuple version in the CITC is the head. (An idea just in case we can't spare a bit :-) Andreas ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC, and compression
What about increasing the size of an existing index entry? Can that be done easily when a new row is added? I'd say it looks pretty much like inserting a new index tuple... Say "value" is the indexed column. Find first page in the index featuring "value". 1 If there is space on the page, add the tuple id to the list of the corresponding index entry (just like creating a new index tuple, but uses less space). else look at next page. If next page has an index tuple with the same indexed value, goto 1 else insert new page and create an index tuple on it I would be worried about the overhead of doing that on compression and decompression. The compression methods mentioned in the article which was passed on the list seemed pretty fast. From IO-limited, the test database became CPU-limited (and a lot faster). ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC, and compression
PFC wrote: > > There were some talks lately about compression. > With a bit of lateral thinking I guess this can be used to contain the > bloat induced by updates. > Of course this is just my hypothesis. > > Compression in indexes : > > Instead of storing (value, tuple identifier) keys in the indexes, store > > (value, [tuple identifier list]) ; ie. all tuples which have the same > indexed value are referenced by the same index tuple, instead of having > one index tuple per actual tuple. > The length of the list would of course be limited to the space actually > > available on an index page ; if many rows have the same indexed value, > several index tuples would be generated so that index tuples fit on index > pages. > This would make the index smaller (more likely to fit in RAM) at the > cost > of a little CPU overhead for index modifications, but would make the index > scans actually use less CPU (no need to compare the indexed value on each > table tuple). What about increasing the size of an existing index entry? Can that be done easily when a new row is added? > Compression in data pages : > > The article that circulated on the list suggested several types of > compression, offset, dictionary, etc. The point is that several row > versions on the same page can be compressed well because these versions > probably have similar column values. > > Just a thought... I would be worried about the overhead of doing that on compression and decompression. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC, and compression
There were some talks lately about compression. With a bit of lateral thinking I guess this can be used to contain the bloat induced by updates. Of course this is just my hypothesis. Compression in indexes : Instead of storing (value, tuple identifier) keys in the indexes, store (value, [tuple identifier list]) ; ie. all tuples which have the same indexed value are referenced by the same index tuple, instead of having one index tuple per actual tuple. The length of the list would of course be limited to the space actually available on an index page ; if many rows have the same indexed value, several index tuples would be generated so that index tuples fit on index pages. This would make the index smaller (more likely to fit in RAM) at the cost of a little CPU overhead for index modifications, but would make the index scans actually use less CPU (no need to compare the indexed value on each table tuple). Compression in data pages : The article that circulated on the list suggested several types of compression, offset, dictionary, etc. The point is that several row versions on the same page can be compressed well because these versions probably have similar column values. Just a thought... ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > > > pass 3: clean heap based on ctid from pass 1 > > > > > > If yo do it this way, you dont need to invent new data structures to > > > pass extra info about CITC internals to passes 2 and 3 > > > > > > On more thing - when should free space map be notified about free space > > > in pages with CITC chains ? > > > > Uh, well, I am thinking we only free CITC space when we are going to use > > it for an UPDATE, rather than free things while doing an operation. It > > is good to keep the cleanup overhead out of the main path as much as > > possible. > > So vacuum should only remove dead CITC chains and leave the ones with > live tuples to CITC internal use ? Yes, it has to. What else would it do? Add index entries? > That would also suggest that pages having live CITC chains and less than > N% of free space should mot be reported to FSM. Parts of the CITC that are not visible can be used for free space by vacuum, but the visible part is left alone. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
Martijn van Oosterhout wrote: -- Start of PGP signed section. > On Mon, Jun 26, 2006 at 10:50:26AM -0400, Bruce Momjian wrote: > > > > I suppose we would also change the index_getmulti() function to return > > > > a set of ctids plus flags so the caller knows to follow the chains, > > > > right? > > > > > > It is probably better to always return the pointer to the head of CITC > > > chain (the one an index points to) and do extra visibility checks and > > > chain-following on each access. This would keep the change internal to > > > tuple fetching functions. > > > > So index_getnext() traverses the chain and returns one member per call. > > Makes sense. Just realize you are in a single index entry returning > > multiple tuples. We will need some record keeping to track that. > > Yes, and for index_getmulti (which doesn't visit the heap at all) we'll > have to change all the users of that (which aren't many, I suppose). > It's probably worth making a utility function to expand them. > > I'm still confused where bitmap index scan fit into all of this. Is > preserving the sequential scan aspect of these a goal with this new > setup? No. I was just pointing out that if you get to the tuple via an index, you get handed the head of the SITC via the index tuple, but if you are doing a sequential scan, you don't get it, so you have to find it, or any other non-visible SITC header. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, E, 2006-06-26 kell 16:58, kirjutas Martijn van Oosterhout: > On Mon, Jun 26, 2006 at 10:50:26AM -0400, Bruce Momjian wrote: > > > > I suppose we would also change the index_getmulti() function to return > > > > a set of ctids plus flags so the caller knows to follow the chains, > > > > right? > > > > > > It is probably better to always return the pointer to the head of CITC > > > chain (the one an index points to) and do extra visibility checks and > > > chain-following on each access. This would keep the change internal to > > > tuple fetching functions. > > > > So index_getnext() traverses the chain and returns one member per call. > > Makes sense. Just realize you are in a single index entry returning > > multiple tuples. We will need some record keeping to track that. > > Yes, and for index_getmulti (which doesn't visit the heap at all) we'll > have to change all the users of that (which aren't many, I suppose). > It's probably worth making a utility function to expand them. > > I'm still confused where bitmap index scan fit into all of this. Is > preserving the sequential scan aspect of these a goal with this new > setup? Bitmap index scan does not have to change much - only the function that gets tuple by its ctid must be able to trace forward chains within the page. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, E, 2006-06-26 kell 10:50, kirjutas Bruce Momjian: > Hannu Krosing wrote: > > ?hel kenal p?eval, E, 2006-06-26 kell 14:56, kirjutas Martijn van > > Oosterhout: > > > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > > > > Correct! We use the same pointers used by normal UPDATEs, except we set > > > > a bit on the old tuple indicating it is a single-index tuple, and we > > > > don't create index entries for the new tuple. Index scan routines will > > > > need to be taught about the new chains, but because only one tuple in > > > > the chain is visible to a single backend, the callers should not need to > > > > be modified. > > > > > > I suppose we would also change the index_getmulti() function to return > > > a set of ctids plus flags so the caller knows to follow the chains, > > > right? > > > > It is probably better to always return the pointer to the head of CITC > > chain (the one an index points to) and do extra visibility checks and > > chain-following on each access. This would keep the change internal to > > tuple fetching functions. > > So index_getnext() traverses the chain and returns one member per call. > Makes sense. Just realize you are in a single index entry returning > multiple tuples. We will need some record keeping to track that. Maybe we need to push visibility checks further down, so that index_getnext() returns only the one heap row that is visible. > > > And for bitmap index scans you would only remember the page in > > > the case of such a tuple, since you can't be sure the exact ctid you've > > > got is the one you want. > > > > no, you should only use the pointer to CITC head outside tuple access > > funtions. And this pointer to CITC head is what is always passed to > > those access functions/macros. > > > > The VACUUM would run its passes thus: > > > > pass 1: run over heap, collect pointers to single dead tuples, and fully > > dead CITC chains (fully dead = no live tuples on this page). Clean up > > old tuples from CITC chains and move live tuples around so that CITC > > points to oldest possibly visible (not vacuumed) tuple. Doing this there > > frees us from need to collect a separate set of pointers for those. Or > > have you planned that old tuples from CITC chains are collected on the > > go/as needed ? Of course we could do both. > > Non-visible CITC members should be freed during an UPDATE on the same > page, so vacuum doesn't have to be involved. Ok. > > pass 2: clean indexes based on ctid from pass 1 > > > > pass 3: clean heap based on ctid from pass 1 > > > > If yo do it this way, you dont need to invent new data structures to > > pass extra info about CITC internals to passes 2 and 3 > > > > On more thing - when should free space map be notified about free space > > in pages with CITC chains ? > > Uh, well, I am thinking we only free CITC space when we are going to use > it for an UPDATE, rather than free things while doing an operation. It > is good to keep the cleanup overhead out of the main path as much as > possible. So vacuum should only remove dead CITC chains and leave the ones with live tuples to CITC internal use ? That would also suggest that pages having live CITC chains and less than N% of free space should mot be reported to FSM. > Also, seems I can't spell algorithms very well: > > Definition: Single-Index-Tuple Chain (SITC) > - > Thinking of vacuum, right now it does these cleanups: > > o non-visible UPDATEs on the same page with no key changes > o non-visible UPDATEs on the same page with key changes > o non-visible UPDATEs on different pages > o DELETEs > o aborted transactions > > The big question is what percentage of dead space is the first one? My > guess is 65%. Can be from 0% to 99.9%, very much dependent on application. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com NOTICE: This communication contains privileged or other confidential information. If you have received it in error, please advise the sender by reply email and immediately delete the message and any attachments without copying or disclosing the contents. ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
On Mon, Jun 26, 2006 at 10:50:26AM -0400, Bruce Momjian wrote: > > > I suppose we would also change the index_getmulti() function to return > > > a set of ctids plus flags so the caller knows to follow the chains, > > > right? > > > > It is probably better to always return the pointer to the head of CITC > > chain (the one an index points to) and do extra visibility checks and > > chain-following on each access. This would keep the change internal to > > tuple fetching functions. > > So index_getnext() traverses the chain and returns one member per call. > Makes sense. Just realize you are in a single index entry returning > multiple tuples. We will need some record keeping to track that. Yes, and for index_getmulti (which doesn't visit the heap at all) we'll have to change all the users of that (which aren't many, I suppose). It's probably worth making a utility function to expand them. I'm still confused where bitmap index scan fit into all of this. Is preserving the sequential scan aspect of these a goal with this new setup? Have a nice day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, E, 2006-06-26 kell 09:10, kirjutas Mark Woodward: > > Ãœhel kenal päeval, R, 2006-06-23 kell 17:27, kirjutas Bruce Momjian: > >> Jonah H. Harris wrote: > >> > On 6/23/06, Tom Lane <[EMAIL PROTECTED]> wrote: > >> > > What I see in this discussion is a huge amount of "the grass must be > >> > > greener on the other side" syndrome, and hardly any recognition that > >> > > every technique has its downsides and complications. > >> > > >> > I'm being totally objective. I don't think we should abandon > >> > PostgreSQL's overall design at all, because we do perform INSERTs and > >> > DELETEs much better than most systems. However, I've looked at many > >> > systems and how they implement UPDATE so that it is a scalable > >> > operation. Sure, there are costs and benefits to each implementation, > >> > but I think we have some pretty brilliant people in this community and > >> > can come up with an elegant design for scalable UPDATEs. > >> > >> I think the UPDATE case is similar to the bitmap index scan or perhaps > >> bitmap indexes on disk --- there are cases we know can not be handled > >> well by our existing code, so we have added (or might add) these > >> features to try to address those difficult cases. > > > > Not really. Bitmap index scan and bitmap index are both new additions > > working well with existing framework. > > > > While the problem of slowdown on frequent updates is real, the suggested > > fix is just plain wrong, as it is based on someones faulty assumption on > > how index lookup works, and very much simplified view of how different > > parts of the system work to implement MVCC. > > Yes, the suggestion was based on MVCC concepts, not a particular > implementation. On the contrary - afaik, it was loosely based on how Oracle does it with its rollback segments, only assuming that rollback segments are kept in heap and that indexes point only to the oldest row version :p > > The original fix he "suggests" was to that imagined behaviour and thus > > ignored all the real problems of such change. > > The original suggestion, was nothing more than a hypothetical for the > purpose of discussion. > > The problem was the steady degradation of performance on frequent updates. > That was the point of discussion. I brought up "one possible way" to > start a "brain storm." The discussion then morphed into critisizing the > example and not addressing the problem. The problem is heatedly discussed every 3-4 months. > Anyway, I think some decent discussion about the problem did happen, and > that is good. Agreed. Maybe this _was_ the best way to bring up the discussion again. