Re: [PERFORM] Index Bloat Problem
Thanks for this description--we have index bloat problems on a massively active (but small) database.This may help shed light on our problems. Sorry for top-posting--challenged email reader. Greg W. From: Jeff Janes jeff.ja...@gmail.com To: Strahinja Kustudić strahin...@nordeus.com Cc: pgsql-performance@postgresql.org Sent: Friday, August 17, 2012 7:33 PM Subject: Re: [PERFORM] Index Bloat Problem On Thu, Aug 16, 2012 at 12:57 PM, Strahinja Kustudić strahin...@nordeus.com wrote: @Jeff I'm not sure if I understand what you mean? I know that we never reuse key ranges. Could you be more clear, or give an example please. If an index leaf page is completely empty because every entry on it were deleted, it will get recycled to be used in some other part of the index. (Eventually--it can take a while, especially if you have long-running transactions). But if the leaf page is only mostly empty, because only most of entries on it were deleted, than it can never be reused, except for entries that naturally fall into its existing key range (which will never happen, if you never reuse key ranges) So if you have a million records with keys 1..100, and do a delete from foo where key between 1 and 99, then 99% of those old index pages will become completely empty and eligible for reuse. But if you do delete from foo where key%1000, then all of the pages will become 99% empty, and none will be eligible for reuse (except the very last one, which can still accept 101 and so on) There has been talk of allowing logically adjacent, mostly empty pages to be merged so that one of them becomes empty, but the way concurrent access to btree indexes was designed this is extremely hard to do safely. Cheers, Jeff -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index Bloat Problem
On Thu, Aug 16, 2012 at 12:57 PM, Strahinja Kustudić strahin...@nordeus.com wrote: @Jeff I'm not sure if I understand what you mean? I know that we never reuse key ranges. Could you be more clear, or give an example please. If an index leaf page is completely empty because every entry on it were deleted, it will get recycled to be used in some other part of the index. (Eventually--it can take a while, especially if you have long-running transactions). But if the leaf page is only mostly empty, because only most of entries on it were deleted, than it can never be reused, except for entries that naturally fall into its existing key range (which will never happen, if you never reuse key ranges) So if you have a million records with keys 1..100, and do a delete from foo where key between 1 and 99, then 99% of those old index pages will become completely empty and eligible for reuse. But if you do delete from foo where key%1000, then all of the pages will become 99% empty, and none will be eligible for reuse (except the very last one, which can still accept 101 and so on) There has been talk of allowing logically adjacent, mostly empty pages to be merged so that one of them becomes empty, but the way concurrent access to btree indexes was designed this is extremely hard to do safely. Cheers, Jeff -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index Bloat Problem
Thanks for the help everyone and sorry for not replying sooner, I was on a business trip. @Hubert pg_reorg looks really interesting and from the first read it looks to be a very good solution for maintenance, but for now I would rather try to slow down, or remove this bloat, so I have to do as less maintenance as possible. @Mark So basically I should decrease the autovacuum nap time from 60s to 10s, reduce the scale factor from 0.2 to 0.1. log_autovacuum_min_duration is already set to 0, which means everything is logged. @Jeff I'm not sure if I understand what you mean? I know that we never reuse key ranges. Could you be more clear, or give an example please. Thanks in advance, Strahinja On Tue, Aug 14, 2012 at 6:14 AM, Jeff Janes jeff.ja...@gmail.com wrote: On Fri, Aug 10, 2012 at 3:15 PM, Strahinja Kustudić strahin...@nordeus.com wrote: For example, yesterday when I checked the database size on the production server it was 30GB, and the restored dump of that database was only 17GB. The most interesting thing is that the data wasn't bloated that much, but the indices were. Some of them were a few times bigger than they should be. For example an index on the production db is 440MB, while that same index after dump/restore is 17MB, and there are many indices with that high difference. Could your pattern of deletions be leaving sparsely populated, but not completely empty, index pages; which your insertions will then never reuse because they never again insert values in that key range? Cheers, Jeff
