Re: [PERFORM] Incorrect estimates on correlated filters
On Aug 13, 2008, at 1:45 PM, Chris Kratz wrote: Yes, I know hints are frowned upon around here. Though, I'd love to have them or something equivalent on this particular query just so the customer can run their important reports. As it is, it's unrunnable. Actually, now that I think about it the last time this was brought up there was discussion about something that doesn't force a particular execution method, but instead provides improved information to the planner. It might be worth pursuing that, as I think there was less opposition to it. -- Decibel!, aka Jim C. Nasby, Database Architect [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828 smime.p7s Description: S/MIME cryptographic signature
Re: [PERFORM] Incorrect estimates on correlated filters
Craig Ringer [EMAIL PROTECTED] writes: It strikes me that there are really two types of query hint possible here. One tells the planner (eg) prefer a merge join here. The other gives the planner more information that it might not otherwise have to work with, so it can improve its decisions. The values used in this join condition are highly correlated. This sounds familiar: http://article.gmane.org/gmane.comp.db.postgresql.devel.general/55730/match=hints Plus ça change... -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's On-Demand Production Tuning -- 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] Incorrect estimates on correlated filters
On Aug 12, 2008, at 4:59 PM, Chris Kratz wrote: Ran into a re-occuring performance problem with some report queries again today. In a nutshell, we have filters on either multiple joined tables, or multiple columns on a single table that are highly correlated. So, the estimates come out grossly incorrect (the planner has no way to know they are correlated). 2000:1 for one I'm looking at right now. Generally this doesn't matter, except in complex reporting queries like these when this is the first join of 40 other joins. Because the estimate is wrong at the lowest level, it snowballs up through the rest of the joins causing the query to run very, very slowly. In many of these cases, forcing nested loops off for the duration of the query fixes the problem. But I have a couple that still are painfully slow and shouldn't be. I've been reading through the archives with others having similar problems (including myself a year ago). Am I right in assuming that at this point there is still little we can do in postgres to speed up this kind of query? Right now the planner has no way to know the correlation between different columns in the same table, let alone columns in different tables. So, it just assumes no correlation and returns incorrectly low estimates in cases like these. The only solution I've come up with so far is to materialize portions of the larger query into subqueries with these correlated filters which are indexed and analyzed before joining into the larger query. This would keep the incorrect estimates from snowballing up through the chain of joins. Are there any other solutions to this problem? Well... you could try and convince certain members of the community that we actually do need some kind of a query hint mechanism... ;) I did make a suggestion a few months ago that involved sorting a table on different columns and recording the correlation of other columns. The scheme isn't perfect, but it would help detect cases like a field populated by a sequence and another field that's insert timestamp; those two fields would correlate highly, and you should even be able to correlate the two histograms; that would allow you to infer that most of the insert times for _id's between 100 and 200 will be between 2008-01-01 00:10 and 2008-01-01 00:20, for example. -- Decibel!, aka Jim C. Nasby, Database Architect [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828 smime.p7s Description: S/MIME cryptographic signature
Re: [PERFORM] Incorrect estimates on correlated filters
On Wed, Aug 13, 2008 at 10:59 AM, Decibel! [EMAIL PROTECTED] wrote: On Aug 12, 2008, at 4:59 PM, Chris Kratz wrote: Ran into a re-occuring performance problem with some report queries again today. In a nutshell, we have filters on either multiple joined tables, or multiple columns on a single table that are highly correlated. So, the estimates come out grossly incorrect (the planner has no way to know they are correlated). 2000:1 for one I'm looking at right now. Generally this doesn't matter, except in complex reporting queries like these when this is the first join of 40 other joins. Because the estimate is wrong at the lowest level, it snowballs up through the rest of the joins causing the query to run very, very slowly. In many of these cases, forcing nested loops off for the duration of the query fixes the problem. But I have a couple that still are painfully slow and shouldn't be. I've been reading through the archives with others having similar problems (including myself a year ago). Am I right in assuming that at this point there is still little we can do in postgres to speed up this kind of query? Right now the planner has no way to know the correlation between different columns in the same table, let alone