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > ?hel kenal p?eval, E, 2006-06-26 kell 14:56, kirjutas Martijn van > Oosterhout: > > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > > > Correct! We use the same pointers used by normal UPDATEs, except we set > > > a bit on the old tuple indicating it is a single-index tuple, and we > > > don't create index entries for the new tuple. Index scan routines will > > > need to be taught about the new chains, but because only one tuple in > > > the chain is visible to a single backend, the callers should not need to > > > be modified. > > > > I suppose we would also change the index_getmulti() function to return > > a set of ctids plus flags so the caller knows to follow the chains, > > right? > > It is probably better to always return the pointer to the head of CITC > chain (the one an index points to) and do extra visibility checks and > chain-following on each access. This would keep the change internal to > tuple fetching functions. So index_getnext() traverses the chain and returns one member per call. Makes sense. Just realize you are in a single index entry returning multiple tuples. We will need some record keeping to track that. > > And for bitmap index scans you would only remember the page in > > the case of such a tuple, since you can't be sure the exact ctid you've > > got is the one you want. > > no, you should only use the pointer to CITC head outside tuple access > funtions. And this pointer to CITC head is what is always passed to > those access functions/macros. > > The VACUUM would run its passes thus: > > pass 1: run over heap, collect pointers to single dead tuples, and fully > dead CITC chains (fully dead = no live tuples on this page). Clean up > old tuples from CITC chains and move live tuples around so that CITC > points to oldest possibly visible (not vacuumed) tuple. Doing this there > frees us from need to collect a separate set of pointers for those. Or > have you planned that old tuples from CITC chains are collected on the > go/as needed ? Of course we could do both. Non-visible CITC members should be freed during an UPDATE on the same page, so vacuum doesn't have to be involved. > pass 2: clean indexes based on ctid from pass 1 > > pass 3: clean heap based on ctid from pass 1 > > If yo do it this way, you dont need to invent new data structures to > pass extra info about CITC internals to passes 2 and 3 > > On more thing - when should free space map be notified about free space > in pages with CITC chains ? Uh, well, I am thinking we only free CITC space when we are going to use it for an UPDATE, rather than free things while doing an operation. It is good to keep the cleanup overhead out of the main path as much as possible. Also, seems I can't spell algorithms very well: Definition: Single-Index-Tuple Chain (SITC) - Thinking of vacuum, right now it does these cleanups: o non-visible UPDATEs on the same page with no key changes o non-visible UPDATEs on the same page with key changes o non-visible UPDATEs on different pages o DELETEs o aborted transactions The big question is what percentage of dead space is the first one? My guess is 65%. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, E, 2006-06-26 kell 14:56, kirjutas Martijn van Oosterhout: > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > > Correct! We use the same pointers used by normal UPDATEs, except we set > > a bit on the old tuple indicating it is a single-index tuple, and we > > don't create index entries for the new tuple. Index scan routines will > > need to be taught about the new chains, but because only one tuple in > > the chain is visible to a single backend, the callers should not need to > > be modified. > > I suppose we would also change the index_getmulti() function to return > a set of ctids plus flags so the caller knows to follow the chains, > right? It is probably better to always return the pointer to the head of CITC chain (the one an index points to) and do extra visibility checks and chain-following on each access. This would keep the change internal to tuple fetching functions. > And for bitmap index scans you would only remember the page in > the case of such a tuple, since you can't be sure the exact ctid you've > got is the one you want. no, you should only use the pointer to CITC head outside tuple access funtions. And this pointer to CITC head is what is always passed to those access functions/macros. The VACUUM would run its passes thus: pass 1: run over heap, collect pointers to single dead tuples, and fully dead CITC chains (fully dead = no live tuples on this page). Clean up old tuples from CITC chains and move live tuples around so that CITC points to oldest possibly visible (not vacuumed) tuple. Doing this there frees us from need to collect a separate set of pointers for those. Or have you planned that old tuples from CITC chains are collected on the go/as needed ? Of course we could do both. pass 2: clean indexes based on ctid from pass 1 pass 3: clean heap based on ctid from pass 1 If yo do it this way, you dont need to invent new data structures to pass extra info about CITC internals to passes 2 and 3 On more thing - when should free space map be notified about free space in pages with CITC chains ? -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
Bruce Momjian wrote: > Bruce Momjian wrote: > > Martijn van Oosterhout wrote: > > -- Start of PGP signed section. > > > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > > > > Correct! We use the same pointers used by normal UPDATEs, except we set > > > > a bit on the old tuple indicating it is a single-index tuple, and we > > > > don't create index entries for the new tuple. Index scan routines will > > > > need to be taught about the new chains, but because only one tuple in > > > > the chain is visible to a single backend, the callers should not need to > > > > be modified. > > > > > > I suppose we would also change the index_getmulti() function to return > > > a set of ctids plus flags so the caller knows to follow the chains, > > > right? And for bitmap index scans you would only remember the page in > > > the case of such a tuple, since you can't be sure the exact ctid you've > > > got is the one you want. > > > > > > Seems doable. > > > > Yes, it just is an issue of where you want to add the complexity --- > > scan entire page when no free space, or only an UPDATE. > > Oh, and because you want to do this when doing an update via sequential > scan as well as an index scan, I am thinking you might need to do the > per-page method because you might not have even seen the head of the > chain yet. With an index scan, finding the head is easy, but for a > sequential scan, it seems more difficult, and we don't have any free > space in the tail of the chain to maintain a pointer to the head. Thinking some more, there will need to be a bit to uniquely identify the head of a CITC. With that, you could just scan the page tuples looking for CITC heads, and checking those to see if they are not visible, and re-using them, rather than doing a full page reorganization where all free spaces is collected in the middle of the page. That should limit the overhead of reuse. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Bruce Momjian wrote: > Martijn van Oosterhout wrote: > -- Start of PGP signed section. > > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > > > Correct! We use the same pointers used by normal UPDATEs, except we set > > > a bit on the old tuple indicating it is a single-index tuple, and we > > > don't create index entries for the new tuple. Index scan routines will > > > need to be taught about the new chains, but because only one tuple in > > > the chain is visible to a single backend, the callers should not need to > > > be modified. > > > > I suppose we would also change the index_getmulti() function to return > > a set of ctids plus flags so the caller knows to follow the chains, > > right? And for bitmap index scans you would only remember the page in > > the case of such a tuple, since you can't be sure the exact ctid you've > > got is the one you want. > > > > Seems doable. > > Yes, it just is an issue of where you want to add the complexity --- > scan entire page when no free space, or only an UPDATE. Oh, and because you want to do this when doing an update via sequential scan as well as an index scan, I am thinking you might need to do the per-page method because you might not have even seen the head of the chain yet. With an index scan, finding the head is easy, but for a sequential scan, it seems more difficult, and we don't have any free space in the tail of the chain to maintain a pointer to the head. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Martijn van Oosterhout wrote: -- Start of PGP signed section. > On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > > Correct! We use the same pointers used by normal UPDATEs, except we set > > a bit on the old tuple indicating it is a single-index tuple, and we > > don't create index entries for the new tuple. Index scan routines will > > need to be taught about the new chains, but because only one tuple in > > the chain is visible to a single backend, the callers should not need to > > be modified. > > I suppose we would also change the index_getmulti() function to return > a set of ctids plus flags so the caller knows to follow the chains, > right? And for bitmap index scans you would only remember the page in > the case of such a tuple, since you can't be sure the exact ctid you've > got is the one you want. > > Seems doable. Yes, it just is an issue of where you want to add the complexity --- scan entire page when no free space, or only an UPDATE. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
> Heikki Linnakangas wrote: >> On Mon, 26 Jun 2006, Jan Wieck wrote: >> >> > On 6/25/2006 10:12 PM, Bruce Momjian wrote: >> >> When you are using the update chaining, you can't mark that index row >> as >> >> dead because it actually points to more than one row on the page, >> some >> >> are non-visible, some are visible. >> > >> > Back up the truck ... you mean in the current code base we have heap >> tuples >> > that are visible in index scans because of heap tuple chaining but >> without >> > index tuples pointing directly at them? >> >> In current code, no. Every heap tuple has corresponding index tuples. >> >> In Bruce's proposal, yes. You would have heap tuples without index >> tuples >> pointing directly at them. An index scan could only find them by >> following >> t_ctid chains. >> >> Correct me if I understood you incorrectly, Bruce. > > Correct! We use the same pointers used by normal UPDATEs, except we set > a bit on the old tuple indicating it is a single-index tuple, and we > don't create index entries for the new tuple. Index scan routines will > need to be taught about the new chains, but because only one tuple in > the chain is visible to a single backend, the callers should not need to > be modified. > > (All tuples in the chain have page item ids. It is just that when they > are freed, the pointers are adjusted so the index points to the chain > head.) > > One problem is that once you find the row you want to update, it is > difficult to see if it is part of a single-index chain because there are > only forward pointers, so I think we have to scan the entire page to > find the chains. To reduce that overhead, I am thinking we free the > non-visible tuples only when the page has no more free space. This > allows us to free not just our own non-visible tuples, but perhaps > others as well. This sort of incorporates the "vacuum row" I suggested. > > We have never been able to free non-visible tuples before because of > index cleanup overhead, but with single-index chains, we can, and reduce > the requirements of vacuum for many workloads. > This is great! ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On Mon, Jun 26, 2006 at 07:17:31AM -0400, Bruce Momjian wrote: > Correct! We use the same pointers used by normal UPDATEs, except we set > a bit on the old tuple indicating it is a single-index tuple, and we > don't create index entries for the new tuple. Index scan routines will > need to be taught about the new chains, but because only one tuple in > the chain is visible to a single backend, the callers should not need to > be modified. I suppose we would also change the index_getmulti() function to return a set of ctids plus flags so the caller knows to follow the chains, right? And for bitmap index scans you would only remember the page in the case of such a tuple, since you can't be sure the exact ctid you've got is the one you want. Seems doable. Have a nice day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
> Ãhel kenal päeval, R, 2006-06-23 kell 17:27, kirjutas Bruce Momjian: >> Jonah H. Harris wrote: >> > On 6/23/06, Tom Lane <[EMAIL PROTECTED]> wrote: >> > > What I see in this discussion is a huge amount of "the grass must be >> > > greener on the other side" syndrome, and hardly any recognition that >> > > every technique has its downsides and complications. >> > >> > I'm being totally objective. I don't think we should abandon >> > PostgreSQL's overall design at all, because we do perform INSERTs and >> > DELETEs much better than most systems. However, I've looked at many >> > systems and how they implement UPDATE so that it is a scalable >> > operation. Sure, there are costs and benefits to each implementation, >> > but I think we have some pretty brilliant people in this community and >> > can come up with an elegant design for scalable UPDATEs. >> >> I think the UPDATE case is similar to the bitmap index scan or perhaps >> bitmap indexes on disk --- there are cases we know can not be handled >> well by our existing code, so we have added (or might add) these >> features to try to address those difficult cases. > > Not really. Bitmap index scan and bitmap index are both new additions > working well with existing framework. > > While the problem of slowdown on frequent updates is real, the suggested > fix is just plain wrong, as it is based on someones faulty assumption on > how index lookup works, and very much simplified view of how different > parts of the system work to implement MVCC. Yes, the suggestion was based on MVCC concepts, not a particular implementation. > > The original fix he "suggests" was to that imagined behaviour and thus > ignored all the real problems of such change. The original suggestion, was nothing more than a hypothetical for the purpose of discussion. The problem was the steady degradation of performance on frequent updates. That was the point of discussion. I brought up "one possible way" to start a "brain storm." The discussion then morphed into critisizing the example and not addressing the problem. Anyway, I think some decent discussion about the problem did happen, and that is good. ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Heikki Linnakangas wrote: > On Mon, 26 Jun 2006, Jan Wieck wrote: > > > On 6/25/2006 10:12 PM, Bruce Momjian wrote: > >> When you are using the update chaining, you can't mark that index row as > >> dead because it actually points to more than one row on the page, some > >> are non-visible, some are visible. > > > > Back up the truck ... you mean in the current code base we have heap tuples > > that are visible in index scans because of heap tuple chaining but without > > index tuples pointing directly at them? > > In current code, no. Every heap tuple has corresponding index tuples. > > In Bruce's proposal, yes. You would have heap tuples without index tuples > pointing directly at them. An index scan could only find them by following > t_ctid chains. > > Correct me if I understood you incorrectly, Bruce. Correct! We use the same pointers used by normal UPDATEs, except we set a bit on the old tuple indicating it is a single-index tuple, and we don't create index entries for the new tuple. Index scan routines will need to be taught about the new chains, but because only one tuple in the chain is visible to a single backend, the callers should not need to be modified. (All tuples in the chain have page item ids. It is just that when they are freed, the pointers are adjusted so the index points to the chain head.) One problem is that once you find the row you want to update, it is difficult to see if it is part of a single-index chain because there are only forward pointers, so I think we have to scan the entire page to find the chains. To reduce that overhead, I am thinking we free the non-visible tuples only when the page has no more free space. This allows us to free not just our own non-visible tuples, but perhaps others as well. We have never been able to free non-visible tuples before because of index cleanup overhead, but with single-index chains, we can, and reduce the requirements of vacuum for many workloads. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Zeugswetter Andreas DCP SD wrote: > > > > On 6/25/2006 10:12 PM, Bruce Momjian wrote: > > > >When you are using the update chaining, you can't mark that index > row > > > >as dead because it actually points to more than one row on the > page, > > > >some are non-visible, some are visible. > > > > > > Back up the truck ... you mean in the current code base we have heap > > > > tuples that are visible in index scans because of heap tuple > chaining > > > but without index tuples pointing directly at them? > > > > I don't know where this idea came from, but it's not true. > > All heap tuples, dead or otherwise, have index entries. > > When using CITC you would be "reusing" the index tuples from the current > heap tuple, so you can only reuse free space or a dead member of a CITC > chain. > You cannot reuse a dead tuple not member of a CITC chain because that > has separate > (invalid) index tuples pointing at it. > > Part of the trick was moving slots (==ctid) around, so I still do not > really see how > you can represent the CITC chain as part of the update chain. > Unless you intend to break dead parts of the update chain ? Maybe that > is ok ? Yes, you have to remove dead (non-visible) parts of the update chain. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