Re: [PERFORM] Index Bloat Problem
On 11/08/12 10:15, Strahinja Kustudić wrote: We have PostgreSQL 9.1 running on Centos 5 on two SSDs, one for indices and one for data. The database is extremely active with reads and writes. We have autovacuum enabled, but we didn't tweak it's aggressiveness. The problem is that after some time the database grows even more than 100% on the file system and most of the growth is because the indices are a few times bigger than they should be, and when this happens, the performance of the DB drops. For example, yesterday when I checked the database size on the production server it was 30GB, and the restored dump of that database was only 17GB. The most interesting thing is that the data wasn't bloated that much, but the indices were. Some of them were a few times bigger than they should be. For example an index on the production db is 440MB, while that same index after dump/restore is 17MB, and there are many indices with that high difference. We could fix the problem if we reindex the DB, but that makes our DB go offline and it's not possible to do in the production enviroment. Is there a way to make the autovacuum daemon more aggressive, since I'm not exactly sure how to do that in this case? Would that even help? Is there another way to remove this index bloat? Some workloads can be difficult to tame. However I would try something like this in postgresql.conf: autovacuum_naptime= 10s autovacuum_vacuum_scale_factor = 0.1 and maybe set log_autovacuum_min_duration so you see what autovacuum is doing. If the above settings don't help, then you could maybe monitor growth and schedule regular REINDEXes on the tables concerned (at some suitably quiet time). Regards Mark -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index Bloat Problem
On Fri, Aug 10, 2012 at 3:15 PM, Strahinja Kustudić strahin...@nordeus.com wrote: For example, yesterday when I checked the database size on the production server it was 30GB, and the restored dump of that database was only 17GB. The most interesting thing is that the data wasn't bloated that much, but the indices were. Some of them were a few times bigger than they should be. For example an index on the production db is 440MB, while that same index after dump/restore is 17MB, and there are many indices with that high difference. Could your pattern of deletions be leaving sparsely populated, but not completely empty, index pages; which your insertions will then never reuse because they never again insert values in that key range? Cheers, Jeff -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index Bloat Problem
On Sat, Aug 11, 2012 at 12:15:11AM +0200, Strahinja Kustudić wrote: Is there a way to make the autovacuum daemon more aggressive, since I'm not exactly sure how to do that in this case? Would that even help? Is there another way to remove this index bloat? http://www.depesz.com/index.php/2011/07/06/bloat-happens/ Best regards, depesz -- The best thing about modern society is how easy it is to avoid contact with it. http://depesz.com/ - Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
Re: [PERFORM] Index bloat problem?
Bill Chandler [EMAIL PROTECTED] writes: Client is reporting that the size of an index is greater than the number of rows in the table (1.9 million vs. 1.5 million). This thread seems to have wandered away without asking the critical question what did you mean by that? It's not possible for an index to have more rows than there are in the table unless something is seriously broken. And there aren't any SQL operations that let you inspect an index directly anyway. So: what is the actual observation that led you to the above conclusion? Facts, please, not inferences. regards, tom lane ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] Index bloat problem?
David Roussel [EMAIL PROTECTED] writes: |dave_data_update_eventsr 1593600.0 40209 |dave_data_update_events_event_id_key i 1912320.0 29271 Hmm ... what PG version is this, and what does VACUUM VERBOSE on that table show? regards, tom lane ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Index bloat problem?
