Re: [PERFORM] LEFT JOIN optimization
On Mon, 12 Sep 2005 00:47:57 +0300, Ksenia Marasanova [EMAIL PROTECTED] wrote: - Seq Scan on user_ (cost=0.00..7430.63 rows=12763 width=245) (actual time=360.431..1120.012 rows=12763 loops=1) If 12000 rows of the given size are stored in more than 7000 pages, then there is a lot of free space in these pages. Try VACUUM FULL ... Servus Manfred ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] LEFT JOIN optimization
* Ksenia Marasanova ([EMAIL PROTECTED]) wrote: Any tips are greatly appreciated. EXPLAIN ANALYZE of the same queries would be much more useful. Thanks, Stephen signature.asc Description: Digital signature
Re: [PERFORM] LEFT JOIN optimization
2005/9/12, Stephen Frost [EMAIL PROTECTED]: * Ksenia Marasanova ([EMAIL PROTECTED]) wrote: Any tips are greatly appreciated. EXPLAIN ANALYZE of the same queries would be much more useful. Thanks, here it is: test=# explain analyze select * from user_ left join church on user_.church_id = church.id; QUERY PLAN --- Hash Left Join (cost=6.44..7626.69 rows=12763 width=325) (actual time=388.573..2016.929 rows=12763 loops=1) Hash Cond: (outer.church_id = inner.id) - Seq Scan on user_ (cost=0.00..7430.63 rows=12763 width=245) (actual time=360.431..1120.012 rows=12763 loops=1) - Hash (cost=5.75..5.75 rows=275 width=80) (actual time=27.985..27.985 rows=0 loops=1) - Seq Scan on church (cost=0.00..5.75 rows=275 width=80) (actual time=0.124..26.953 rows=275 loops=1) Total runtime: 2025.946 ms (6 rows) test=# set enable_seqscan='false'; SET test=# explain analyze select * from user_ left join church on user_.church_id = church.id; QUERY PLAN -- Merge Right Join (cost=0.00..44675.77 rows=12763 width=325) (actual time=0.808..2119.099 rows=12763 loops=1) Merge Cond: (outer.id = inner.church_id) - Index Scan using chirch_pkey on church (cost=0.00..17.02 rows=275 width=80) (actual time=0.365..5.471 rows=275 loops=1) - Index Scan using user__church_id on user_ (cost=0.00..44500.34 rows=12763 width=245) (actual time=0.324..1243.348 rows=12763 loops=1) Total runtime: 2131.364 ms (5 rows) I followed some tips on the web and vacuum-ed database, I think the query is faster now, almost acceptable, but still interesting to know if it possible to optimize it... Thanks again, -- Ksenia ---(end of broadcast)--- TIP 4: Have you searched our list archives? http://archives.postgresql.org
Re: [PERFORM] LEFT JOIN optimization
* Ksenia Marasanova ([EMAIL PROTECTED]) wrote: test=# explain analyze select * from user_ left join church on user_.church_id = church.id; [...] Total runtime: 2025.946 ms (6 rows) test=# set enable_seqscan='false'; SET test=# explain analyze select * from user_ left join church on user_.church_id = church.id; [...] Total runtime: 2131.364 ms (5 rows) I followed some tips on the web and vacuum-ed database, I think the query is faster now, almost acceptable, but still interesting to know if it possible to optimize it... I have to say that it does seem a bit slow for only 12,000 rows.. What's the datatype of user_.church_id and church.id? Are you sure you really want all 12,000 rows every time you run that query? Perhaps there's a 'where' clause you could apply with an associated index to limit the query to just what you actually need? You'll noticed from above, though, that the non-index scan is faster. I'd expect that when using a left-join query: you have to go through the entire table on an open left-join like that, a sequencial scan is going to be the best way to do that. The fact that church.id is hashed makes the solution the planner came up with almost certainly the best one possible. Are you sure a left-join is what you want? Sounds like maybe you've moved (for some reason) from a regular join to a left join with a filtering in the application which is probably a bad move... If you can use at least some filtering in the database I expect that'd help.. Thanks, Stephen signature.asc Description: Digital signature