> > On 6/25/2006 10:12 PM, Bruce Momjian wrote: > > >When you are using the update chaining, you can't mark that index row > > >as dead because it actually points to more than one row on the page, > > >some are non-visible, some are visible. > > > > Back up the truck ... you mean in the current code base we have heap > > tuples that are visible in index scans because of heap tuple chaining > > but without index tuples pointing directly at them? > > I don't know where this idea came from, but it's not true. > All heap tuples, dead or otherwise, have index entries. When using CITC you would be "reusing" the index tuples from the current heap tuple, so you can only reuse free space or a dead member of a CITC chain. You cannot reuse a dead tuple not member of a CITC chain because that has separate (invalid) index tuples pointing at it. Part of the trick was moving slots (==ctid) around, so I still do not really see how you can represent the CITC chain as part of the update chain. Unless you intend to break dead parts of the update chain ? Maybe that is ok ? Andreas ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Alvaro Herrera wrote: > Jan Wieck wrote: > > On 6/25/2006 10:12 PM, Bruce Momjian wrote: > > >When you are using the update chaining, you can't mark that index row as > > >dead because it actually points to more than one row on the page, some > > >are non-visible, some are visible. > > > > Back up the truck ... you mean in the current code base we have heap > > tuples that are visible in index scans because of heap tuple chaining > > but without index tuples pointing directly at them? > > I don't know where this idea came from, but it's not true. All heap > tuples, dead or otherwise, have index entries. Unless the idea is to > extend update chaining to mean something different from the current > meaning. It does mean something different. Single-Index-Tuple Chain (CITC) is a special type of update chaining where the updates are all on the same row, and a single index entry points to the entire chain. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
Jan Wieck wrote: > On 6/25/2006 10:12 PM, Bruce Momjian wrote: > > When you are using the update chaining, you can't mark that index row as > > dead because it actually points to more than one row on the page, some > > are non-visible, some are visible. > > Back up the truck ... you mean in the current code base we have heap > tuples that are visible in index scans because of heap tuple chaining > but without index tuples pointing directly at them? No, this would be new code added. The basic idea is with the new same-page update chaining, a single index points to the head of a chain, not to a single tuple, so you can't mark a tuple as pointing to dead rows if any of the tuples in the chain are visible. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
On Mon, 26 Jun 2006, Jan Wieck wrote: On 6/25/2006 10:12 PM, Bruce Momjian wrote: When you are using the update chaining, you can't mark that index row as dead because it actually points to more than one row on the page, some are non-visible, some are visible. Back up the truck ... you mean in the current code base we have heap tuples that are visible in index scans because of heap tuple chaining but without index tuples pointing directly at them? In current code, no. Every heap tuple has corresponding index tuples. In Bruce's proposal, yes. You would have heap tuples without index tuples pointing directly at them. An index scan could only find them by following t_ctid chains. Correct me if I understood you incorrectly, Bruce. - Heikki ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
Jan Wieck wrote: > On 6/25/2006 10:12 PM, Bruce Momjian wrote: > >When you are using the update chaining, you can't mark that index row as > >dead because it actually points to more than one row on the page, some > >are non-visible, some are visible. > > Back up the truck ... you mean in the current code base we have heap > tuples that are visible in index scans because of heap tuple chaining > but without index tuples pointing directly at them? I don't know where this idea came from, but it's not true. All heap tuples, dead or otherwise, have index entries. Unless the idea is to extend update chaining to mean something different from the current meaning. -- Alvaro Herrerahttp://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
On 6/25/2006 10:12 PM, Bruce Momjian wrote: When you are using the update chaining, you can't mark that index row as dead because it actually points to more than one row on the page, some are non-visible, some are visible. Back up the truck ... you mean in the current code base we have heap tuples that are visible in index scans because of heap tuple chaining but without index tuples pointing directly at them? Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Jan Wieck wrote: > > [item1]...[tuple1] > > > > becomes on UPDATE: > >--> > > [item1]...[tuple1][tuple2] > > -> > > > > on another UPDATE, if tuple1 is no longer visible: > > > >--> > > [item1]...[tuple1][tuple2] > > <-- > > > >> Another problem with this is that even if you find such row, it doesn't > >> spare you the index traversal. The dead row whos item id you're reusing > >> might have resulted from an insert that aborted or crashed before it > >> finished creating all index entries. Or some of its index entries might > >> already be flagged known dead, and you better reset those flags. > > > > You can only reuse heap rows that were created and expired by committed > > transactions. In fact, you can only UPDATE a row that was created by a > > committed transaction. You cannot _reuse_ any row, but only a row that > > is being UPDATEd. Also, it cannot be known dead because it are are in > > the process of updating it. > > Now you lost me. What do you mean "a row that is being UPDATEd"? The row > (version) being UPDATEd right now cannot be expired, or why would you > update that one? And if your transaction rolls back later, the row you > update right now must be the one surviving. It can only be a non-visible version of the row earlier in the UPDATE chain, not the actual one being updated. > Any row that was created by a committed transaction does indeed have all > the index entries created. But if it is deleted and expired, that means > that the transaction that stamped xmax has committed and is outside of > every existing snapshot. You can only reuse a slot that is used by a > tuple that satisfies the vacuum snapshot. And a tuple that satisfies > that snapshot has potentially index entries flagged known dead. When you are using the update chaining, you can't mark that index row as dead because it actually points to more than one row on the page, some are non-visible, some are visible. > > I am thinking my idea was not fully understood. Hopefully this email > > helps. > > I must be missing something because I still don't see how it can work. I just posted pseudo-code. Hope that helps. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
On 6/25/2006 5:18 PM, Bruce Momjian wrote: Jan Wieck wrote: >> An update that results in all the same values of every indexed column of >> a known deleted invisible tuple. This reused tuple can by definition not >> be the one currently updated. So unless it is a table without a primary >> key, this assumes that at least 3 versions of the same row exist within >> the same block. How likely is that to happen? > > Good question. You take the current tuple, and make another one on the > same page. Later, an update can reuse the original tuple if it is no > longer visible to anyone (by changing the item id), so you only need two > tuples, not three. My hope is that a repeated update would eventually > move to a page that enough free space for two (or more) versions. > > Does that help explain it? > That's exactly what I meant. You need space for 3 or more tuple versions within one page and the luck that one of them is invisible at the time of the update. I don't know how likely or unlikely this is in reality, but it doesn't sound very promising to me so far. Why three? I explained using only two heap tuples: For some reason I counted in the new tuple ... sorry that. Yes, it can work with two tuples. [item1]...[tuple1] becomes on UPDATE: --> [item1]...[tuple1][tuple2] -> on another UPDATE, if tuple1 is no longer visible: --> [item1]...[tuple1][tuple2] <-- Another problem with this is that even if you find such row, it doesn't spare you the index traversal. The dead row whos item id you're reusing might have resulted from an insert that aborted or crashed before it finished creating all index entries. Or some of its index entries might already be flagged known dead, and you better reset those flags. You can only reuse heap rows that were created and expired by committed transactions. In fact, you can only UPDATE a row that was created by a committed transaction. You cannot _reuse_ any row, but only a row that is being UPDATEd. Also, it cannot be known dead because it are are in the process of updating it. Now you lost me. What do you mean "a row that is being UPDATEd"? The row (version) being UPDATEd right now cannot be expired, or why would you update that one? And if your transaction rolls back later, the row you update right now must be the one surviving. Any row that was created by a committed transaction does indeed have all the index entries created. But if it is deleted and expired, that means that the transaction that stamped xmax has committed and is outside of every existing snapshot. You can only reuse a slot that is used by a tuple that satisfies the vacuum snapshot. And a tuple that satisfies that snapshot has potentially index entries flagged known dead. I am thinking my idea was not fully understood. Hopefully this email helps. I must be missing something because I still don't see how it can work. Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] vacuum, performance, and MVCC
bruce wrote: > Why three? I explained using only two heap tuples: > > [item1]...[tuple1] > > becomes on UPDATE: >--> > [item1]...[tuple1][tuple2] > -> > > on another UPDATE, if tuple1 is no longer visible: > >--> > [item1]...[tuple1][tuple2] > <-- > Here is some pseudo-code that implements this: Definition: Single-Index-Tuple Chain (CITC) Do old and new UPDATE rows have unchanged indexed columns? Is old row member of CITC? Point item id at first CITC visible tuple in chain Mark previous invisible CITC tuples as freespace Does page have free space? Add new tuple on the same page as old Mark old tuple as CITC Do not create index entries VACUUM would have to be taught about CITC, and CREATE INDEX would have to create entries in other indexes for cases where its new indexed columns change inside a CITC. Conceptually, CITC allows a single index entry to point to multiple UPDATEd rows, allowing non-visible tuples to be recycled (and reused for future UPDATEs) without affecting the index. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > ?hel kenal p?eval, P, 2006-06-25 kell 14:24, kirjutas Bruce Momjian: > > Jan Wieck wrote: > > > >> Sure, but index reuse seems a lot easier, as there is nothing > > > >> additional > > > >> to remember or clean out when doing it. > > > > > > > > Yes, seems so. TODO added: > > > > > > > > * Reuse index tuples that point to heap tuples that are not > > > > visible to > > > > anyone? > > > > > > > >> When reusing a heap tuple you have to clean out all index entries > > > >> pointing to it. > > > > > > > > Well, not for UPDATE for no key changes on the same page, if we do that. > > > > > > > > > > An update that results in all the same values of every indexed column of > > > a known deleted invisible tuple. This reused tuple can by definition not > > > be the one currently updated. So unless it is a table without a primary > > > key, this assumes that at least 3 versions of the same row exist within > > > the same block. How likely is that to happen? > > > > Good question. You take the current tuple, and make another one on the > > same page. Later, an update can reuse the original tuple if it is no > > longer visible to anyone (by changing the item id), so you only need two > > tuples, not three. My hope is that a repeated update would eventually > > move to a page that enough free space for two (or more) versions. > > I can confirm that this is exactly what happens when running an > update-heavy load with frequent vacuums. Eventually most rows get their > own db pages or share the same page with 2-3 rows. And there will be > lots of unused (filed up, or cleaned and not yet reused) pages. Right, that was my guess because heavily updated rows start to move around in the table, and because UPDATE tries to stay on the same page, once it the row hits a mostly-empty page, it stays there. > The overall performance could be made a little better by tuning the > system to not put more than N new rows on the same page at initial > insert or when the row move to a new page during update. Currently > several new rows are initially put on the same page and then move around > during repeated updates until they slow(ish)ly claim their own page. We have a fillfactor patch that will be in 8.2. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Jan Wieck wrote: > >> An update that results in all the same values of every indexed column of > >> a known deleted invisible tuple. This reused tuple can by definition not > >> be the one currently updated. So unless it is a table without a primary > >> key, this assumes that at least 3 versions of the same row exist within > >> the same block. How likely is that to happen? > > > > Good question. You take the current tuple, and make another one on the > > same page. Later, an update can reuse the original tuple if it is no > > longer visible to anyone (by changing the item id), so you only need two > > tuples, not three. My hope is that a repeated update would eventually > > move to a page that enough free space for two (or more) versions. > > > > Does that help explain it? > > > > That's exactly what I meant. You need space for 3 or more tuple versions > within one page and the luck that one of them is invisible at the time > of the update. I don't know how likely or unlikely this is in reality, > but it doesn't sound very promising to me so far. Why three? I explained using only two heap tuples: [item1]...[tuple1] becomes on UPDATE: --> [item1]...[tuple1][tuple2] -> on another UPDATE, if tuple1 is no longer visible: --> [item1]...[tuple1][tuple2] <-- > Another problem with this is that even if you find such row, it doesn't > spare you the index traversal. The dead row whos item id you're reusing > might have resulted from an insert that aborted or crashed before it > finished creating all index entries. Or some of its index entries might > already be flagged known dead, and you better reset those flags. You can only reuse heap rows that were created and expired by committed transactions. In fact, you can only UPDATE a row that was created by a committed transaction. You cannot _reuse_ any row, but only a row that is being UPDATEd. Also, it cannot be known dead because it are are in the process of updating it. I am thinking my idea was not fully understood. Hopefully this email helps. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, P, 2006-06-25 kell 06:52, kirjutas Mark Woodward: > I'm not sure why vacuum can't run similarly to the way it does now. What do you mean ? Currently vacuum runs a three-step process 1) runs a full scan over heap and collects all dead tuple ctids from heap 2) run full scans over all indexes of the relation and removes any pointers pointing to dead tuples. 3) runs another full scan over heap and removes the tuples in the list collected at step 1. There is no modifications done to live tuples (ok, they *may* get frozen if they are above certain age, but this is not relevant to current discussion). -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, P, 2006-06-25 kell 14:24, kirjutas Bruce Momjian: > Jan Wieck wrote: > > >> Sure, but index reuse seems a lot easier, as there is nothing additional > > >> to remember or clean out when doing it. > > > > > > Yes, seems so. TODO added: > > > > > > * Reuse index tuples that point to heap tuples that are not visible to > > > anyone? > > > > > >> When reusing a heap tuple you have to clean out all index entries > > >> pointing to it. > > > > > > Well, not for UPDATE for no key changes on the same page, if we do that. > > > > > > > An update that results in all the same values of every indexed column of > > a known deleted invisible tuple. This reused tuple can by definition not > > be the one currently updated. So unless it is a table without a primary > > key, this assumes that at least 3 versions of the same row exist within > > the same block. How likely is that to happen? > > Good question. You take the current tuple, and make another one on the > same page. Later, an update can reuse the original tuple if it is no > longer visible to anyone (by changing the item id), so you only need two > tuples, not three. My hope is that a repeated update would eventually > move to a page that enough free space for two (or more) versions. I can confirm that this is exactly what happens when running an update-heavy load with frequent vacuums. Eventually most rows get their own db pages or share the same page with 2-3 rows. And there will be lots of unused (filed up, or cleaned and not yet reused) pages. The overall performance could be made a little better by tuning the system to not put more than N new rows on the same page at initial insert or when the row move to a new page during update. Currently several new rows are initially put on the same page and then move around during repeated updates until they slow(ish)ly claim their own page. > Does that help explain it? > -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