On Fri, 22 Apr 2005 10:06:33 -0400, Tom Lane [EMAIL PROTECTED] said: David Roussel [EMAIL PROTECTED] writes: |dave_data_update_eventsr 1593600.0 40209 |dave_data_update_events_event_id_key i 1912320.0 29271 Hmm ... what PG version is this, and what does VACUUM VERBOSE on that table show? PG 7.4 The disparity seems to have sorted itself out now, so hampering futher investigations. I guess the regular inserts of new data, and the nightly deletion and index recreation did it. However, we did suffer reduced performance and the strange cardinality for several days before it went away. For what it's worth.. ndb=# vacuum verbose iso_pjm_data_update_events; INFO: vacuuming public.iso_pjm_data_update_events INFO: index iso_pjm_data_update_events_event_id_key now contains 1912320 row versions in 29271 pages DETAIL: 21969 index pages have been deleted, 2 are currently reusable. CPU 6.17s/0.88u sec elapsed 32.55 sec. INFO: index iso_pjm_data_update_events_lds_idx now contains 1912320 row versions in 7366 pages DETAIL: 0 index pages have been deleted, 0 are currently reusable. CPU 3.52s/0.57u sec elapsed 14.35 sec. INFO: index iso_pjm_data_update_events_obj_id_idx now contains 1912320 row versions in 7366 pages DETAIL: 0 index pages have been deleted, 0 are currently reusable. CPU 3.57s/0.58u sec elapsed 12.87 sec. INFO: iso_pjm_data_update_events: found 0 removable, 1912320 nonremovable row versions in 40209 pages DETAIL: 159384 dead row versions cannot be removed yet. There were 745191 unused item pointers. 0 pages are entirely empty. CPU 18.26s/3.62u sec elapsed 74.35 sec. VACUUM After each insert is does this... VACUUM ANALYZE iso_pjm_DATA_UPDATE_EVENTS VACUUM ANALYZE iso_pjm_CONTROL Each night it does this... BEGIN DROP INDEX iso_pjm_control_obj_id_idx DROP INDEX iso_pjm_control_real_name_idx DROP INDEX iso_pjm_data_update_events_lds_idx DROP INDEX iso_pjm_data_update_events_obj_id_idx CREATE UNIQUE INDEX iso_pjm_control_obj_id_idx ON iso_pjm_control(obj_id) CLUSTER iso_pjm_control_obj_id_idx ON iso_pjm_control CREATE UNIQUE INDEX iso_pjm_control_real_name_idx ON iso_pjm_control(real_name) CREATE INDEX iso_pjm_data_update_events_lds_idx ON iso_pjm_data_update_events(lds) CREATE INDEX iso_pjm_data_update_events_obj_id_idx ON iso_pjm_data_update_events(obj_id) COMMIT Note there is no reference to iso_pjm_data_update_events_event_id_key which is the index that went wacky on us. Does that seem weird to you? Thanks David ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] Index bloat problem?
You would be interested in http://archives.postgresql.org/pgsql-hackers/2005-04/msg00565.php On Thu, Apr 21, 2005 at 03:33:05PM -0400, Dave Chapeskie wrote: On Thu, Apr 21, 2005 at 11:28:43AM -0700, Josh Berkus wrote: Michael, Every five minutes, DBCC INDEXDEFRAG will report to the user an estimated percentage completed. DBCC INDEXDEFRAG can be terminated at any point in the process, and *any completed work is retained.* Keen. Sounds like something for our TODO list. -- Josh Berkus Aglio Database Solutions San Francisco See http://archives.postgresql.org/pgsql-general/2005-03/msg01465.php for my thoughts on a non-blocking alternative to REINDEX. I got no replies to that message. :-( I've almost got a working solution integrated in the backend that does correct WAL logging and everything. (Writing the code to write and replay WAL logs for complicated operations can be very annoying!) For now I've gone with a syntax of: REINDEX INDEX btree_index_name INCREMENTAL; (For now it's not a proper index AM (accessor method), instead the generic index code knows this is only supported for btrees and directly calls the btree_compress function.) It's not actually a REINDEX per-se in that it doesn't rebuild the whole index. It holds brief exclusive locks on the index while it shuffles items around to pack the leaf pages fuller. There were issues with the code I attached to the above message that have been resolved with the new code. With respect to the numbers provided in that e-mail the new code also recycles more pages than before. Once I've finished it up I'll prepare and post a patch. -- Dave Chapeskie OpenPGP Key ID: 0x3D2B6B34 ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED] -- Jim C. Nasby, Database Consultant [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828 Windows: Where do you want to go today? Linux: Where do you want to go tomorrow? FreeBSD: Are you guys coming, or what? ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Index bloat problem?
David Roussel [EMAIL PROTECTED] writes: Note there is no reference to iso_pjm_data_update_events_event_id_key which is the index that went wacky on us. Does that seem weird to you? What that says is that that index doesn't belong to that table. You sure it wasn't a chance coincidence of names that made you think it did? regards, tom lane ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Index bloat problem?
