Re: [HACKERS] Ordered Append Node
Tom Lane [EMAIL PROTECTED] writes: Gregory Stark [EMAIL PROTECTED] writes: I've been hacking on the idea of an Append node which maintains the ordering of its subtables merging their records in order. I finally got round to looking at this ... A lot of things to chew on. Thanks very much. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's On-Demand Production Tuning -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Ordered Append Node
Gregory Stark [EMAIL PROTECTED] writes: I've been hacking on the idea of an Append node which maintains the ordering of its subtables merging their records in order. I finally got round to looking at this ... 1) I still haven't completely figured out what to do with equivalence classes. My hack of just stuffing all the append subrel vars into there seems to work fine. I need to understand what's going on to see if there's really a problem with it or not. This still makes me itchy, but it might be all right, because we'd never really consider a plan for a single subnode as representing a useful plan for the whole query. What it would need is some additions to the README files (ahem) describing what's going on. 2) I'm not sure this code will work when the append rel is a target (ie UPDATE and DELETE stmts). It won't, at least not for the case where the appendrel is an inheritance tree in which the children are unlike the parent. You have missed updating the estate-es_result_relation_info and estate-es_junkFilter to match the tuple actually returned. In a plain Append it is only necessary to update those when switching from one subnode to the next, so we handle it in exec_append_initialize_next. In an ordered Append you'd have to track which subnode each entry in the heap came from (easy, but you aren't doing it now) and update those fields for every tuple returned. If you need an example of what I'm talking about: create table p1(f1 int, f2 int); create table p2(f3 int, f4 int); create table c() inherits(p1, p2); ... insert some data ... update p2 set ... Of course a plain UPDATE p2 wouldn't have much interest in ordered-Append plans, but you could force the failure by doing a joining update with a mergejoin plan. 4) I haven't handled mark/restore or random access. I think they could be handled and they'll probably be worth the complexity but I'm not sure. I don't think it'd be nearly as easy as you think, eg just how are you going to back up the merge? A heap merge doesn't normally remember the past few tuples it emitted. I'd definitely recommend not trying that in v1. BTW, I concur with Heikki's suggestion that this might be better done as a separate executor node type. There'd be a certain amount of duplicated boilerplate, but that would force you to look at each and every reference to Append nodes and decide what's needed. There are a whole bunch of places you've missed touching that know something about what Append nodes can do, and you'll need to look at them all in any case. 5) Is it considering too many paths? Hard to say for sure, but I concur that that needs thinking about. One point is that as soon as you have even one subnode that hasn't got a cheap-startup-cost path, it probably stops being interesting to try to build a cheap-startup-cost ordered-Append path. But exactly what's the threshold of cheap enough, I'm not sure. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Ordered Append Node
Hello Gregory, Gregory Stark wrote: It is kind of like a merge join but not quite. It's interleaving rows rather than matching them up. It's more like the final merge of a sort which also uses a heap to efficiently find the next value from the source tapes. Well, maybe my point here is: why do you need the heap to sort? The inputs you've got are already sorted, you only need to merge them. To me this sounds very much like the final stage of a merge sort, which would not require much memory. IMO, a merge sort could easier be implemented by a binary tree zipper node, as I had in mind. Leading to a plan like that (well, hey, this is all made up): Zipper (cost..., sort by public.t.i) - Zipper (cost .., sort by public.t.i) - Zipper (cost .., sort by public.t.i) - Index Scan using ti1 on t1 - Index Scan using t12 on t2 - Index Scan using ti2 on t3 - Zipper (cost .., sort by public.t.i) - Index Scan using ti4 on t4 - Index Scan using ti5 on t5 Every zipper node would simply have to keep the two top tuples from its inputs in memory, compare them and return the best. But maybe that's exactly how Knuth's polyphase merge sort internally also merge their inputs (or runs). And perhaps it makes sense to show the user just one simple append node instead of throwing a tree of Zipper nodes at her. ;-) Not necessarily but it is something Postgres supports and I don't think we want to break it. Actually it's useful for partitioned tables if you build the new partition in a separate table and then add it to the partitioned table. In that case you may have gone through several steps of adding columns and dropping them to get the structure to line up. Agreed, especially because lining up the columns isn't that hard after all. OTOH I think Postgres is way too flexible in how it allows partitioning to be done and thus it often can't optimize it properly. I'd very much like to teach it a stricter and simpler to use partitioning scheme than what we have with constraint exclusion. But that's pipe dreaming, and your improvement to the append node is certainly a good step towards the right direction, keep up the good work! Regards Markus ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] Ordered Append Node
* Markus Schiltknecht: uses a heap to efficiently find the next value from the source tapes. Well, maybe my point here is: why do you need the heap to sort? I think you need it because there are potentially many input types. -- Florian Weimer[EMAIL PROTECTED] BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99 ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] Ordered Append Node