On 6/25/2006 2:24 PM, Bruce Momjian wrote: Jan Wieck wrote: >> Sure, but index reuse seems a lot easier, as there is nothing additional >> to remember or clean out when doing it. > > Yes, seems so. TODO added: > > * Reuse index tuples that point to heap tuples that are not visible to > anyone? > >> When reusing a heap tuple you have to clean out all index entries >> pointing to it. > > Well, not for UPDATE for no key changes on the same page, if we do that. > An update that results in all the same values of every indexed column of a known deleted invisible tuple. This reused tuple can by definition not be the one currently updated. So unless it is a table without a primary key, this assumes that at least 3 versions of the same row exist within the same block. How likely is that to happen? Good question. You take the current tuple, and make another one on the same page. Later, an update can reuse the original tuple if it is no longer visible to anyone (by changing the item id), so you only need two tuples, not three. My hope is that a repeated update would eventually move to a page that enough free space for two (or more) versions. Does that help explain it? That's exactly what I meant. You need space for 3 or more tuple versions within one page and the luck that one of them is invisible at the time of the update. I don't know how likely or unlikely this is in reality, but it doesn't sound very promising to me so far. Another problem with this is that even if you find such row, it doesn't spare you the index traversal. The dead row whos item id you're reusing might have resulted from an insert that aborted or crashed before it finished creating all index entries. Or some of its index entries might already be flagged known dead, and you better reset those flags. Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
Heikki Linnakangas wrote: > On Sat, 24 Jun 2006, Bruce Momjian wrote: > > > Because having them be on the same page is the only way you can update > > the page item pointer so when you recycle the row, you the indexes are > > now pointing to the new version. Pages look like: > > > > [marker][item1][item2][item3]...[tuple1][tuple2][tuple3] > > > > and indexes only point to items, not to tuples. This allows tuples to > > be compacted on the page without affecting the indexes. > > > > If tuple1 is updated to tuple2, once tuple1 is no longer visible to any > > backends, you can modify item1 to point to tuple2, and you can mark the > > space used by tuple1 as reusable: > > > > [marker][item1(tuple2)][item2][item3]...[free][tuple2][tuple3] > > Ok, now I think I get it. So the limitation of old and new tuple being on > the same page is required to make it possible to remove the old tuple > without touching the indexes? Yes, modifying the heap page item pointer is required for reuse. > If you move the new tuple (logically, by modifying item pointers) on > vacuum, isn't there a risk that a concurrent seqscan misses it? Well, you lock the page while updating the item pointer. Because the versions are on the same page, a single page lock should be fine. > > If you can't expire the old row because one of the indexed columns was > > modified, I see no reason to try to reduce the additional index entries. > > It won't enable early expiration, but it means less work to do on update. > If there's a lot of indexes, not having to add so many index tuples can be > a significant saving. Already added to TODO. * Reuse index tuples that point to heap tuples that are not visible to anyone? > To summarise, we have two issues related to frequent updates: > 1. Index and heap bloat, requiring frequent vacuum. > 2. Updates are more expensive than on other DBMSs, because we have to add > a new index tuple in every index, even if none of the indexed columns are > modified. > > Tom suggested that we just improve vacuum and autovacuum, and someone > brought up the dead space map idea again. Those are all worthwhile things > to do and help with vacuuming after deletes as well as updates, but they > only address issue 1. Mark's suggestion (assuming that it would've > worked) as well as yours address both, but only for updates. Agreed. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
Jan Wieck wrote: > >> Sure, but index reuse seems a lot easier, as there is nothing additional > >> to remember or clean out when doing it. > > > > Yes, seems so. TODO added: > > > > * Reuse index tuples that point to heap tuples that are not visible to > > anyone? > > > >> When reusing a heap tuple you have to clean out all index entries > >> pointing to it. > > > > Well, not for UPDATE for no key changes on the same page, if we do that. > > > > An update that results in all the same values of every indexed column of > a known deleted invisible tuple. This reused tuple can by definition not > be the one currently updated. So unless it is a table without a primary > key, this assumes that at least 3 versions of the same row exist within > the same block. How likely is that to happen? Good question. You take the current tuple, and make another one on the same page. Later, an update can reuse the original tuple if it is no longer visible to anyone (by changing the item id), so you only need two tuples, not three. My hope is that a repeated update would eventually move to a page that enough free space for two (or more) versions. Does that help explain it? -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, 24 Jun 2006, Bruce Momjian wrote: Because having them be on the same page is the only way you can update the page item pointer so when you recycle the row, you the indexes are now pointing to the new version. Pages look like: [marker][item1][item2][item3]...[tuple1][tuple2][tuple3] and indexes only point to items, not to tuples. This allows tuples to be compacted on the page without affecting the indexes. If tuple1 is updated to tuple2, once tuple1 is no longer visible to any backends, you can modify item1 to point to tuple2, and you can mark the space used by tuple1 as reusable: [marker][item1(tuple2)][item2][item3]...[free][tuple2][tuple3] Ok, now I think I get it. So the limitation of old and new tuple being on the same page is required to make it possible to remove the old tuple without touching the indexes? If you move the new tuple (logically, by modifying item pointers) on vacuum, isn't there a risk that a concurrent seqscan misses it? If you can't expire the old row because one of the indexed columns was modified, I see no reason to try to reduce the additional index entries. It won't enable early expiration, but it means less work to do on update. If there's a lot of indexes, not having to add so many index tuples can be a significant saving. To summarise, we have two issues related to frequent updates: 1. Index and heap bloat, requiring frequent vacuum. 2. Updates are more expensive than on other DBMSs, because we have to add a new index tuple in every index, even if none of the indexed columns are modified. Tom suggested that we just improve vacuum and autovacuum, and someone brought up the dead space map idea again. Those are all worthwhile things to do and help with vacuuming after deletes as well as updates, but they only address issue 1. Mark's suggestion (assuming that it would've worked) as well as yours address both, but only for updates. - Heikki ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/2006 4:10 PM, Hannu Krosing wrote: Ühel kenal päeval, L, 2006-06-24 kell 15:44, kirjutas Jan Wieck: >> That fixes the symptom, not the problem. The problem is performance >> steadily degrades over time. > > No, you got it backwards. The performance degradation is the symptom. > The problem is that there are too many dead tuples in the table. There > is one way to solve that problem -- remove them, which is done by > running vacuum. Precisely. > There are some problems with vacuum itself, that I agree with. For > example it would be good if a long-running vacuum wouldn't affect a > vacuum running in another table because of the long-running transaction > effect it has. It would be good if vacuum could be run partially over a > table. It would be good if there was a way to speed up vacuum by using > a dead space map or something. It would be good if vacuum wouldn't waste time on blocks that don't have any possible work in them. Vacuum has two main purposes. A) remove dead rows and B) freeze xids. Once a block has zero deleted rows and all xids are frozen, there is nothing to do with this block and vacuum should skip it until a transaction updates that block. This requires 2 bits per block, which is 32K per 1G segment of a heap. Clearing the bits is done when the block is marked dirty. This way vacuum would not waste any time and IO on huge slow changing tables. That part, sequentially scanning huge tables that didn't change much is what keeps us from running vacuum every couple of seconds. Seems like a plan. Still, there is another problem which is not solved by map approach only, at least with current implementation of vacuum. This is the fact that we need to do full scan over index(es) to clean up pointers to removed tuples. And huge tables tend to have huge indexes. Right, now that you say it I remember why this wasn't so easy as it sounded at the beginning. Obviously there is no other way to find an index tuple without a sequential scan other than doing an index scan. So vacuum would have to estimate based on the bitmaps if it could be beneficial (huge table, little vacuumable pages) to actually remove/flag single index tuples before removing the heap tuple. This can be done in advance to removing the heap tuple because index tuples might not be there to begin with. However, that is a very costly thing to do and not trivial to implement. Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
On 6/25/2006 12:27 PM, Bruce Momjian wrote: Hannu Krosing wrote: > > Maybe we could start from reusing the index tuples which point to > > invisible tuples ? The index is not MVCC anyway, so maybe it is easier > > to do in-place replacement there ? > > > > This probably has the same obstacles which have prevented us from > > removing those in the first place (removing instead of marking as > > invisible). Does it cause some locking issues ? Or does it go against > > some other constraints of our index lookups ? > > > > I think that just setting the invisible bit in an index leaf node causes > > nearly as much disk io as removing the node. > > > > If we could delete/reuse old index tuples, it would solve a sizable > > chunk of index-growth problem, especially for cases where referenced key > > value does not change. > > I think heap _and_ index reuse is the only useful direction. Index or > heap reuse alone seems too marginal for the added complexity. Sure, but index reuse seems a lot easier, as there is nothing additional to remember or clean out when doing it. Yes, seems so. TODO added: * Reuse index tuples that point to heap tuples that are not visible to anyone? When reusing a heap tuple you have to clean out all index entries pointing to it. Well, not for UPDATE for no key changes on the same page, if we do that. An update that results in all the same values of every indexed column of a known deleted invisible tuple. This reused tuple can by definition not be the one currently updated. So unless it is a table without a primary key, this assumes that at least 3 versions of the same row exist within the same block. How likely is that to happen? Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
On 6/25/2006 6:52 AM, Mark Woodward wrote: On 6/24/2006 9:23 AM, Mark Woodward wrote: On Sat, 24 Jun 2006, Mark Woodward wrote: I'm probably mistaken, but aren't there already forward references in tuples to later versions? If so, I'm only sugesting reversing the order and referencing the latest version. I thought I understood your idea, but now you lost me again. I thought what you want is that the older heap tuple has a pointer to the newer one. Which it already has, it's called t_ctid. Perfect! Can you try to explain more carefully how the whole thing would work? What would an index tuple point to? What pointers would a heap tuple have? What would an index scan do to find the row version it's interested in? What exactly would an update do? Since we already allocate space for some notion of linked list, then all I'm suggesting is we reverse the order, sort of. Currently it looks like this: ver001->ver002->ver003->...-verN That's what t_ctid does now, right? Well, that's sort of stupid. Why not have it do this: ver001->verN->...->ver003->ver002->| ^-/ This will speed up almost *all* queries when there are more than two version of rows. OK, here is the behavior of an update: (1) Find the latest version of the row (2) Duplicate row and modify as per plan (3) Set the t_ctid of the new row to the last "latest" (4) Set the t_ctid of the first row to that of the new row (5) Attempt to index the row (6) If the first version of the row is in the index already (ver001) Don't modify the index, otherwise, add the new version (just as before) When you vacuum, simply make the latest version (verN) the key row (ver001). This isn't done "simply". Currently, vacuum collects a trivial array of ctid's it is removing and every now and then does a bulk remove of the index tuples pointing to them. Now lets consider a table with two indexed columns with the following row versions resulting from an insert and 3 updates to that same row: v1: a,b v2: a,c v3: a,d v4: b,d In your new scheme, there would be two index tuples for column 1 (one pointing to v1, one pointing to v4) and 3 index tuples for column 2 (one for each different value pointing to v1, v2 and v3). Did I get that right so far? If vacuum now can remove v1, it has to update index 1 to point to v2 and remove the pointer to v1 from index 2. If it can remove v1 and v2, it has to update index 1 to point to v3 and remove v1 and v2 from index 2. If it can remove v1, v2 and v3 it must delete the index 1 tuple pointing to v1, delete the index 2 entries pointing to v1 and v2 and update the index 2 entry for v3 to point to v4. Figuring out which index tuples to remove and which ones to update can only be done by comparing each and every indexed columns old and new values. To do so, vacuum will have to fetch all the row versions, which can be scattered all over the place, with all possible locking issues including but not limited to deadlocks. I'm not sure why vacuum can't run similarly to the way it does now. What's that supposed to mean? Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > > > Maybe we could start from reusing the index tuples which point to > > > invisible tuples ? The index is not MVCC anyway, so maybe it is easier > > > to do in-place replacement there ? > > > > > > This probably has the same obstacles which have prevented us from > > > removing those in the first place (removing instead of marking as > > > invisible). Does it cause some locking issues ? Or does it go against > > > some other constraints of our index lookups ? > > > > > > I think that just setting the invisible bit in an index leaf node causes > > > nearly as much disk io as removing the node. > > > > > > If we could delete/reuse old index tuples, it would solve a sizable > > > chunk of index-growth problem, especially for cases where referenced key > > > value does not change. > > > > I think heap _and_ index reuse is the only useful direction. Index or > > heap reuse alone seems too marginal for the added complexity. > > Sure, but index reuse seems a lot easier, as there is nothing additional > to remember or clean out when doing it. Yes, seems so. TODO added: * Reuse index tuples that point to heap tuples that are not visible to anyone? > When reusing a heap tuple you have to clean out all index entries > pointing to it. Well, not for UPDATE for no key changes on the same page, if we do that. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