Quoting Bill Chandler [EMAIL PROTECTED]: Running PostgreSQL 7.4.2, Solaris. Client is reporting that the size of an index is greater than the number of rows in the table (1.9 million vs. 1.5 million). Index was automatically created from a 'bigserial unique' column. We have been running 'VACUUM ANALYZE' very regularly. In fact, our vacuum schedule has probably been overkill. We have been running on a per-table basis after every update (many per day, only inserts occurring) and after every purge (one per day, deleting a day's worth of data). What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? I gather you mean, out-of-the-ordinary for most apps, but not for this client? In case nobody else has asked: is your max_fsm_pages big enough to handle all the deleted pages, across ALL tables hit by the purge? If not, you're haemorrhaging pages, and VACUUM is probably warning you about exactly that. If that's not a problem, you might want to consider partitioning the data. Take a look at inherited tables. For me, they're a good approximation of clustered indexes (sigh, miss'em) and equivalent to table spaces. My app is in a similar boat to yours: up to 1/3 of a 10M-row table goes away every day. For each of the child tables that is a candidate to be dropped, there is a big prologue txn, whichs moves (INSERT then DELETE) the good rows into a child table that is NOT to be dropped. Then BANG pull the plug on the tables you don't want. MUCH faster than DELETE: the dropped tables' files' disk space goes away in one shot, too. Just my 2c. ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
[PERFORM] Index bloat problem?
All, Running PostgreSQL 7.4.2, Solaris. Client is reporting that the size of an index is greater than the number of rows in the table (1.9 million vs. 1.5 million). Index was automatically created from a 'bigserial unique' column. Database contains several tables with exactly the same columns (including 'bigserial unique' column). This is the only table where this index is out of line with the actual # of rows. Queries on this table take 40 seconds to retrieve 2000 rows as opposed to 1-2 seconds on the other tables. We have been running 'VACUUM ANALYZE' very regularly. In fact, our vacuum schedule has probably been overkill. We have been running on a per-table basis after every update (many per day, only inserts occurring) and after every purge (one per day, deleting a day's worth of data). It is theoretically possible that at some time a process was run that deleted all rows in the table followed by a VACUUM FULL. In this case we would have dropped/recreated our own indexes on the table but not the index automatically created for the bigserial column. If that happened, could that cause these symptoms? What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? thanks, Bill __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] Index bloat problem?
Bill, What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? Yes. You'd want to run REINDEX after and event like that. As you should now. -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Index bloat problem?
--- Josh Berkus josh@agliodbs.com wrote: Bill, What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? Yes. You'd want to run REINDEX after and event like that. As you should now. -- Josh Berkus Aglio Database Solutions San Francisco Thank you. Though I must say, that is very discouraging. REINDEX is a costly operation, timewise and due to the fact that it locks out other processes from proceeding. Updates are constantly coming in and queries are occurring continuously. A REINDEX could potentially bring the whole thing to a halt. Honestly, this seems like an inordinate amount of babysitting for a production application. I'm not sure if the client will be willing to accept it. Admittedly my knowledge of the inner workings of an RDBMS is limited, but could somebody explain to me why this would be so? If you delete a bunch of rows why doesn't the index get updated at the same time? Is this a common issue among all RDBMSs or is it something that is PostgreSQL specific? Is there any way around it? thanks, Bill __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Index bloat problem?
Bill, Honestly, this seems like an inordinate amount of babysitting for a production application. I'm not sure if the client will be willing to accept it. Well, then, tell them not to delete 75% of the rows in a table at once. I imagine that operation brought processing to a halt, too. If the client isn't willing to accept the consequences of their own bad data management, I'm not really sure what you expect us to do about it. Admittedly my knowledge of the inner workings of an RDBMS is limited, but could somebody explain to me why this would be so? If you delete a bunch of rows why doesn't the index get updated at the same time? It does get updated. What doesn't happen is the space getting reclaimed. In a *normal* data situation, the dead nodes are recycled for new rows. But doing a massive delete operation upsets that, and generally needs to be followed by a REINDEX. Is this a common issue among all RDBMSs or is it something that is PostgreSQL specific? Speaking from experience, this sort of thing affects MSSQL as well, although the maintenance routines are different. -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 3: 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: [PERFORM] Index bloat problem?
