Re: [PERFORM] Query planner is using wrong index.
--- Tom Lane [EMAIL PROTECTED] wrote: Brian Herlihy [EMAIL PROTECTED] writes: My options seem to be - Fudge the analysis results so that the selectivity estimate changes. I have tested reducing n_distinct, but this doesn't seem to help. - Combine the columns into one column, allowing postgres to calculate the combined selectivity. - Drop the (p2, p3) index. But I need this for other queries. Have you considered reordering the pkey to be (p2,p3,p1) and then dropping the (p2,p3) index? regards, tom lane Hi Tom, I've considered it. Unfortunately I need to do lookups on (p1) and (p1,p2) as well as (p1, p2, p3). The solution I've gone with is to create an index on (p2 || '/' || p3). This is unique for each p2/p3 combination, because p2 cannot contain the '/' character. I'm assuming that this index will be no slower to generate than one on (p2, p3), as concatenation is very cheap. Having the index on an expression hides it from the optimizer, which is then forced to use the primary key instead. It works perfectly now! There were only 2 queries in the system which need this index, so it was no problem to change them. Thankyou very much for all your time and patience! Before I go, I have a question - From discussions on the Postgresql irc channel, and from reading the TODO list on the website, I am under the impression that there are no plans to allow optimizer hints, such as use index table_pkey. Is this really true? Such a feature would make life inestimably easier for your end-users, particularly me :) Thanks, Brian ---(end of broadcast)--- TIP 6: explain analyze is your friend
Spotting planner errors (was Re: [PERFORM] Query planner is using wrong index.)
Tom Lane wrote: Brian Herlihy [EMAIL PROTECTED] writes: Before I go, I have a question - From discussions on the Postgresql irc channel, and from reading the TODO list on the website, I am under the impression that there are no plans to allow optimizer hints, such as use index table_pkey. Is this really true? I personally don't think it's a good idea: the time spent in designing, implementing, and maintaining a usable hint system would be significant, and IMHO the effort is better spent on *fixing* the optimizer problems than working around them. Tom - does the planner/executor know it's got row estimates wrong? That is, if I'm not running an EXPLAIN ANALYSE is there a point at which we could log planner estimate for X out by factor of Y? -- Richard Huxton Archonet Ltd ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: Spotting planner errors (was Re: [PERFORM] Query planner is using wrong index.)
Richard Huxton dev@archonet.com writes: Tom - does the planner/executor know it's got row estimates wrong? That is, if I'm not running an EXPLAIN ANALYSE is there a point at which we could log planner estimate for X out by factor of Y? Not at the moment, but you could certainly imagine changing the executor to count rows even without EXPLAIN ANALYZE, and then complain during plan shutdown. Not sure how helpful that would be; there would be a lot of noise from common cases such as executing underneath a LIMIT node. regards, tom lane ---(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
[PERFORM] Query planner is using wrong index.
Hi, I have a problem with the choice of index made by the query planner. My table looks like this: CREATE TABLE t ( p1 varchar not null, p2 varchar not null, p3 varchar not null, i1 integer, i2 integer, i3 integer, i4 integer, i5 integer, d1 date, d2 date, d3 date, PRIMARY KEY (p1, p2, p3) ); I have also created an index on (p2, p3), as some of my lookups are on these only. All the integers and dates are data values. The table has around 9 million rows. I am using postgresl 7.4.7 I have set statistics to 1000 on the p1, p2 and p3 columns, and run vacuum full analyse. However, I still see query plans like this: db=# explain select * from t where p1 = 'something' and p2 = 'fairly_common' and p3 = 'fairly_common'; QUERY PLAN --- Index Scan using p2p3 on t (cost=0.00..6.01 rows=1 width=102) Index Cond: (((p2)::text = 'fairly_common'::text) AND ((p3)::text = 'fairly_common'::text)) Filter: ((p1)::text = 'something'::text) (3 rows) The problem appears to be this: db=# explain select * from t where p2 = 'fairly_common' and p3 = 'fairly_common'; QUERY PLAN --- Index Scan using p2p3 on t (cost=0.00..6.01 rows=1 width=102) Index Cond: (((p2)::text = 'fairly_common'::text) AND ((p3)::text = 'fairly_common'::text)) (3 rows) The query planner thinks that this will return only 1 row. In fact, these index lookups sometimes return up to 500 rows, which then must be filtered by p1. This can take 2 or 3 seconds to execute for what should be a simple primary key lookup. For VERY common values of p2 and p3, the query planner chooses the primary key, because these values are stored explicitly in the analyse results. For rare values there is no problem, because the query runs quickly. But for fairly common values, there is a problem. I would like the query planner to use the primary key for all of these lookups. How can I enforce this? Thanks, Brian ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Query planner is using wrong index.