> On 6/24/2006 9:23 AM, Mark Woodward wrote: > >>> On Sat, 24 Jun 2006, Mark Woodward wrote: >>> I'm probably mistaken, but aren't there already forward references in tuples to later versions? If so, I'm only sugesting reversing the order and referencing the latest version. >>> >>> I thought I understood your idea, but now you lost me again. I thought >>> what you want is that the older heap tuple has a pointer to the >>> newer one. Which it already has, it's called t_ctid. >> >> Perfect! >>> >>> Can you try to explain more carefully how the whole thing would work? >>> What would an index tuple point to? What pointers would a heap tuple >>> have? What would an index scan do to find the row version it's >>> interested >>> in? What exactly would an update do? >> >> >> Since we already allocate space for some notion of linked list, then all >> I'm suggesting is we reverse the order, sort of. Currently it looks like >> this: >> >> ver001->ver002->ver003->...-verN >> >> That's what t_ctid does now, right? Well, that's sort of stupid. Why not >> have it do this: >> >> ver001->verN->...->ver003->ver002->| >> ^-/ >> >> This will speed up almost *all* queries when there are more than two >> version of rows. >> >> OK, here is the behavior of an update: >> (1) Find the latest version of the row >> (2) Duplicate row and modify as per plan >> (3) Set the t_ctid of the new row to the last "latest" >> (4) Set the t_ctid of the first row to that of the new row >> (5) Attempt to index the row >> (6) If the first version of the row is in the index already (ver001) >> Don't >> modify the index, otherwise, add the new version (just as before) >> >> When you vacuum, simply make the latest version (verN) the key row >> (ver001). > > This isn't done "simply". Currently, vacuum collects a trivial array of > ctid's it is removing and every now and then does a bulk remove of the > index tuples pointing to them. Now lets consider a table with two > indexed columns with the following row versions resulting from an insert > and 3 updates to that same row: > >v1: a,b >v2: a,c >v3: a,d >v4: b,d > > In your new scheme, there would be two index tuples for column 1 (one > pointing to v1, one pointing to v4) and 3 index tuples for column 2 (one > for each different value pointing to v1, v2 and v3). Did I get that > right so far? > > If vacuum now can remove v1, it has to update index 1 to point to v2 and > remove the pointer to v1 from index 2. If it can remove v1 and v2, it > has to update index 1 to point to v3 and remove v1 and v2 from index 2. > If it can remove v1, v2 and v3 it must delete the index 1 tuple pointing > to v1, delete the index 2 entries pointing to v1 and v2 and update the > index 2 entry for v3 to point to v4. Figuring out which index tuples to > remove and which ones to update can only be done by comparing each and > every indexed columns old and new values. To do so, vacuum will have to > fetch all the row versions, which can be scattered all over the place, > with all possible locking issues including but not limited to deadlocks. I'm not sure why vacuum can't run similarly to the way it does now. ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, L, 2006-06-24 kell 19:36, kirjutas Bruce Momjian: > Hannu Krosing wrote: > > ?hel kenal p?eval, R, 2006-06-23 kell 13:08, kirjutas Tom Lane: > > > > > > Bottom line: there's still lots of low-hanging fruit. Why are people > > > feeling that we need to abandon or massively complicate our basic > > > architecture to make progress? > > > > Maybe we could start from reusing the index tuples which point to > > invisible tuples ? The index is not MVCC anyway, so maybe it is easier > > to do in-place replacement there ? > > > > This probably has the same obstacles which have prevented us from > > removing those in the first place (removing instead of marking as > > invisible). Does it cause some locking issues ? Or does it go against > > some other constraints of our index lookups ? > > > > I think that just setting the invisible bit in an index leaf node causes > > nearly as much disk io as removing the node. > > > > If we could delete/reuse old index tuples, it would solve a sizable > > chunk of index-growth problem, especially for cases where referenced key > > value does not change. > > I think heap _and_ index reuse is the only useful direction. Index or > heap reuse alone seems too marginal for the added complexity. Sure, but index reuse seems a lot easier, as there is nothing additional to remember or clean out when doing it. When reusing a heap tuple you have to clean out all index entries pointing to it. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
Hannu Krosing wrote: > ?hel kenal p?eval, R, 2006-06-23 kell 13:08, kirjutas Tom Lane: > > Csaba Nagy <[EMAIL PROTECTED]> writes: > > >> Surprisingly its mostly WAL traffic, the heap/index pages themselves are > > >> often not yet synced to disk by time of vacuum, so no additional traffic > > >> there. If you had made 5 updates per page and then vacuum it, then you > > >> make effectively 1 extra WAL write meaning 20% increase in WAL traffic. > > > > > Is this also holding about read traffic ? I thought vacuum will make a > > > full table scan... for big tables a full table scan is always badly > > > influencing the performance of the box. If the full table scan would be > > > avoided, then I wouldn't mind running vacuum in a loop... > > > > If you're doing heavy updates of a big table then it's likely to end up > > visiting most of the table anyway, no? There is talk of keeping a map > > of dirty pages, but I think it'd be a win for infrequently-updated > > tables, not ones that need constant vacuuming. > > > > I think a lot of our problems in this area could be solved with fairly > > straightforward tuning efforts on the existing autovacuum > > infrastructure. In particular, someone should be looking into > > recommendable default vacuum-cost-delay settings so that a background > > vacuum doesn't affect performance too much. > > One thing that would help updates quite a lot in some scenarios is > keeping the pages only partially-filled, so that most updates could keep > the new version in the same page. I think that has also been discussed > as an option to vacuum and maybe as part of initial inserts. Maybe some > of it even ended up as a todo item. We have a patch in the queue for index fillfactor which will be in 8.2. I am also hoping the frequently updated rows will migrate out to the empty pages. > > Another problem with the > > current autovac infrastructure is that it doesn't respond very well to > > the case where there are individual tables that need constant attention > > as well as many that don't. If you have N databases then you can visit > > a particular table at most once every N*autovacuum_naptime seconds, and > > *every* table in the entire cluster gets reconsidered at that same rate. > > I'm not sure if we need the ability to have multiple autovac daemons > > running at the same time, > > My patch enabling effective continuous vacuum of fast-update tables, > while still being able to vacuum huge slowly changing ones is still not > applied. Without that patch there is no reason to vacuum the small and > fast changingg tables while vacuum on bigger tables is running, as it > won't clean out dead tuples anyway. I think it will be applied, but I am looking for someone else to eyeball it since Tom has come concerns. > > but we definitely could use something with a > > more flexible table-visiting pattern. Perhaps it would be enough to > > look through the per-table stats for each database before selecting the > > database to autovacuum in each cycle, instead of going by "least > > recently autovacuumed". > > > > Bottom line: there's still lots of low-hanging fruit. Why are people > > feeling that we need to abandon or massively complicate our basic > > architecture to make progress? > > Maybe we could start from reusing the index tuples which point to > invisible tuples ? The index is not MVCC anyway, so maybe it is easier > to do in-place replacement there ? > > This probably has the same obstacles which have prevented us from > removing those in the first place (removing instead of marking as > invisible). Does it cause some locking issues ? Or does it go against > some other constraints of our index lookups ? > > I think that just setting the invisible bit in an index leaf node causes > nearly as much disk io as removing the node. > > If we could delete/reuse old index tuples, it would solve a sizable > chunk of index-growth problem, especially for cases where referenced key > value does not change. I think heap _and_ index reuse is the only useful direction. Index or heap reuse alone seems too marginal for the added complexity. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Mark Woodward wrote: > The update behavior of PostgreSQL is probably the *last* serious issue. > Debate all you want, vacuum mitigates the problem to varying levels, > fixing the problem will be a huge win. If the update behavior gets fixed, > I can't think of a single issue with postgresql that would be a show > stopper. Nah, it's just *your* pet peeve. Everyone has theirs. Some people may share yours, of course. I agree it's a problem, but from there to saying "it's _the last_ issue" there's a lot of distance. Your idea of reusing a tuple's self pointer (t_ctid) does not work BTW, because the self pointer must point to self. The case where the pointer does not point to exactly the same tuple, it must point to a newer version. If you change that invariant, a lot of things break; see for example heap_get_lastest_tid. -- Alvaro Herrerahttp://www.CommandPrompt.com/ The PostgreSQL Company - Command Prompt, Inc. ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
Heikki Linnakangas wrote: > On Sat, 24 Jun 2006, Bruce Momjian wrote: > > > OK, I have an idea. Right now, an UPDATE where the old and new rows are > > on the same page have two index entries. What if we made only one index > > entry for both? We already have UPDATE chaining, where the old row > > points to the new one. If no key columns change, we set a bit in the > > heap that the chaining points to the old and new row (both on the same > > page), so an index scan uses one index entry to see the old and new row, > > and once the old row is no longer visible, the page index id can be > > updated to point to the new row and the old row can be marked free and > > perhaps used for a future UPDATE. (UPDATE already tries to do keep > > updates on the same heap page.) > > In fact, that's what I originally thought Mark was suggesting. A couple of > points: > > Why the limitation of old and new row being on the same page? Because having them be on the same page is the only way you can update the page item pointer so when you recycle the row, you the indexes are now pointing to the new version. Pages look like: [marker][item1][item2][item3]...[tuple1][tuple2][tuple3] and indexes only point to items, not to tuples. This allows tuples to be compacted on the page without affecting the indexes. If tuple1 is updated to tuple2, once tuple1 is no longer visible to any backends, you can modify item1 to point to tuple2, and you can mark the space used by tuple1 as reusable: [marker][item1(tuple2)][item2][item3]...[free][tuple2][tuple3] > This only works if none of the updated columns are indexed. That's a bit > annoying. It would be nice to be able to create new index tuples in those The hope is that a commonly updated tuple will eventually be on a page where there is sufficient free space for updated version to stay on there. For an active server, there might be several updated versions of rows on the same page. > indexes that contain one of the changed columns, but not in others. If you can't expire the old row because one of the indexed columns was modified, I see no reason to try to reduce the additional index entries. > What happens if you create a new index that contains one of > the changed columns? Uh, I had not thought of that. You could easily create two index entries for the old and new rows, but then the heap bit saying there is only one index row would be inaccurate for the new index. I suppose you could create new rows in all indexes and clear the heap bit. -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
Martha Stewart called it a Good Thing when [EMAIL PROTECTED] (Jan Wieck) wrote:
> On 6/22/2006 2:37 PM, Alvaro Herrera wrote:
>
>> Adding back pgsql-hackers.
>> Mark Woodward wrote:
>>> > Mark Woodward wrote:
>>> >
>>> >> Hmm, OK, then the problem is more serious than I suspected.
>>> >> This means that every index on a row has to be updated on every
>>> >> transaction that modifies that row. Is that correct?
>>> >
>>> > Add an index entry, yes.
>>> >
>>> >> I am attaching some code that shows the problem with regard to
>>> >> applications such as web server session management, when run, each
>>> >> second
>>> >> the system can handle fewer and fewer connections. Here is a brief
>>> >> output:
>>> >> [...]
>>> >> There has to be a more linear way of handling this scenario.
>>> >
>>> > So vacuum the table often.
>>> That fixes the symptom, not the problem. The problem is performance
>>> steadily degrades over time.
>> No, you got it backwards. The performance degradation is the
>> symptom.
>> The problem is that there are too many dead tuples in the table. There
>> is one way to solve that problem -- remove them, which is done by
>> running vacuum.
>
> Precisely.
>
>> There are some problems with vacuum itself, that I agree with. For
>> example it would be good if a long-running vacuum wouldn't affect a
>> vacuum running in another table because of the long-running transaction
>> effect it has. It would be good if vacuum could be run partially over a
>> table. It would be good if there was a way to speed up vacuum by using
>> a dead space map or something.
>
> It would be good if vacuum wouldn't waste time on blocks that don't
> have any possible work in them. Vacuum has two main purposes. A)
> remove dead rows and B) freeze xids. Once a block has zero deleted
> rows and all xids are frozen, there is nothing to do with this block
> and vacuum should skip it until a transaction updates that block.
>
> This requires 2 bits per block, which is 32K per 1G segment of a
> heap. Clearing the bits is done when the block is marked dirty. This
> way vacuum would not waste any time and IO on huge slow changing
> tables. That part, sequentially scanning huge tables that didn't
> change much is what keeps us from running vacuum every couple of
> seconds.
This is, in effect, the "VACUUM Space Map."
I see one unfortunate thing about that representation of it, namely
that it would in effect require that non-frozen pages be kept on the
VSM for potentially a long time.
Based on *present* VACUUM strategy, at least.
Would it not be the case, here, that any time a page could be
"frozen," it would have to be? In effect, we are always trying to run
VACUUM FREEZE?
--
output = ("cbbrowne" "@" "gmail.com")
http://cbbrowne.com/info/finances.html
Rules of the Evil Overlord #72. "If all the heroes are standing
together around a strange device and begin to taunt me, I will pull
out a conventional weapon instead of using my unstoppable superweapon
on them.