Bill, Honestly, this seems like an inordinate amount of babysitting for a production application. I'm not sure if the client will be willing to accept it. Well, then, tell them not to delete 75% of the rows in a table at once. I imagine that operation brought processing to a halt, too. Admittedly my knowledge of the inner workings of an RDBMS is limited, but could somebody explain to me why this would be so? If you delete a bunch of rows why doesn't the index get updated at the same time? It does get updated. What doesn't happen is the space getting reclaimed. In a *normal* data situation, those dead nodes would be replaced with new index nodes. However, a mass-delete-in-one-go messes that system up. Is this a common issue among all RDBMSs or is it something that is PostgreSQL specific? Speaking from experience, this sort of thing affects MSSQL as well, although the maintenance routines are different. -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 8: explain analyze is your friend
Re: [PERFORM] Index bloat problem?
Is: REINDEX DATABASE blah supposed to rebuild all indices in the database, or must you specify each table individualy? (I'm asking because I just tried it and it only did system tables) Alex Turner netEconomist On 4/21/05, Josh Berkus josh@agliodbs.com wrote: Bill, What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? Yes. You'd want to run REINDEX after and event like that. As you should now. -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Index bloat problem?
Alex, REINDEX DATABASE blah supposed to rebuild all indices in the database, or must you specify each table individualy? (I'm asking because I just tried it and it only did system tables) DATABASE Recreate all system indexes of a specified database. Indexes on user tables are not processed. Also, indexes on shared system catalogs are skipped except in stand-alone mode (see below). http://www.postgresql.org/docs/8.0/static/sql-reindex.html -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Re: [PERFORM] Index bloat problem?
josh@agliodbs.com (Josh Berkus) writes: Bill, What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? Yes. You'd want to run REINDEX after and event like that. As you should now. Based on Tom's recent comments, I'd be inclined to handle this via doing a CLUSTER, which has the triple heroism effect of: a) Reorganizing the entire table to conform with the relevant index order, b) Having the effect of VACUUM FULL, and c) Having the effect of REINDEX all in one command. It has all of the oops, that blocked me for 20 minutes effect of REINDEX and VACUUM FULL, but at least it doesn't have the effect twice... -- (format nil [EMAIL PROTECTED] cbbrowne acm.org) http://www.ntlug.org/~cbbrowne/sap.html Rules of the Evil Overlord #78. I will not tell my Legions of Terror And he must be taken alive! The command will be: ``And try to take him alive if it is reasonably practical.'' http://www.eviloverlord.com/ ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Index bloat problem?
Is this a common issue among all RDBMSs or is it something that is PostgreSQL specific? Speaking from experience, this sort of thing affects MSSQL as well, although the maintenance routines are different. Yes, this is true with MSSQL too, however sql server implements a defrag index that doesn't lock up the table.. http://msdn.microsoft.com/library/default.asp?url=/library/en-us/tsqlref/ts_dbcc_30o9.asp DBCC INDEXDEFRAG can defragment clustered and nonclustered indexes on tables and views. DBCC INDEXDEFRAG defragments the leaf level of an index so that the physical order of the pages matches the left-to-right logical order of the leaf nodes, thus improving index-scanning performance. Every five minutes, DBCC INDEXDEFRAG will report to the user an estimated percentage completed. DBCC INDEXDEFRAG can be terminated at any point in the process, and *any completed work is retained.* -michael ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Index bloat problem?
Michael, Every five minutes, DBCC INDEXDEFRAG will report to the user an estimated percentage completed. DBCC INDEXDEFRAG can be terminated at any point in the process, and *any completed work is retained.* Keen. Sounds like something for our TODO list. -- Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 4: Don't 'kill -9' the postmaster
Re: [PERFORM] Index bloat problem?