On fim, 2006-04-06 at 12:35 +1000, Brian Herlihy wrote: I have a problem with the choice of index made by the query planner. My table looks like this: CREATE TABLE t ( p1 varchar not null, p2 varchar not null, p3 varchar not null, i1 integer, i2 integer, i3 integer, i4 integer, i5 integer, d1 date, d2 date, d3 date, PRIMARY KEY (p1, p2, p3) ); I have also created an index on (p2, p3), as some of my lookups are on these only. All the integers and dates are data values. The table has around 9 million rows. I am using postgresl 7.4.7 I have set statistics to 1000 on the p1, p2 and p3 columns, and run vacuum full analyse. However, I still see query plans like this: ... db=# explain select * from t where p2 = 'fairly_common' and p3 = 'fairly_common'; QUERY PLAN --- Index Scan using p2p3 on t (cost=0.00..6.01 rows=1 width=102) Index Cond: (((p2)::text = 'fairly_common'::text) AND ((p3)::text = 'fairly_common'::text)) (3 rows) please show us an actual EXPLAIN ANALYZE this will show us more. I would like the query planner to use the primary key for all of these lookups. How can I enforce this? How would that help? have you tested to see if it would actualy be better? gnari ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Query planner is using wrong index.
--- Ragnar [EMAIL PROTECTED] wrote: On fim, 2006-04-06 at 12:35 +1000, Brian Herlihy wrote: I have a problem with the choice of index made by the query planner. My table looks like this: CREATE TABLE t ( p1 varchar not null, p2 varchar not null, p3 varchar not null, i1 integer, i2 integer, i3 integer, i4 integer, i5 integer, d1 date, d2 date, d3 date, PRIMARY KEY (p1, p2, p3) ); I have also created an index on (p2, p3), as some of my lookups are on these only. All the integers and dates are data values. The table has around 9 million rows. I am using postgresl 7.4.7 I have set statistics to 1000 on the p1, p2 and p3 columns, and run vacuum full analyse. However, I still see query plans like this: ... db=# explain select * from t where p2 = 'fairly_common' and p3 = 'fairly_common'; QUERY PLAN --- Index Scan using p2p3 on t (cost=0.00..6.01 rows=1 width=102) Index Cond: (((p2)::text = 'fairly_common'::text) AND ((p3)::text = 'fairly_common'::text)) (3 rows) please show us an actual EXPLAIN ANALYZE this will show us more. I would like the query planner to use the primary key for all of these lookups. How can I enforce this? How would that help? have you tested to see if it would actualy be better? gnari Yes, the primary key is far better. I gave it the ultimate test - I dropped the (p2, p3) index. It's blindingly fast when using the PK, which is what I expect from Postgresql :) This query is part of an import process, which has been getting increasingly slow as the table has grown. I first discovered the problem when I noticed queries which should be simple PK lookups taking up to 2.5 seconds on an idle system. I discussed this problem in the Postgres IRC channel, and it turns out to be due to an inaccurate selectivity estimate. The columns p2 and p3 are highly correlated, which is why I often get hundreds of rows even after specifying values for both these columns. However, the query optimizer assumes the columns are not correlated. It calculates the selectivity for each column seperately, then multiplies them to get the combined selectivity for specifying both p2 and p3. This results in an estimate of 1 row, which makes the (p2,p3) index look as good as the (p1,p2,p3) index. I'm aware now that there is no way to force use of a particular index in Postgres. I've also been told that there is no way to have the optimizer take into account correlation between column values. My options seem to be - Fudge the analysis results so that the selectivity estimate changes. I have tested reducing n_distinct, but this doesn't seem to help. - Combine the columns into one column, allowing postgres to calculate the combined selectivity. - Drop the (p2, p3) index. But I need this for other queries. None of these are good solutions. So I am hoping that there is a better way to go about this! Thanks, Brian ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Query planner is using wrong index.