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, Jun 24, 2006 at 10:04:43PM +0300, Hannu Krosing wrote: > Maybe we could start from reusing the index tuples which point to > invisible tuples ? The index is not MVCC anyway, so maybe it is easier > to do in-place replacement there ? > > This probably has the same obstacles which have prevented us from > removing those in the first place (removing instead of marking as > invisible). Does it cause some locking issues ? Or does it go against > some other constraints of our index lookups ? The problem with updating an index is that you have to do it in a way that concurrent scans (forwards and backwards) don't get confused because the tuple they stopped on vanished. AIUI, the current approach is two step. The first time round you mark it deleted but don't actually delete it. Thus, any scan currently stopped on that tuple won't have a problem. Sometime later you remove the actual tuple, once you know there's no scan stopped on it (because no scan will deliberatly stop on a deleted tuple). I forget the actual locking steps that ensure this though. > If we could delete/reuse old index tuples, it would solve a sizable > chunk of index-growth problem, especially for cases where referenced key > value does not change. I think we recently changed the code to always scan an index a page at a time so maybe scans no longer stop in the middle of a page anymore... Or perhaps that was VACUUM only. Have a noce day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, L, 2006-06-24 kell 15:44, kirjutas Jan Wieck: > >> That fixes the symptom, not the problem. The problem is performance > >> steadily degrades over time. > > > > No, you got it backwards. The performance degradation is the symptom. > > The problem is that there are too many dead tuples in the table. There > > is one way to solve that problem -- remove them, which is done by > > running vacuum. > > Precisely. > > > There are some problems with vacuum itself, that I agree with. For > > example it would be good if a long-running vacuum wouldn't affect a > > vacuum running in another table because of the long-running transaction > > effect it has. It would be good if vacuum could be run partially over a > > table. It would be good if there was a way to speed up vacuum by using > > a dead space map or something. > > It would be good if vacuum wouldn't waste time on blocks that don't have > any possible work in them. Vacuum has two main purposes. A) remove dead > rows and B) freeze xids. Once a block has zero deleted rows and all xids > are frozen, there is nothing to do with this block and vacuum should > skip it until a transaction updates that block. > > This requires 2 bits per block, which is 32K per 1G segment of a heap. > Clearing the bits is done when the block is marked dirty. This way > vacuum would not waste any time and IO on huge slow changing tables. > That part, sequentially scanning huge tables that didn't change much is > what keeps us from running vacuum every couple of seconds. Seems like a plan. Still, there is another problem which is not solved by map approach only, at least with current implementation of vacuum. This is the fact that we need to do full scan over index(es) to clean up pointers to removed tuples. And huge tables tend to have huge indexes. As indexes have no MVCC info inside them, it may be possible to start reusing index entries pointing to rows that are invisible to all running transactions. Currently we just mark these index entries as dead, but maybe there is a way to reuse them. This could solve the index bloat problem for may cases. Another possible solution for indexes with mostly dead pointers is doing a reindex, but this will become possible only after we have implemented a concurrent, non-blocking CREATE INDEX. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On 6/22/2006 2:37 PM, Alvaro Herrera wrote: Adding back pgsql-hackers. Mark Woodward wrote: > Mark Woodward wrote: > >> Hmm, OK, then the problem is more serious than I suspected. >> This means that every index on a row has to be updated on every >> transaction that modifies that row. Is that correct? > > Add an index entry, yes. > >> I am attaching some code that shows the problem with regard to >> applications such as web server session management, when run, each >> second >> the system can handle fewer and fewer connections. Here is a brief >> output: >> [...] >> There has to be a more linear way of handling this scenario. > > So vacuum the table often. That fixes the symptom, not the problem. The problem is performance steadily degrades over time. No, you got it backwards. The performance degradation is the symptom. The problem is that there are too many dead tuples in the table. There is one way to solve that problem -- remove them, which is done by running vacuum. Precisely. There are some problems with vacuum itself, that I agree with. For example it would be good if a long-running vacuum wouldn't affect a vacuum running in another table because of the long-running transaction effect it has. It would be good if vacuum could be run partially over a table. It would be good if there was a way to speed up vacuum by using a dead space map or something. It would be good if vacuum wouldn't waste time on blocks that don't have any possible work in them. Vacuum has two main purposes. A) remove dead rows and B) freeze xids. Once a block has zero deleted rows and all xids are frozen, there is nothing to do with this block and vacuum should skip it until a transaction updates that block. This requires 2 bits per block, which is 32K per 1G segment of a heap. Clearing the bits is done when the block is marked dirty. This way vacuum would not waste any time and IO on huge slow changing tables. That part, sequentially scanning huge tables that didn't change much is what keeps us from running vacuum every couple of seconds. Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, R, 2006-06-23 kell 17:27, kirjutas Bruce Momjian: > Jonah H. Harris wrote: > > On 6/23/06, Tom Lane <[EMAIL PROTECTED]> wrote: > > > What I see in this discussion is a huge amount of "the grass must be > > > greener on the other side" syndrome, and hardly any recognition that > > > every technique has its downsides and complications. > > > > I'm being totally objective. I don't think we should abandon > > PostgreSQL's overall design at all, because we do perform INSERTs and > > DELETEs much better than most systems. However, I've looked at many > > systems and how they implement UPDATE so that it is a scalable > > operation. Sure, there are costs and benefits to each implementation, > > but I think we have some pretty brilliant people in this community and > > can come up with an elegant design for scalable UPDATEs. > > I think the UPDATE case is similar to the bitmap index scan or perhaps > bitmap indexes on disk --- there are cases we know can not be handled > well by our existing code, so we have added (or might add) these > features to try to address those difficult cases. Not really. Bitmap index scan and bitmap index are both new additions working well with existing framework. While the problem of slowdown on frequent updates is real, the suggested fix is just plain wrong, as it is based on someones faulty assumption on how index lookup works, and very much simplified view of how different parts of the system work to implement MVCC. The original fix he "suggests" was to that imagined behaviour and thus ignored all the real problems of such change. All the next suggestions were variations of the first ones, and failed to address or even research any problems brought up. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
Ühel kenal päeval, R, 2006-06-23 kell 13:08, kirjutas Tom Lane: > Csaba Nagy <[EMAIL PROTECTED]> writes: > >> Surprisingly its mostly WAL traffic, the heap/index pages themselves are > >> often not yet synced to disk by time of vacuum, so no additional traffic > >> there. If you had made 5 updates per page and then vacuum it, then you > >> make effectively 1 extra WAL write meaning 20% increase in WAL traffic. > > > Is this also holding about read traffic ? I thought vacuum will make a > > full table scan... for big tables a full table scan is always badly > > influencing the performance of the box. If the full table scan would be > > avoided, then I wouldn't mind running vacuum in a loop... > > If you're doing heavy updates of a big table then it's likely to end up > visiting most of the table anyway, no? There is talk of keeping a map > of dirty pages, but I think it'd be a win for infrequently-updated > tables, not ones that need constant vacuuming. > > I think a lot of our problems in this area could be solved with fairly > straightforward tuning efforts on the existing autovacuum > infrastructure. In particular, someone should be looking into > recommendable default vacuum-cost-delay settings so that a background > vacuum doesn't affect performance too much. One thing that would help updates quite a lot in some scenarios is keeping the pages only partially-filled, so that most updates could keep the new version in the same page. I think that has also been discussed as an option to vacuum and maybe as part of initial inserts. Maybe some of it even ended up as a todo item. > Another problem with the > current autovac infrastructure is that it doesn't respond very well to > the case where there are individual tables that need constant attention > as well as many that don't. If you have N databases then you can visit > a particular table at most once every N*autovacuum_naptime seconds, and > *every* table in the entire cluster gets reconsidered at that same rate. > I'm not sure if we need the ability to have multiple autovac daemons > running at the same time, My patch enabling effective continuous vacuum of fast-update tables, while still being able to vacuum huge slowly changing ones is still not applied. Without that patch there is no reason to vacuum the small and fast changingg tables while vacuum on bigger tables is running, as it won't clean out dead tuples anyway. > but we definitely could use something with a > more flexible table-visiting pattern. Perhaps it would be enough to > look through the per-table stats for each database before selecting the > database to autovacuum in each cycle, instead of going by "least > recently autovacuumed". > > Bottom line: there's still lots of low-hanging fruit. Why are people > feeling that we need to abandon or massively complicate our basic > architecture to make progress? Maybe we could start from reusing the index tuples which point to invisible tuples ? The index is not MVCC anyway, so maybe it is easier to do in-place replacement there ? This probably has the same obstacles which have prevented us from removing those in the first place (removing instead of marking as invisible). Does it cause some locking issues ? Or does it go against some other constraints of our index lookups ? I think that just setting the invisible bit in an index leaf node causes nearly as much disk io as removing the node. If we could delete/reuse old index tuples, it would solve a sizable chunk of index-growth problem, especially for cases where referenced key value does not change. -- Hannu Krosing Database Architect Skype Technologies OÜ Akadeemia tee 21 F, Tallinn, 12618, Estonia Skype me: callto:hkrosing Get Skype for free: http://www.skype.com ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/2006 9:23 AM, Mark Woodward wrote: On Sat, 24 Jun 2006, Mark Woodward wrote: I'm probably mistaken, but aren't there already forward references in tuples to later versions? If so, I'm only sugesting reversing the order and referencing the latest version. I thought I understood your idea, but now you lost me again. I thought what you want is that the older heap tuple has a pointer to the newer one. Which it already has, it's called t_ctid. Perfect! Can you try to explain more carefully how the whole thing would work? What would an index tuple point to? What pointers would a heap tuple have? What would an index scan do to find the row version it's interested in? What exactly would an update do? Since we already allocate space for some notion of linked list, then all I'm suggesting is we reverse the order, sort of. Currently it looks like this: ver001->ver002->ver003->...-verN That's what t_ctid does now, right? Well, that's sort of stupid. Why not have it do this: ver001->verN->...->ver003->ver002->| ^-/ This will speed up almost *all* queries when there are more than two version of rows. OK, here is the behavior of an update: (1) Find the latest version of the row (2) Duplicate row and modify as per plan (3) Set the t_ctid of the new row to the last "latest" (4) Set the t_ctid of the first row to that of the new row (5) Attempt to index the row (6) If the first version of the row is in the index already (ver001) Don't modify the index, otherwise, add the new version (just as before) When you vacuum, simply make the latest version (verN) the key row (ver001). This isn't done "simply". Currently, vacuum collects a trivial array of ctid's it is removing and every now and then does a bulk remove of the index tuples pointing to them. Now lets consider a table with two indexed columns with the following row versions resulting from an insert and 3 updates to that same row: v1: a,b v2: a,c v3: a,d v4: b,d In your new scheme, there would be two index tuples for column 1 (one pointing to v1, one pointing to v4) and 3 index tuples for column 2 (one for each different value pointing to v1, v2 and v3). Did I get that right so far? If vacuum now can remove v1, it has to update index 1 to point to v2 and remove the pointer to v1 from index 2. If it can remove v1 and v2, it has to update index 1 to point to v3 and remove v1 and v2 from index 2. If it can remove v1, v2 and v3 it must delete the index 1 tuple pointing to v1, delete the index 2 entries pointing to v1 and v2 and update the index 2 entry for v3 to point to v4. Figuring out which index tuples to remove and which ones to update can only be done by comparing each and every indexed columns old and new values. To do so, vacuum will have to fetch all the row versions, which can be scattered all over the place, with all possible locking issues including but not limited to deadlocks. Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, 24 Jun 2006, Bruce Momjian wrote: OK, I have an idea. Right now, an UPDATE where the old and new rows are on the same page have two index entries. What if we made only one index entry for both? We already have UPDATE chaining, where the old row points to the new one. If no key columns change, we set a bit in the heap that the chaining points to the old and new row (both on the same page), so an index scan uses one index entry to see the old and new row, and once the old row is no longer visible, the page index id can be updated to point to the new row and the old row can be marked free and perhaps used for a future UPDATE. (UPDATE already tries to do keep updates on the same heap page.) In fact, that's what I originally thought Mark was suggesting. A couple of points: Why the limitation of old and new row being on the same page? This only works if none of the updated columns are indexed. That's a bit annoying. It would be nice to be able to create new index tuples in those indexes that contain one of the changed columns, but not in others. What happens if you create a new index that contains one of the changed columns? - Heikki ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
> On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: >> > On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: >> >> In the scenario, as previously outlined: >> >> >> >> ver001->verN->...->ver003->ver2->| >> >> ^-/ >> > >> > So you want to always keep an old version around? >> >> Prior to vacuum, it will be there anyway, and after vacuum, the new >> version will become ver001. > > So you do intend to move verN into ver001's slot? What about the > other conditions you had mentioned where you have to follow > PostgreSQL's current behavior? How are you going to have a pointer > chain in that case? Who said anything about moving anything. When vacuum comes along, it cleans out previous versions of rows. Very little will change. ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