--- [EMAIL PROTECTED] wrote: I gather you mean, out-of-the-ordinary for most apps, but not for this client? Actually, no. The normal activity is to delete 3-5% of the rows per day, followed by a VACUUM ANALYZE. Then over the course of the day (in multiple transactions) about the same amount are INSERTed (each transaction followed by a VACUUM ANALYZE on just the updated table). So 75% deletion is just out of the ordinary for this app. However, on occasion, deleting 75% of rows is a legitimate action for the client to take. It would be nice if they didn't have to remember to do things like REINDEX or CLUSTER or whatever on just those occasions. In case nobody else has asked: is your max_fsm_pages big enough to handle all the deleted pages, across ALL tables hit by the purge? If not, you're haemorrhaging pages, and VACUUM is probably warning you about exactly that. This parameter is most likely set incorrectly. So that could be causing problems. Could that be a culprit for the index bloat, though? If that's not a problem, you might want to consider partitioning the data. Take a look at inherited tables. For me, they're a good approximation of clustered indexes (sigh, miss'em) and equivalent to table spaces. My app is in a similar boat to yours: up to 1/3 of a 10M-row table goes away every day. For each of the child tables that is a candidate to be dropped, there is a big prologue txn, whichs moves (INSERT then DELETE) the good rows into a child table that is NOT to be dropped. Then BANG pull the plug on the tables you don't want. MUCH faster than DELETE: the dropped tables' files' disk space goes away in one shot, too. Just my 2c. Thanks. Bill __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Index bloat problem?
Same thing happens in Oracle ALTER INDEX blah rebuild To force a rebuild. It will mark the free blocks as 'free' below the PCTFREE value for the tablespace. Basically If you build an index with entries. and each entry is 1/4 of a block, the database will write 2500 blocks to the disk. If you delete a random 75% of the index values, you will now have 2500 blocks that have 75% free space. The database will reuse that free space in those blocks as you insert new values, but until then, you still have 2500 blocks worth of data on a disk, that is only 25% full. Rebuilding the index forces the system to physically re-allocate all that data space, and now you have just 2499 entries, that use 625 blocks. I'm not sure that 'blocks' is the correct term in postgres, it's segments in Oracle, but the concept remains the same. Alex Turner netEconomist On 4/21/05, Bill Chandler [EMAIL PROTECTED] wrote: --- Josh Berkus josh@agliodbs.com wrote: Bill, What about if an out-of-the-ordinary number of rows were deleted (say 75% of rows in the table, as opposed to normal 5%) followed by a 'VACUUM ANALYZE'? Could things get out of whack because of that situation? Yes. You'd want to run REINDEX after and event like that. As you should now. -- Josh Berkus Aglio Database Solutions San Francisco Thank you. Though I must say, that is very discouraging. REINDEX is a costly operation, timewise and due to the fact that it locks out other processes from proceeding. Updates are constantly coming in and queries are occurring continuously. A REINDEX could potentially bring the whole thing to a halt. Honestly, this seems like an inordinate amount of babysitting for a production application. I'm not sure if the client will be willing to accept it. Admittedly my knowledge of the inner workings of an RDBMS is limited, but could somebody explain to me why this would be so? If you delete a bunch of rows why doesn't the index get updated at the same time? Is this a common issue among all RDBMSs or is it something that is PostgreSQL specific? Is there any way around it? thanks, Bill __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org ---(end of broadcast)--- TIP 6: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] Index bloat problem?
On Thu, Apr 21, 2005 at 11:28:43AM -0700, Josh Berkus wrote: Michael, Every five minutes, DBCC INDEXDEFRAG will report to the user an estimated percentage completed. DBCC INDEXDEFRAG can be terminated at any point in the process, and *any completed work is retained.* Keen. Sounds like something for our TODO list. -- Josh Berkus Aglio Database Solutions San Francisco See http://archives.postgresql.org/pgsql-general/2005-03/msg01465.php for my thoughts on a non-blocking alternative to REINDEX. I got no replies to that message. :-( I've almost got a working solution integrated in the backend that does correct WAL logging and everything. (Writing the code to write and replay WAL logs for complicated operations can be very annoying!) For now I've gone with a syntax of: REINDEX INDEX btree_index_name INCREMENTAL; (For now it's not a proper index AM (accessor method), instead the generic index code knows this is only supported for btrees and directly calls the btree_compress function.) It's not actually a REINDEX per-se in that it doesn't rebuild the whole index. It holds brief exclusive locks on the index while it shuffles items around to pack the leaf pages fuller. There were issues with the code I attached to the above message that have been resolved with the new code. With respect to the numbers provided in that e-mail the new code also recycles more pages than before. Once I've finished it up I'll prepare and post a patch. -- Dave Chapeskie OpenPGP Key ID: 0x3D2B6B34 ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Index bloat problem?