columns in different tables. So, it just assumes no correlation and returns incorrectly low estimates in cases like these. The only solution I've come up with so far is to materialize portions of the larger query into subqueries with these correlated filters which are indexed and analyzed before joining into the larger query. This would keep the incorrect estimates from snowballing up through the chain of joins. Are there any other solutions to this problem? Well... you could try and convince certain members of the community that we actually do need some kind of a query hint mechanism... ;) I did make a suggestion a few months ago that involved sorting a table on different columns and recording the correlation of other columns. The scheme isn't perfect, but it would help detect cases like a field populated by a sequence and another field that's insert timestamp; those two fields would correlate highly, and you should even be able to correlate the two histograms; that would allow you to infer that most of the insert times for _id's between 100 and 200 will be between 2008-01-01 00:10 and 2008-01-01 00:20, for example. -- Decibel!, aka Jim C. Nasby, Database Architect [EMAIL PROTECTED] Give your computer some brain candy! www.distributed.net Team #1828 Thanks for the reply, Yes, I know hints are frowned upon around here. Though, I'd love to have them or something equivalent on this particular query just so the customer can run their important reports. As it is, it's unrunnable. Unfortunately, if I don't think the sorting idea would help in the one case I'm looking at which involves filters on two tables that are joined together. The filters happen to be correlated such that about 95% of the rows from each filtered table are actually returned after the join. Unfortunately, the planner thinks we will get 1 row back. I do have to find a way to make these queries runnable. I'll keep looking. Thanks, -Chris
Re: [PERFORM] Incorrect estimates on correlated filters
Chris Kratz wrote: Unfortunately, if I don't think the sorting idea would help in the one case I'm looking at which involves filters on two tables that are joined together. The filters happen to be correlated such that about 95% of the rows from each filtered table are actually returned after the join. Unfortunately, the planner thinks we will get 1 row back. Maybe you can wrap that part of the query in a SQL function and set its estimated cost to the real values with ALTER FUNCTION ... ROWS. -- Alvaro Herrerahttp://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support -- 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] Incorrect estimates on correlated filters
Decibel! wrote: Well... you could try and convince certain members of the community that we actually do need some kind of a query hint mechanism... ;) It strikes me that there are really two types of query hint possible here. One tells the planner (eg) prefer a merge join here. The other gives the planner more information that it might not otherwise have to work with, so it can improve its decisions. The values used in this join condition are highly correlated. Is there anything wrong with the second approach? It shouldn't tend to suppress planner bug reports etc. Well, not unless people use it to lie to the planner, and I expect results from that would be iffy at best. It just provides information to supplement Pg's existing stats system to handle cases where it's not able to reasonably collect the required information. -- Craig Ringer -- Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
[PERFORM] Incorrect estimates on correlated filters
Hello All, Ran into a re-occuring performance problem with some report queries again today. In a nutshell, we have filters on either multiple joined tables, or multiple columns on a single table that are highly correlated. So, the estimates come out grossly incorrect (the planner has no way to know they are correlated). 2000:1 for one I'm looking at right now. Generally this doesn't matter, except in complex reporting queries like these when this is the first join of 40 other joins. Because the estimate is wrong at the lowest level, it snowballs up through the rest of the joins causing the query to run very, very slowly. In many of these cases, forcing nested loops off for the duration of the query fixes the problem. But I have a couple that still are painfully slow and shouldn't be. I've been reading through the archives with others having similar problems (including myself a year ago). Am I right in assuming that at this point there is still little we can do in postgres to speed up this kind of query? Right now the planner has no way to know the correlation between different columns in the same table, let alone columns in different tables. So, it just assumes no correlation and returns incorrectly low estimates in cases like these. The only solution I've come up with so far is to materialize portions of the larger query into subqueries with these correlated filters which are indexed and analyzed before joining into the larger query. This would keep the incorrect estimates from snowballing up through the chain of joins. Are there any other solutions to this problem? Thanks, -Chris