bruce wrote: > Tom Lane wrote: > > Bruce Momjian <[EMAIL PROTECTED]> writes: > > > I think at some point we have to admit that _polling_ the tables, which > > > is what autovacuum does, just isn't going to work well, no matter how > > > much it is tweeked, and another approach should be considered for > > > certain workload cases. > > > > Autovacuum polls in its current, first-generation implementation; > > what I said upthread was it needs to be smarter than that. I am not > > sure how you get from that to the conclusion that the very next step > > is to abandon the vacuuming approach altogether. > > I am not ready to abandon autovacuum, but as I stated later the UPDATE > with no key change case is common enought that it could be handled > better without involving autovacuum and its limitations. > > As I remember, most databases have problem with DELETE/INSERT cycles, > but we seem to be hit by UPDATE performance more than most, and more > than is wise. In an attempt to get some concrete on these ideas... ;-) I think the major complexity in doing an in-place UPDATE when no key columns change is allowing rollback on transaction abort (or backend crash), and properly handling visibility for transactions in progress. If the old and new rows are on the same heap page (perhaps a necessary limitation), you can easily update the heap item id to point to the new heap row. All indexes will then point to the new row, and sequential scans will still see both rows (which is good). That leave the rollback issue (undoing the item id change), and having index scans for current backends still see the old row. OK, I have an idea. Right now, an UPDATE where the old and new rows are on the same page have two index entries. What if we made only one index entry for both? We already have UPDATE chaining, where the old row points to the new one. If no key columns change, we set a bit in the heap that the chaining points to the old and new row (both on the same page), so an index scan uses one index entry to see the old and new row, and once the old row is no longer visible, the page index id can be updated to point to the new row and the old row can be marked free and perhaps used for a future UPDATE. (UPDATE already tries to do keep updates on the same heap page.) FYI, the reason heap cleanup is possible once you go with a single index entry for old and new rows is because there is no index cleanup (and single-row index cleanup is very expensive). -- Bruce Momjian [EMAIL PROTECTED] EnterpriseDBhttp://www.enterprisedb.com + If your life is a hard drive, Christ can be your backup. + ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
What I see in this discussion is a huge amount of "the grass must be greener on the other side" syndrome, and hardly any recognition that every technique has its downsides and complications. Sure ;) MVCC generates dead rows, by its very nature ; however I see two trends in this : 1* A large transaction that updates/deletes many rows. For instance suppose you UPDATE an entire table whose size is larger than memory. Old row versions have to be kept somewhere until commit, be it in the table itself or in some accessory undo-log. So, there will be a lot of harddisk grinding anyway, be it MVCC or Oracle-style, or whatever. MVCC will bloat the table and indexes, then VACUUM will shrink them. Update-in-place systems will bloat an undo log. It seems to me the current MVCC+VACUUM is the right tool for this job, requiring about the same amount of IO that the others. Vacuum scans sequentially, so it's the best way to process large volumes of data. 2* Many short transactions update rows in a table Like the sessions problem, for instance. Current VACUUM sucks for this case, I guess that's known. --- So, we have two different problems, and one tool which is adapted to one problem only. Should the tool (Vacuum) be fixed to handle both problems, making it more complex and difficult to maintain, or should another tool be created specifically for the second problem ? Problem 2 is very different from problem 1. The only case when they meet is when there is a continuous stream of small updates running concurrently with a long transaction. So, what is the ideal tool for case 2 ? We'd want a vacuuming machine that can be run very often, or even better, continuously. The table can be large, larger than the disk cache, so scanning it is not an option. The updates are probably randomly distributed into the table. Therefore, VACUUMing a long time after these transactions are commited and the pages are no longer in cache would require a lot of random seeks, which is also bad. Besides, we want this vacuum to be continuous and transparent. The best time to vacuum pages is, thus, when they are still in the background writer's memory, or the disk cache, waiting to be flushed to disk. There, they can be re-read, vacuumed and re-written with no seek penalty, only additional WAL traffic. However the amount of WAL traffic in bytes/s is less important that the frequency of WAL syncs. Emitting more WAL data shouldn't be a problem if those sync writes are coalesced with the sync writes of current reansactions. So, I guess the best solution for case 2 is to have the background writer perform on-the-fly VACUUM : An UPDATE or DELETE transaction hands over dirty pages to be written to the bgwriter. It also tells the bgwriter the ID of the current transaction and flags specifying if they contain candidate dead rows. The bgwriter should have a sufficiently large buffer in order to be able to keep these pages in memory until all the transactions that can see the dead rows in these pages are finished. Then, the pages are vacuumed and written. The key is the size of the buffer. It should be large enough to contain enough pages so that it is actually possible to vacuum something out of them before writing them. However if the buffer capacity is exceeded (for instance, because there is a long running transaction), this is not a problem : the pages are simply written to disk normally, they will contain dead rows, which will need to be handled lated by the standard VACUUM. I think this would maximize code reuse by using the current bgwriter's architecture... did I miss something ? ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: > On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: >> In the scenario, as previously outlined: >> >> ver001->verN->...->ver003->ver2->| >> ^-/ > > So you want to always keep an old version around? Prior to vacuum, it will be there anyway, and after vacuum, the new version will become ver001. So you do intend to move verN into ver001's slot? What about the other conditions you had mentioned where you have to follow PostgreSQL's current behavior? How are you going to have a pointer chain in that case? -- Jonah H. Harris, Software Architect | phone: 732.331.1300 EnterpriseDB Corporation| fax: 732.331.1301 33 Wood Ave S, 2nd Floor| [EMAIL PROTECTED] Iselin, New Jersey 08830| http://www.enterprisedb.com/ ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
> On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: >> In the scenario, as previously outlined: >> >> ver001->verN->...->ver003->ver2->| >> ^-/ > > So you want to always keep an old version around? Prior to vacuum, it will be there anyway, and after vacuum, the new version will become ver001. ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: In the scenario, as previously outlined: ver001->verN->...->ver003->ver2->| ^-/ So you want to always keep an old version around? -- Jonah H. Harris, Software Architect | phone: 732.331.1300 EnterpriseDB Corporation| fax: 732.331.1301 33 Wood Ave S, 2nd Floor| [EMAIL PROTECTED] Iselin, New Jersey 08830| http://www.enterprisedb.com/ ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, Jun 24, 2006 at 09:23:28AM -0400, Mark Woodward wrote: > > Can you try to explain more carefully how the whole thing would work? > > What would an index tuple point to? What pointers would a heap tuple > > have? What would an index scan do to find the row version it's interested > > in? What exactly would an update do? > > > Since we already allocate space for some notion of linked list, then all > I'm suggesting is we reverse the order, sort of. Currently it looks like > this: > > ver001->ver002->ver003->...-verN > > That's what t_ctid does now, right? Well, that's sort of stupid. Why not > have it do this: > > ver001->verN->...->ver003->ver002->| > ^-/ You don't say where the index points or the order, but I'm assuming from your diagram that ver1 is the oldest, verN is the newest. Currently there is an index entry for each version, but in your version there is only an index entry for ver1, right? > This will speed up almost *all* queries when there are more than two > version of rows. > > OK, here is the behavior of an update: > (1) Find the latest version of the row You mean, find the version of the row which satisfies your snapshot. If the version pointed to by the index is it, you're done. Otherwise you follow the chain. The most common option being one step, because ver01 is likely to be invisible. > (2) Duplicate row and modify as per plan > (3) Set the t_ctid of the new row to the last "latest" > (4) Set the t_ctid of the first row to that of the new row > (5) Attempt to index the row > (6) If the first version of the row is in the index already (ver001) Don't > modify the index, otherwise, add the new version (just as before) This looks OK, I guess. I wouldn't know about locking... Have a nice day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/06, Mark Woodward wrote: ver001->verN->...->ver003->ver002->| ^-/ This will speed up almost *all* queries when there are more than two version of rows. OK, here is the behavior of an update: (1) Find the latest version of the row (2) Duplicate row and modify as per plan (3) Set the t_ctid of the new row to the last "latest" (4) Set the t_ctid of the first row to that of the new row (5) Attempt to index the row (6) If the first version of the row is in the index already (ver001) Don't modify the index, otherwise, add the new version (just as before) I am not sure I understand your algorithm. If we take as a starting point the following situation of a fresh tuple, in very schematic form it looks like: Heap: TIDT_CTID XMIN XMAX Col1 Col2 xxx1xxx1ttt1 null1 1 Index on Col1: 1xxx1 Index on Col2: 1xxx1 Now, after an update to this tuple changing the Value2 field, in what state should the heap, index 1 and index 2 be? If I understand you correctly, you want it to be: Heap: TIDT_CTID XMIN XMAX Col1 Col2 xxx1xxx2ttt1 ttt21 1 xxx2xxx1ttt2 null1 2 Index on Col1: 1xxx2 Index on Col2: 1xxx1 2xxx2 Three questions: 1. Do I understand your intention correctly? 2. Could you extend this for an update to increment value2 (because the T_CTID logic gets a bit unclear for me there). 3. The net benefit of this would be 1 less entry in the index on Col1? Jochem ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
> On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: >> Currently it looks like this: >> >> ver001->ver002->ver003->...-verN >> >> That's what t_ctid does now, right? Well, that's sort of stupid. Why not >> have it do this: >> >> ver001->verN->...->ver003->ver002->| > > Heh, because that's crazy. The first time you insert a key into the > index it will point to v1 of a tuple... say after 5 updates you have > v2,v3,v4,v5... your c_tid pointer chain looks like v1 > (original)->v2->v3->v4-v5 (newest). However, your whole idea is based > on not having to do another index insert for unchanged keys, so the > index still points to v1... which means you have to follow the c_tid > chain to get to the newest version just like a sequential scan. I > don't see how you think you can reverse pointer it. In the scenario, as previously outlined: ver001->verN->...->ver003->ver2->| ^-/ The index points to version 1 (ver001) which points to the latest version (verN). > >> This will speed up almost *all* queries when there are more than two >> version of rows. > > Nope. Of course it will. > >> When you vacuum, simply make the latest version (verN) the key row >> (ver001). > > How are you going to do this without a ton of locking... remember, the > index is pointing to v1 with a tid... so you'll have to physically > move the newest version v5 to v1's tid from wherever it was... like a > vacuum full on steroids. Unless of course, you rebuild the index... > but that's not a solution either. I don't understand how you can assume this. In fact, it wil proably reduce locking and disk IO by not having to modify indexes. \ ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/06, Jonah H. Harris <[EMAIL PROTECTED]> wrote: Grr... need coffee... s/c_tid/ctid/g -- Jonah H. Harris, Software Architect | phone: 732.331.1300 EnterpriseDB Corporation| fax: 732.331.1301 33 Wood Ave S, 2nd Floor| [EMAIL PROTECTED] Iselin, New Jersey 08830| http://www.enterprisedb.com/ ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On 6/24/06, Mark Woodward <[EMAIL PROTECTED]> wrote: Currently it looks like this: ver001->ver002->ver003->...-verN That's what t_ctid does now, right? Well, that's sort of stupid. Why not have it do this: ver001->verN->...->ver003->ver002->| Heh, because that's crazy. The first time you insert a key into the index it will point to v1 of a tuple... say after 5 updates you have v2,v3,v4,v5... your c_tid pointer chain looks like v1 (original)->v2->v3->v4-v5 (newest). However, your whole idea is based on not having to do another index insert for unchanged keys, so the index still points to v1... which means you have to follow the c_tid chain to get to the newest version just like a sequential scan. I don't see how you think you can reverse pointer it. This will speed up almost *all* queries when there are more than two version of rows. Nope. When you vacuum, simply make the latest version (verN) the key row (ver001). How are you going to do this without a ton of locking... remember, the index is pointing to v1 with a tid... so you'll have to physically move the newest version v5 to v1's tid from wherever it was... like a vacuum full on steroids. Unless of course, you rebuild the index... but that's not a solution either. -- Jonah H. Harris, Software Architect | phone: 732.331.1300 EnterpriseDB Corporation| fax: 732.331.1301 33 Wood Ave S, 2nd Floor| [EMAIL PROTECTED] Iselin, New Jersey 08830| http://www.enterprisedb.com/ ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
> On Sat, 24 Jun 2006, Mark Woodward wrote: > >> I'm probably mistaken, but aren't there already forward references in >> tuples to later versions? If so, I'm only sugesting reversing the order >> and referencing the latest version. > > I thought I understood your idea, but now you lost me again. I thought > what you want is that the older heap tuple has a pointer to the > newer one. Which it already has, it's called t_ctid. Perfect! > > Can you try to explain more carefully how the whole thing would work? > What would an index tuple point to? What pointers would a heap tuple > have? What would an index scan do to find the row version it's interested > in? What exactly would an update do? Since we already allocate space for some notion of linked list, then all I'm suggesting is we reverse the order, sort of. Currently it looks like this: ver001->ver002->ver003->...-verN That's what t_ctid does now, right? Well, that's sort of stupid. Why not have it do this: ver001->verN->...->ver003->ver002->| ^-/ This will speed up almost *all* queries when there are more than two version of rows. OK, here is the behavior of an update: (1) Find the latest version of the row (2) Duplicate row and modify as per plan (3) Set the t_ctid of the new row to the last "latest" (4) Set the t_ctid of the first row to that of the new row (5) Attempt to index the row (6) If the first version of the row is in the index already (ver001) Don't modify the index, otherwise, add the new version (just as before) When you vacuum, simply make the latest version (verN) the key row (ver001). There are, no doubt, issues that need to be resolved (I can think of a coouple off the top of my head), but overall I think it is workable and don't think this will affect performance in the simple case and improve performance in the cases where there are more than one or two version of a row. ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, 24 Jun 2006, Mark Woodward wrote: I'm probably mistaken, but aren't there already forward references in tuples to later versions? If so, I'm only sugesting reversing the order and referencing the latest version. I thought I understood your idea, but now you lost me again. I thought what you want is that the older heap tuple has a pointer to the newer one. Which it already has, it's called t_ctid. Can you try to explain more carefully how the whole thing would work? What would an index tuple point to? What pointers would a heap tuple have? What would an index scan do to find the row version it's interested in? What exactly would an update do? - Heikki ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