Quoting Bill Chandler [EMAIL PROTECTED]: ... The normal activity is to delete 3-5% of the rows per day, followed by a VACUUM ANALYZE. ... However, on occasion, deleting 75% of rows is a legitimate action for the client to take. In case nobody else has asked: is your max_fsm_pages big enough to handle all the deleted pages, across ALL tables hit by the purge? This parameter is most likely set incorrectly. So that could be causing problems. Could that be a culprit for the index bloat, though? Look at the last few lines of vacuum verbose output. It will say something like: free space map: 55 relations, 88416 pages stored; 89184 total pages needed Allocated FSM size: 1000 relations + 100 pages = 5920 kB shared memory. 100 here is [max_fsm_pages] from my postgresql.conf. If the total pages needed is bigger than the pages fsm is allocated for, then you are bleeding. -- Dreams come true, not free. -- S.Sondheim, ITW ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Index bloat problem?
Mischa, Thanks. Yes, I understand that not having a large enough max_fsm_pages is a problem and I think that it is most likely the case for the client. What I wasn't sure of was if the index bloat we're seeing is the result of the bleeding you're talking about or something else. If I deleted 75% of the rows but had a max_fsm_pages setting that still exceeded the pages required (as indicated in VACUUM output), would that solve my indexing problem or would I still need to REINDEX after such a purge? regards, Bill --- Mischa Sandberg [EMAIL PROTECTED] wrote: Quoting Bill Chandler [EMAIL PROTECTED]: ... The normal activity is to delete 3-5% of the rows per day, followed by a VACUUM ANALYZE. ... However, on occasion, deleting 75% of rows is a legitimate action for the client to take. In case nobody else has asked: is your max_fsm_pages big enough to handle all the deleted pages, across ALL tables hit by the purge? This parameter is most likely set incorrectly. So that could be causing problems. Could that be a culprit for the index bloat, though? Look at the last few lines of vacuum verbose output. It will say something like: free space map: 55 relations, 88416 pages stored; 89184 total pages needed Allocated FSM size: 1000 relations + 100 pages = 5920 kB shared memory. 100 here is [max_fsm_pages] from my postgresql.conf. If the total pages needed is bigger than the pages fsm is allocated for, then you are bleeding. -- Dreams come true, not free. -- S.Sondheim, ITW __ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com ---(end of broadcast)--- TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
Re: [PERFORM] Index bloat problem?
Bill, If I deleted 75% of the rows but had a max_fsm_pages setting that still exceeded the pages required (as indicated in VACUUM output), would that solve my indexing problem or would I still need to REINDEX after such a purge? Depends on the performance you're expecting.The FSM relates the the re-use of nodes, not taking up free space. So after you've deleted 75% of rows, the index wouldn't shrink. It just wouldn't grow when you start adding rows. -- --Josh Josh Berkus Aglio Database Solutions San Francisco ---(end of broadcast)--- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Index bloat problem?
Bill Chandler wrote: Mischa, Thanks. Yes, I understand that not having a large enough max_fsm_pages is a problem and I think that it is most likely the case for the client. What I wasn't sure of was if the index bloat we're seeing is the result of the bleeding you're talking about or something else. If I deleted 75% of the rows but had a max_fsm_pages setting that still exceeded the pages required (as indicated in VACUUM output), would that solve my indexing problem or would I still need to REINDEX after such a purge? regards, Bill I don't believe VACUUM re-packs indexes. It just removes empty index pages. So if you have 1000 index pages all with 1 entry in them, vacuum cannot reclaim any pages. REINDEX re-packs the pages to 90% full. fsm just needs to hold enough pages that all requests have free space that can be used before your next vacuum. It is just a map letting postgres know where space is available for a new fill. John =:- signature.asc Description: OpenPGP digital signature