Hi, Florian Weimer wrote: Florian Weimer wrote: I think you need it because there are potentially many input types. Eh, tapes. Aha.. Given the partitioning case, I'd expect all rows to have an equal tuple descriptor. Maybe this is a matter of what to optimize, then? Could you elaborate on what use case you have in mind? You need a priority queue to figure out from which tape (partition) you need to remove the next tuple. And why do you need lots of heap memory to do that? Anything wrong with the zipper approach I've outlined upthread? Am I missing something? Regards Markus ---(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] Ordered Append Node
* Markus Schiltknecht: You need a priority queue to figure out from which tape (partition) you need to remove the next tuple. And why do you need lots of heap memory to do that? Anything wrong with the zipper approach I've outlined upthread? heap == priority queue here, I guess. Looking at your zipper again, it's actually an implementation of a heap. -- Florian Weimer[EMAIL PROTECTED] BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99 ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] Ordered Append Node
Heikki Linnakangas [EMAIL PROTECTED] writes: Markus Schiltknecht wrote: Florian Weimer wrote: Given the partitioning case, I'd expect all rows to have an equal tuple descriptor. Maybe this is a matter of what to optimize, then? Could you elaborate on what use case you have in mind? You need a priority queue to figure out from which tape (partition) you need to remove the next tuple. And why do you need lots of heap memory to do that? Anything wrong with the zipper approach I've outlined upthread? We're talking about a binary heap, with just one node per partition. AFAICT it's roughly the same data structure as the zipper tree you envisioned, but not implemented with separate executor nodes for each level. Not quite the same since the Executor-based implementation would have a static tree structure based on the partitions. Even if the partitions are all empty except for one or two you would still have to push the result records through all the nodes for the empty partitions. A heap only has the next record from each input. If an input has reached eof then the heap doesn't have an entry for that input. So if one of the inputs is empty (often the parent of the inheritance tree) it doesn't require a test anywhere to propagate the record up past it. I also did an optimization similar to the bounded-sort case where we check if the next tuple from the same input which last contributed the result record comes before the top element of the heap. That avoids having to do an insert and siftup only to pull out the same record you just inserted. In theory this is not an optimization but in practice I think partitioned tables will often contain contiguous blocks of key values and queries will often be joining against that key and therefore often want to order by it. Ideally we would also be able to do this in the planner. If the planner could determine from the constraints that all the key values from each partition are non-overlapping and order them properly then it could generate a regular append node with a path order without the overhead of the run-time comparisons. But that requires a) dealing with the problem of the parent table which has no constraints and b) an efficient way to prove that constraints don't overlap and order them properly. The latter looks like an O(n^2) problem to me, though it's a planning problem which might be worth making slow in exchange for even a small speedup at run-time. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's Slony Replication support! ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [HACKERS] Ordered Append Node
Gregory Stark [EMAIL PROTECTED] writes: Heikki Linnakangas [EMAIL PROTECTED] writes: Markus Schiltknecht wrote: And why do you need lots of heap memory to do that? Anything wrong with the zipper approach I've outlined upthread? We're talking about a binary heap, with just one node per partition. AFAICT it's roughly the same data structure as the zipper tree you envisioned, but not implemented with separate executor nodes for each level. Not quite the same since the Executor-based implementation would have a static tree structure based on the partitions. Even if the partitions are all empty except for one or two you would still have to push the result records through all the nodes for the empty partitions. Also, the overhead per executor node visit is not exactly trivial. I think that zipper scheme would be quite slow compared to a standard heap merge within a single node. regards, tom lane ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] Ordered Append Node
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Gregory Stark wrote: But that requires a) dealing with the problem of the parent table which has no constraints and b) an efficient way to prove that constraints don't overlap and order them properly. The latter looks like an O(n^2) problem to me, though it's a planning problem which might be worth making slow in exchange for even a small speedup at run-time. Is it really worthwile to optimize away the heap access by thinking about what the child tables hold? If the tables are read using IO, I think the complete plan would turn out to be IO-bound, and the heap is of no interest. If the tables reside in memory, the heap still only slows the process down by O(log(number of tables)) which usually won't be that much imho. Nonetheless, in the case of range partitioning, a sort on the lower ends of the ranges and a linear test of neighbouring ranges for overlap, skipping emtpy ranges, should work in O(n log(n)). Jens-W. Schicke -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFHRu2xzhchXT4RR5ARAu//AKCZWZj680RhnbivbU/UqLBvsigBggCgq0Tw GB+OYl0VOidmzVcK6ckhFBw= =gbt7 -END PGP SIGNATURE- ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] Ordered Append Node