On fim, 2006-04-06 at 19:27 +1000, Brian Herlihy wrote: --- Ragnar [EMAIL PROTECTED] wrote: On fim, 2006-04-06 at 12:35 +1000, Brian Herlihy wrote: ... PRIMARY KEY (p1, p2, p3) ... I have also created an index on (p2, p3), as some of my lookups are on these only. ... db=# explain select * from t where p2 = 'fairly_common' and p3 = 'fairly_common'; please show us an actual EXPLAIN ANALYZE I would like the query planner to use the primary key for all of these lookups. have you tested to see if it would actualy be better? Yes, the primary key is far better. I gave it the ultimate test - I dropped the (p2, p3) index. It's blindingly fast when using the PK, I have problems understanding exactly how an index on (p1,p2,p3) can be faster than and index on (p2,p3) for a query not involving p1. can you demonstrate this with actual EXPLAIN ANALYZES ? something like: EXPLAIN ANALYZE select * from t where p2 = ? and p3 = ?; BEGIN; DROP INDEX p2p3; EXPLAIN ANALYZE select * from t where p2 = ? and p3 = ?; ROLLBACK; maybe your p2p3 index needs REINDEX ? My options seem to be - Fudge the analysis results so that the selectivity estimate changes. I have tested reducing n_distinct, but this doesn't seem to help. - Combine the columns into one column, allowing postgres to calculate the combined selectivity. - Drop the (p2, p3) index. But I need this for other queries. None of these are good solutions. So I am hoping that there is a better way to go about this! I think we must detemine exactly what the problem is before devising complex solutions gnari ---(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: [PERFORM] Query planner is using wrong index.
--- Ragnar [EMAIL PROTECTED] wrote: On fim, 2006-04-06 at 19:27 +1000, Brian Herlihy wrote: Yes, the primary key is far better. I gave it the ultimate test - I dropped the (p2, p3) index. It's blindingly fast when using the PK, I have problems understanding exactly how an index on (p1,p2,p3) can be faster than and index on (p2,p3) for a query not involving p1. can you demonstrate this with actual EXPLAIN ANALYZES ? something like: EXPLAIN ANALYZE select * from t where p2 = ? and p3 = ?; BEGIN; DROP INDEX p2p3; EXPLAIN ANALYZE select * from t where p2 = ? and p3 = ?; ROLLBACK; maybe your p2p3 index needs REINDEX ? Here's the output. The timings after caching are repeatable (varying only by 10% or so). Query before caching: db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls='; QUERY PLAN --- Index Scan using p2_p3_idx on t (cost=0.00..6.02 rows=1 width=102) (actual time=2793.247..2793.247 rows=0 loops=1) Index Cond: (((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Filter: ((p1)::text = 'a'::text) Total runtime: 2793.303 ms (4 rows) Query after caching: db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls='; QUERY PLAN --- Index Scan using p2_p3_idx on t (cost=0.00..6.02 rows=1 width=102) (actual time=0.617..0.617 rows=0 loops=1) Index Cond: (((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Filter: ((p1)::text = 'a'::text) Total runtime: 0.665 ms (4 rows) === At this point I did DROP INDEX p2_p3_idx Query after dropping index: db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls='; QUERY PLAN -- Index Scan using t_pkey on t (cost=0.00..6.02 rows=1 width=102) (actual time=95.188..95.188 rows=0 loops=1) Index Cond: (((p1)::text = 'a'::text) AND ((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Total runtime: 95.239 ms (3 rows) Query after dropping index, fully cached: db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls='; QUERY PLAN -- Index Scan using t_pkey on t (cost=0.00..6.02 rows=1 width=102) (actual time=0.030..0.030 rows=0 loops=1) Index Cond: (((p1)::text = 'a'::text) AND ((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Total runtime: 0.077 ms (3 rows) And one where the query planner chooses the primary key instead. Both p2 and p3 are present as Most Common Values in pg_statistics: Query before fully cached: db# explain analyze SELECT * FROM t WHERE p1 = 'b' AND p2 = 'www.google.com' AND p3 = 'search?hl=lr=q='; QUERY PLAN -- Index Scan using t_pkey on t (cost=0.00..6.02 rows=1 width=102) (actual time=212.092..212.100 rows=1 loops=1) Index Cond: (((p1)::text = 'b'::text) AND ((p2)::text = 'www.google.com'::text) AND ((p3)::text = 'search?hl=lr=q='::text)) Total runtime: 212.159 ms (3 rows) Query after fully cached: db# explain analyze SELECT * FROM t WHERE p1 = 'b' AND p2 = 'www.google.com' AND p3 = 'search?hl=lr=q='; QUERY PLAN -- Index Scan using t_pkey on t
Re: [PERFORM] Query planner is using wrong index.
On fös, 2006-04-07 at 00:01 +1000, Brian Herlihy wrote: --- Ragnar [EMAIL PROTECTED] wrote: On fim, 2006-04-06 at 19:27 +1000, Brian Herlihy wrote: Yes, the primary key is far better. I gave it the ultimate test - I dropped the (p2, p3) index. It's blindingly fast when using the PK, I have problems understanding exactly how an index on (p1,p2,p3) can be faster than and index on (p2,p3) for a query not involving p1. db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls='; this is different from what you said earlier. in your original post you showed a problem query without any reference to p1 in the WHERE clause. this confused me. Index Scan using p2_p3_idx on t (cost=0.00..6.02 rows=1 width=102) (actual time=2793.247..2793.247 rows=0 loops=1) Index Cond: (((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Filter: ((p1)::text = 'a'::text) Total runtime: 2793.303 ms (4 rows) try to add an ORDER BY clause: explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls=' ORDER BY p1,p2,p3; this might push the planner into using the primary key gnari ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [PERFORM] Query planner is using wrong index.