On Fri, Jun 23, 2006 at 03:10:39PM -0400, Mark Woodward wrote: > This is NOT an "in-place" update. The whole MVCC strategy of keeping old > versions around doesn't change. The only thing that does change is one > level of indirection. Rather than keep references to all versions of all > rows in indexes, keep only a reference to the first or "key" row of each > row, and have the first version of a row form the head of a linked list to > subsequent versions of each row. The list will be in decending order. I thought of another issue with this. If you move away from storing each row in the indexes, you can pretty much forget bitmap index scans. They pretty much rely on every row being represented, not just a subset. Until you go to the heap you don't know if a tuple will match, which is precisely what the bitmap scan is trying to avoid. You could follow the links, but that destroys the nice sequential access properties. > In the vast majority of cases, the overhead of this action will be > trivial. In an unmodified row, you're there. In a modified row, you have > one extra lookup. In extream cases, you may have to go back a few > versions, but I don't see that as a common behavior. I wonder if looking at old versions is really all that uncommon. A large reporting query which runs for hours will probably be looking at a lot of old versions. These are the queries that will be hit the hardest. If you're trying to avoid index bloat, I wonder if it wouldn't be better to tackle this from the other end. In indexes, allow a row to carry multiple CTIDs. That way a new version only requires adding six more bytes, rather than a whole new tuple. Have a nice day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, Jun 24, 2006 at 08:14:10AM -0400, Mark Woodward wrote: > > On 6/23/2006 3:10 PM, Mark Woodward wrote: > > > >> This is NOT an "in-place" update. The whole MVCC strategy of keeping old > >> versions around doesn't change. The only thing that does change is one > >> level of indirection. Rather than keep references to all versions of all > >> rows in indexes, keep only a reference to the first or "key" row of each > >> row, and have the first version of a row form the head of a linked list > >> to > >> subsequent versions of each row. The list will be in decending order. > > > > Where exactly do you intend to keep all those links (for a table with N > > indexes)? > > > > I'm probably mistaken, but aren't there already forward references in > tuples to later versions? If so, I'm only sugesting reversing the order > and referencing the latest version. You can't do that. The links exist so that in READ COMMITTED mode you can always find the newest version. You would need to add additional links to go backwards. Have a nice day, -- Martijn van Oosterhout http://svana.org/kleptog/ > From each according to his ability. To each according to his ability to > litigate. signature.asc Description: Digital signature
Re: [HACKERS] vacuum, performance, and MVCC
> On 6/23/2006 3:10 PM, Mark Woodward wrote: > >> This is NOT an "in-place" update. The whole MVCC strategy of keeping old >> versions around doesn't change. The only thing that does change is one >> level of indirection. Rather than keep references to all versions of all >> rows in indexes, keep only a reference to the first or "key" row of each >> row, and have the first version of a row form the head of a linked list >> to >> subsequent versions of each row. The list will be in decending order. > > Where exactly do you intend to keep all those links (for a table with N > indexes)? > I'm probably mistaken, but aren't there already forward references in tuples to later versions? If so, I'm only sugesting reversing the order and referencing the latest version. ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] vacuum, performance, and MVCC
On Fri, 23 Jun 2006, Jonah H. Harris wrote: On 6/23/06, Mark Woodward <[EMAIL PROTECTED]> wrote: Rather than keep references to all versions of all rows in indexes, keep only a reference to the first or "key" row of each row, and have the first version of a row form the head of a linked list to subsequent versions of each row. The list will be in decending order. By all means, please go ahead and try it because it's not quite that easy. You're going to run into serious locking and contention issues this way. In the end, it's not much better than running a sequential scan to query a row that's been updated several thousand times on a table that hasn't been vacuumed... follow that pointer :) Can you elaborate what kind of locking and contention issues you're thinking of? You could update the index tuple to point to a newer version of the row, when an index scan determines that the heap tuple it points to is not visible to anyone. We already check that to update the XMAX_COMMITTED hint bit. Updating the index tuple comes with a cost, of course, but alleviates the "follow that pointer" issue. The biggest challenge that I see is that an index scan would somehow need to know when to follow the t_ctid chain and when not. If you follow the pointer and there's another index tuple for the row, the scan could see the same tuple twice. Some kind of bookkeeping would be needed to solve that. Also, vacuuming would become a bit more complex, since it would need to update the index tuples to point to newer row versions instead of just removing them. All in all, I think this solution to the "an update needs to update all indexes, even when none of the indexed columns changed" issue requires less changes than implementing Oracle style rollback segments and/or an undo log. - Heikki ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] vacuum, performance, and MVCC
On Sat, Jun 24, 2006 at 03:29:47AM -0400, Jan Wieck wrote: > >It sounds like everybody agrees that things need to be fixed, and genuinely > >motivated people are trying to offer what they have to the table. > One singe core team member responds vaguely in a way, you feel being > supportive of your case, and you conclude that "everybody agrees"? > Sorry, x'use me? > There are a couple of side effects on this "update in place" issue that > aren't even mentioned yet. Nobody with any significant in depth > knowledge of the Postgres non-overwriting storage engine concept seems > to suggest any move towards a storage system, that does updates in place > that require "undo" operations in case of a process/system failure. > You're ready to "fix" all those places to support the undo you need? You > must have a big table. Jan: Who on the list has claimed that nothing is broken and nothing needs to be improved? Are you making this claim? Shutting down ideas is not constructive. Encouraging those with ideas to step up and do more than talk could be. Cheers, mark -- [EMAIL PROTECTED] / [EMAIL PROTECTED] / [EMAIL PROTECTED] __ . . _ ._ . . .__. . ._. .__ . . . .__ | Neighbourhood Coder |\/| |_| |_| |/|_ |\/| | |_ | |/ |_ | | | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada One ring to rule them all, one ring to find them, one ring to bring them all and in the darkness bind them... http://mark.mielke.cc/ ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
On 6/23/2006 9:56 PM, [EMAIL PROTECTED] wrote: On Fri, Jun 23, 2006 at 03:08:34PM -0400, Bruce Momjian wrote: Tom Lane wrote: > ... > suggesting. We're having a hard enough time debugging and optimizing > *one* storage model. I think the correct path forward is to stick with > the same basic storage model and vacuuming concept, and address the > known performance issues with better-optimized vacuuming. No, it will > never be perfect for every scenario, but we can certainly make it much > better than it is now, without risking killing the project by > introducing undebuggable, unmaintainable complexity. Well, are you suggesting we just stop improving the database? I am sure not. But, your suggestion is that we can't do better without incurring more complexity (true), and that complexity will not be worth it. I don't agree with that until I see some proposals, and shutting down discussion because they will add complexity or are fruitless seems unwise. It sounds like everybody agrees that things need to be fixed, and genuinely motivated people are trying to offer what they have to the table. One singe core team member responds vaguely in a way, you feel being supportive of your case, and you conclude that "everybody agrees"? Sorry, x'use me? There are a couple of side effects on this "update in place" issue that aren't even mentioned yet. Nobody with any significant in depth knowledge of the Postgres non-overwriting storage engine concept seems to suggest any move towards a storage system, that does updates in place that require "undo" operations in case of a process/system failure. You're ready to "fix" all those places to support the undo you need? You must have a big table. Jan Tom already has enough on his plate, as do most others here - so unless a competent champion can take up the challenge, discussion is all we have. I'm not liking the "we should do it this way," "no, we should do it that." My read of the situation is that both may be useful, and that both should be pursued. But one set of people can't pursue both. Is any who is able, able to take up this challenge? Perhaps more than one, from both major directions? (vacuum on one side, and improved storage on the other) Somebody with the time and skill, who can work through the design discussions on one of the aspects? I want to contribute soon, and this is the sort of thing that interests me - but I still don't have time yet, and there would be no guarantee that I succeeded. Somebody else? :-) Cheers, mark -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] vacuum, performance, and MVCC
On 6/23/2006 3:10 PM, Mark Woodward wrote: This is NOT an "in-place" update. The whole MVCC strategy of keeping old versions around doesn't change. The only thing that does change is one level of indirection. Rather than keep references to all versions of all rows in indexes, keep only a reference to the first or "key" row of each row, and have the first version of a row form the head of a linked list to subsequent versions of each row. The list will be in decending order. Where exactly do you intend to keep all those links (for a table with N indexes)? Jan -- #==# # It's easier to get forgiveness for being wrong than for being right. # # Let's break this rule - forgive me. # #== [EMAIL PROTECTED] # ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] vacuum, performance, and MVCC
> Just out of curiosity Mark, didn't you write your session daemon so
> that you don't have to put sessions in postgres anymore?
The original project started as a shared key/value system for a beowulf
cluster in the late 90s, but got reworked to be a session handler for PHP
when I worked with Stig, MSession.
> Or are you
> just giving that as an example of a very wide, very heavily updated
> table? My session tables have been an extreme case of this problem,
> but no other table that I have is so adversely affected by this
> behavior. My decision was not to pull postgres out entirely, just
> using other session handlers.
I have been working as a consultant since 2001, and prior to that, as CTO
at at a dot.com startup. MSession (the previous name) was used to
circumvent shortcomings in PostgreSQL, specificially the problem we are
talking about.
As a consultant, I have to convince the customer that "all is well." My
MCache system does not guarentee that no session data lost, nor does
memcached or other non-ACID system.
The technical arguments we can make, no matter how correct, leave us on
the defensive when asked "What if the server crashes, do you lose data?"
of course the answer is "yes." Then we get drawn into a conversation about
transient and unimportant data vs persistent and valuable data. At which
point you've lost the customer. A solid SQL database is the defacto
standard, perhaps not the best choice, but unavoidable.
The update behavior of PostgreSQL is probably the *last* serious issue.
Debate all you want, vacuum mitigates the problem to varying levels,
fixing the problem will be a huge win. If the update behavior gets fixed,
I can't think of a single issue with postgresql that would be a show
stopper.
>
> Rick
>
> On Jun 22, 2006, at 7:59 AM, Mark Woodward wrote:
>
>>> After a long battle with technology, [EMAIL PROTECTED] ("Mark
>>> Woodward"), an earthling, wrote:
> Clinging to sanity, [EMAIL PROTECTED] ("Mark Woodward")
> mumbled into
> her beard:
>> [snip]
>
> 1. The index points to all the versions, until they get
> vacuumed out.
It can't point to "all" versions, it points to the last "current"
version
as updated by vacuum, or the first version of the row.
>>>
>>> No, it points to *all* the versions.
>>>
>>> Suppose I take a table with two rows:
>>>
>>> INFO: analyzing "public.test"
>>> INFO: "test": 1 pages, 2 rows sampled, 2 estimated total rows
>>> VACUUM
>>>
>>> Then, over and over, I remove and insert one entry with the same PK:
>>>
>>> sample=# delete from test where id = 2;insert into test (id)
>>> values (2);
>>> DELETE 1
>>
>> [snip]
>>>
>>> Now, I vacuum it.
>>>
>>> sample=# vacuum verbose analyze test;
>>> INFO: vacuuming "public.test"
>>> INFO: index "test_id_key" now contains 2 row versions in 2 pages
>>> DETAIL: 10 index row versions were removed.
>>> 0 index pages have been deleted, 0 are currently reusable.
>>> CPU 0.00s/0.00u sec elapsed 0.00 sec.
>>> INFO: "test": removed 10 row versions in 1 pages
>>> DETAIL: CPU 0.00s/0.00u sec elapsed 0.00 sec.
>>> INFO: "test": found 10 removable, 2 nonremovable row versions in
>>> 1 pages
>>> DETAIL: 0 dead row versions cannot be removed yet.
>>> There were 0 unused item pointers.
>>> 0 pages are entirely empty.
>>> CPU 0.00s/0.00u sec elapsed 0.00 sec.
>>> INFO: analyzing "public.test"
>>> INFO: "test": 1 pages, 2 rows sampled, 2 estimated total rows
>>> VACUUM
>>>
>>> Notice that the index contained 10 versions of that one row.
>>>
>>> It pointed to *ALL* the versions.
>>
>> Hmm, OK, then the problem is more serious than I suspected.
>> This means that every index on a row has to be updated on every
>> transaction that modifies that row. Is that correct?
>>
>> I am attaching some code that shows the problem with regard to
>> applications such as web server session management, when run, each
>> second
>> the system can handle fewer and fewer connections. Here is a brief
>> output:
>>
>> [EMAIL PROTECTED]:~/pgfoo$ ./footest
>> 1307 sessions per second, elapsed: 1
>> 1292 sessions per second, elapsed: 2
>> 1287 sessions per second, elapsed: 3
>>
>> 1216 sessions per second, elapsed: 25
>> 1213 sessions per second, elapsed: 26
>> 1208 sessions per second, elapsed: 27
>>
>> 1192 sessions per second, elapsed: 36
>> 1184 sessions per second, elapsed: 37
>> 1183 sessions per second, elapsed: 38
>>
>> 1164 sessions per second, elapsed: 58
>> 1170 sessions per second, elapsed: 59
>> 1168 sessions per second, elapsed: 60
>>
>> As you can see, in about a minute at high load, this very simple table
>> lost about 10% of its performance, and I've seen worse based on update
>> frequency. Before you say this is an obscure problem, I can tell
>> you it
>> isn't. I have worked with more than a few projects that had to
>> switch away
>> from PostgreSQL because of this behavior.
>>
>> Obviously this is not a problem with small sites, but this is a real
>> problem with an
Re: [HACKERS] vacuum, performance, and MVCC
On Fri, Jun 23, 2006 at 03:08:34PM -0400, Bruce Momjian wrote: > Tom Lane wrote: > > ... > > suggesting. We're having a hard enough time debugging and optimizing > > *one* storage model. I think the correct path forward is to stick with > > the same basic storage model and vacuuming concept, and address the > > known performance issues with better-optimized vacuuming. No, it will > > never be perfect for every scenario, but we can certainly make it much > > better than it is now, without risking killing the project by > > introducing undebuggable, unmaintainable complexity. > Well, are you suggesting we just stop improving the database? I am sure > not. But, your suggestion is that we can't do better without incurring > more complexity (true), and that complexity will not be worth it. I > don't agree with that until I see some proposals, and shutting down > discussion because they will add complexity or are fruitless seems > unwise. It sounds like everybody agrees that things need to be fixed, and genuinely motivated people are trying to offer what they have to the table. Tom already has enough on his plate, as do most others here - so unless a competent champion can take up the challenge, discussion is all we have. I'm not liking the "we should do it this way," "no, we should do it that." My read of the situation is that both may be useful, and that both should be pursued. But one set of people can't pursue both. Is any who is able, able to take up this challenge? Perhaps more than one, from both major directions? (vacuum on one side, and improved storage on the other) Somebody with the time and skill, who can work through the design discussions on one of the aspects? I want to contribute soon, and this is the sort of thing that interests me - but I still don't have time yet, and there would be no guarantee that I succeeded. Somebody else? :-) Cheers, mark -- [EMAIL PROTECTED] / [EMAIL PROTECTED] / [EMAIL PROTECTED] __ . . _ ._ . . .__. . ._. .__ . . . .__ | Neighbourhood Coder |\/| |_| |_| |/|_ |\/| | |_ | |/ |_ | | | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada One ring to rule them all, one ring to find them, one ring to bring them all and in the darkness bind them... http://mark.mielke.cc/ ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] vacuum, performance, and MVCC
On Jun 22, 2006, at 2:36 PM, Mark Woodward wrote: What you seem not to grasp at this point is a large web-farm, about 10 or more servers running PHP, Java, ASP, or even perl. The database is usually the most convenient and, aside from the particular issue we are talking about, best suited. The answer is sticky sessions : each user is assigned to one and only one webserver in the cluster and his session is maintained locally, in RAM. No locks, no need to manage distributed session... I actually have a good number of years of experience in this topic, and memcached or file system files are NOT the best solutions for a server farm. If sessions are distributed, certainly, but if sessions are sticky to their own server ? And what if a particulr server goes down? or gets too high a percentage of the load? Yes, I don't think that sticky sessions are the answer. But phps session handling behavior could be greatly improved on. ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