Gregory Stark wrote: Ideally we would also be able to do this in the planner. If the planner could determine from the constraints that all the key values from each partition are non-overlapping and order them properly then it could generate a regular append node with a path order without the overhead of the run-time comparisons. But that requires a) dealing with the problem of the parent table which has no constraints and ... It would help even if you could only prove it for some partitions. You could use a regular append node for the partitions you know not to overlap, and merge the output of that with the new kind of ordered append node. We'd see that the parent table is empty on the first invocation, and after that the ordered append node would just pass through the tuples. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [HACKERS] Ordered Append Node
On Fri, 2007-11-23 at 12:36 +, Gregory Stark wrote: I also did an optimization similar to the bounded-sort case where we check if the next tuple from the same input which last contributed the result record comes before the top element of the heap. That avoids having to do an insert and siftup only to pull out the same record you just inserted. In theory this is not an optimization but in practice I think partitioned tables will often contain contiguous blocks of key values and queries will often be joining against that key and therefore often want to order by it. If it is an option, you could also do this by a new method on the heap which adds a new entry and removes the resulting new head in one atomic operation. That would work with one single comparison for the less than current head situation, and it would not need to repeat that comparison if that fails. Also it could directly remove the head and balance the tree in one go. Cheers, Csaba. ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] Ordered Append Node
Hi, Florian Weimer wrote: I think you need it because there are potentially many input types. Given the partitioning case, I'd expect all rows to have an equal tuple descriptor. Maybe this is a matter of what to optimize, then? Could you elaborate on what use case you have in mind? Regards Markus ---(end of broadcast)--- TIP 7: You can help support the PostgreSQL project by donating at http://www.postgresql.org/about/donate
Re: [HACKERS] Ordered Append Node
Markus Schiltknecht wrote: Florian Weimer wrote: Given the partitioning case, I'd expect all rows to have an equal tuple descriptor. Maybe this is a matter of what to optimize, then? Could you elaborate on what use case you have in mind? You need a priority queue to figure out from which tape (partition) you need to remove the next tuple. And why do you need lots of heap memory to do that? Anything wrong with the zipper approach I've outlined upthread? We're talking about a binary heap, with just one node per partition. AFAICT it's roughly the same data structure as the zipper tree you envisioned, but not implemented with separate executor nodes for each level. -- Heikki Linnakangas EnterpriseDB 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] Ordered Append Node
* Markus Schiltknecht: Florian Weimer wrote: I think you need it because there are potentially many input types. Eh, tapes. Given the partitioning case, I'd expect all rows to have an equal tuple descriptor. Maybe this is a matter of what to optimize, then? Could you elaborate on what use case you have in mind? You need a priority queue to figure out from which tape (partition) you need to remove the next tuple. -- Florian Weimer[EMAIL PROTECTED] BFK edv-consulting GmbH http://www.bfk.de/ Kriegsstraße 100 tel: +49-721-96201-1 D-76133 Karlsruhe fax: +49-721-96201-99 ---(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] Ordered Append Node
But that requires a) dealing with the problem of the parent table which has no constraints and ... Imho we should provide a mechanism that forces the parent to be empty and let the planner know. e.g. a false constraint on parent ONLY. Andreas ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] Ordered Append Node
Gregory Stark wrote: Not quite the same since the Executor-based implementation would have a static tree structure based on the partitions. Even if the partitions are all empty except for one or two you would still have to push the result records through all the nodes for the empty partitions. A heap only has the next record from each input. If an input has reached eof then the heap doesn't have an entry for that input. So if one of the inputs is empty (often the parent of the inheritance tree) it doesn't require a test anywhere to propagate the record up past it. Ah, so the binary tree (binary heap?) gets adjusted dynamically. Very clever! (OTOH probably a micro optimization, but as code is already there, use it, yeah!) I also did an optimization similar to the bounded-sort case where we check if the next tuple from the same input which last contributed the result record comes before the top element of the heap. That avoids having to do an insert and siftup only to pull out the same record you just inserted. In theory this is not an optimization but in practice I think partitioned tables will often contain contiguous blocks of key values and queries will often be joining against that key and therefore often want to order by it. Hm... that assumes range partitioning, no? If you partition among three partitions by id modulo 3, tuples are most probably coming from one partition after the other, i.e.: 1 2 3 1 2 3 1 2 3 ... And with hash partitioning, you're