--- Ragnar [EMAIL PROTECTED] wrote: On fös, 2006-04-07 at 00:01 +1000, Brian Herlihy wrote: Index Scan using p2_p3_idx on t (cost=0.00..6.02 rows=1 width=102) (actual time=2793.247..2793.247 rows=0 loops=1) Index Cond: (((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Filter: ((p1)::text = 'a'::text) Total runtime: 2793.303 ms (4 rows) try to add an ORDER BY clause: explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls=' ORDER BY p1,p2,p3; this might push the planner into using the primary key gnari Thankyou very much, that works very well for select. However, I need it to work for update as well. Is there an equivalent way to force use of an index for updates? Here are the results for select: db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls=' order by p1,p2,p3; QUERY PLAN -- Index Scan using t_pkey on t (cost=0.00..6.02 rows=1 width=102) (actual time=32.519..32.519 rows=0 loops=1) Index Cond: (((p1)::text = 'a'::text) AND ((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Total runtime: 32.569 ms (3 rows) db# explain analyze select * from t WHERE p1 = 'a' and p2 = 'uk.altavista.com' AND p3 = 'web/results?itag=q=kgs=kls='; QUERY PLAN --- Index Scan using p2_p3_idx on t (cost=0.00..6.02 rows=1 width=102) (actual time=2790.364..2790.364 rows=0 loops=1) Index Cond: (((p2)::text = 'uk.altavista.com'::text) AND ((p3)::text = 'web/results?itag=q=kgs=kls='::text)) Filter: ((p1)::text = 'a'::text) Total runtime: 2790.420 ms (4 rows) But I cannot add an order by to an update. The other idea I came up with last night was to change p2_p3_idx so it indexes a value derived from p2 and p3, rather than p2 and p3 themselves. This would hide this index from the optimizer, forcing it to use the primary key. I am really surprised that I have to go through such contortions just to use the primary key! This area of Postgres needs improvement. Thanks, Brian ---(end of broadcast)--- TIP 3: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
Re: [PERFORM] Query planner is using wrong index.
-Original Message- From: [EMAIL PROTECTED] [mailto:pgsql-performance- [EMAIL PROTECTED] On Behalf Of Brian Herlihy Sent: Thursday, April 06, 2006 6:56 PM To: pgsql-performance@postgresql.org Subject: Re: [PERFORM] Query planner is using wrong index. [Snip] I am really surprised that I have to go through such contortions just to use the primary key! This area of Postgres needs improvement. Of course you mentioned that you are using 7.4.7. You might want to try upgrading to 8.1.3. There have been a lot of improvements to the performance since 7.4. I don't know if your specific problem was fixed, but it's worth a try. Also you might want to at least upgrade to 7.4.12 for the bug fixes. ---(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: [PERFORM] Query planner is using wrong index.
--- Dave Dutcher [EMAIL PROTECTED] wrote: -Original Message- To: pgsql-performance@postgresql.org Subject: Re: [PERFORM] Query planner is using wrong index. [Snip] I am really surprised that I have to go through such contortions just to use the primary key! This area of Postgres needs improvement. Of course you mentioned that you are using 7.4.7. You might want to try upgrading to 8.1.3. There have been a lot of improvements to the performance since 7.4. I don't know if your specific problem was fixed, but it's worth a try. Also you might want to at least upgrade to 7.4.12 for the bug fixes. Thanks for the suggestions. I've verified the same problem in 8.1.3 as well, after my initial post. It was actually in 8.1.3 that I first discovered the problem. I noticed this item in the TODO list: - Allow accurate statistics to be collected on indexes with more than one column or expression indexes, perhaps using per-index statistics This is what I need! But until that is added, I need a way to use the primary key with the current version :) Thanks, Brian ---(end of broadcast)--- TIP 2: Don't 'kill -9' the postmaster
Re: [PERFORM] Query planner is using wrong index.
Brian Herlihy [EMAIL PROTECTED] writes: My options seem to be - Fudge the analysis results so that the selectivity estimate changes. I have tested reducing n_distinct, but this doesn't seem to help. - Combine the columns into one column, allowing postgres to calculate the combined selectivity. - Drop the (p2, p3) index. But I need this for other queries. Have you considered reordering the pkey to be (p2,p3,p1) and then dropping the (p2,p3) index? regards, tom lane ---(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