completely unable to predict the ordering. Ideally we would also be able to do this in the planner. If the planner could determine from the constraints that all the key values from each partition are non-overlapping and order them properly then it could generate a regular append node with a path order without the overhead of the run-time comparisons. Agreed. But that requires a) dealing with the problem of the parent table which has no constraints and b) an efficient way to prove that constraints don't overlap and order them properly. The latter looks like an O(n^2) problem to me, though it's a planning problem which might be worth making slow in exchange for even a small speedup at run-time. Well, I think someday, Postgres needs better support for vertical data partitioning in general. Dealing with constraints and inheritance is way too flexible and prone to error. I'll shortly start a new thread about that, to outline my current thoughts about that topic. Regards Markus ---(end of broadcast)--- TIP 6: explain analyze is your friend
Re: [HACKERS] Ordered Append Node
Hi, Heikki Linnakangas wrote: AFAICT it's roughly the same data structure as the zipper tree you envisioned, but not implemented with separate executor nodes for each level. Aha, that sounds good to me. ;-) As I've already mentioned, I think it's even better to simply show the user a single append node, instead of lots of small nodes from a binary tree merger. Regards Markus ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [HACKERS] Ordered Append Node
Hello Gregory, Gregory Stark wrote: I've been hacking on the idea of an Append node which maintains the ordering of its subtables merging their records in order. This is important for partitioned tables since otherwise a lot of plans are not available such as merge joins. Cool! Some time ago, I've been trying something very similar, but didn't get very far. I'd like to share my thoughts anyway. First of, I envisioned that append node to be applicable also to unordered reading from multiple partitions, simply interleaving between the partitions. The logic I've followed is to do as follows: 1) Go through all subrels asking for any interesting pathkey lists. Gather up the union of all of these. I also tried to modify the Append node first, then figured that it might be better to base on the merge join node instead. While this seems farther away, I had the hope that a binary tree of such 'plain merge' nodes would require less comparisons in total. Plus, I found it a lot simpler to cope with exactly two input relations instead of n, as with the append node. :-) 2) Go back through all the subrels and for each accumulated pathkey list ask for the best path for that subrel for that pathkey list. 3) Generate an two append paths for each of these pathkey lists, one using the best startup_cost subpath and one with the best total_cost subpath available for the pathkey list for each child rel. If there is none available take the best unordered path and put a sort node above it. 4) Additionally advertise the traditional unordered append node which our parent could choose to put a sort node above same as ever. 5) Append plan nodes look a lot like Sort plan nodes glued onto an Append plan node, with sort function oids and so on. 6) Append exec state nodes look a lot like a (a few fields from) tuplesortstate glued onto an Append node. They have the ScanKey array and the fmgr sort functions. They also have an array of TupleTableSlot and a heap of indexes into that array. 8) ExecAppend does something similar to tuplesort's bounded sort (I fear I'm going to get typecasted) or more to the point, similar to the final merge of a merge sort. It directly returns the slot the child rel last returned to it. Uh.. well, yeah. I guess you have a better understanding of the planner and executor that I do. Open questions: 1) I still haven't completely figured out what to do with equivalence classes. My hack of just stuffing all the append subrel vars into there seems to work fine. I need to understand what's going on to see if there's really a problem with it or not. 2) I'm not sure this code will work when the append rel is a target (ie UPDATE and DELETE stmts). 3) It does seem to work when the columns in the subrels don't line up but I didn't do anything special to handle this case. Uh.. is there any use case for that? WRT partitioning certainly not, is there? I'll have a look at your patch. Regards Markus ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [HACKERS] Ordered Append Node
Markus Schiltknecht [EMAIL PROTECTED] writes: 1) Go through all subrels asking for any interesting pathkey lists. Gather up the union of all of these. I also tried to modify the Append node first, then figured that it might be better to base on the merge join node instead. While this seems farther away, I had the hope that a binary tree of such 'plain merge' nodes would require less comparisons in total. It is kind of like a merge join but not quite. It's interleaving rows rather than matching them up. It's more like the final merge of a sort which also uses a heap to efficiently find the next value from the source tapes. 3) It does seem to work when the columns in the subrels don't line up but I didn't do anything special to handle this case. Uh.. is there any use case for that? WRT partitioning certainly not, is there? Not necessarily but it is something Postgres supports and I don't think we want to break it. Actually it's useful for partitioned tables if you build the new partition in a separate table and then add it to the partitioned table. In that case you may have gone through several steps of adding columns and dropping them to get the structure to line up. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's PostGIS support! ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
