On 31 Srpen 2011, 16:49, Andy Colson wrote: > On 8/31/2011 9:35 AM, Tore Halvorsen wrote: >> Hi, >> >> I'm trying to optimize a query where I have two tables that both have a >> timestamp column. I want the result where either of the timestamps is >> after a specified time. In a reduced form, like this: >> >> >> CREATE TABLE a >> ( >> id serial NOT NULL PRIMARY KEY, >> time timestamp without time zone NOT NULL DEFAULT now() >> ); >> >> CREATE INDEX a_time_idx ON a USING btree (time DESC NULLS LAST); >> >> CREATE TABLE b >> ( >> id serial NOT NULL PRIMARY KEY, >> time timestamp without time zone NOT NULL DEFAULT now() >> ); >> >> CREATE INDEX b_time_idx ON b USING btree (time DESC NULLS LAST); >> >> --- generate some data >> insert into a(time) >> select now() - '10 year'::interval * random() from generate_series(1, >> 1000000, 1); >> >> insert into b(time) >> select now() - '10 year'::interval * random() from generate_series(1, >> 1000000, 1); >> >> -- Using constraint works as expected, and uses the time index. >> select * from a join b using(id) >> where a.time >= '2011-08-15'; >> >> -- ... both ways... >> select * from a join b using(id) >> where b.time >= '2011-08-15'; >> >> -- However, if I'm trying to do this for both times at once, the time >> index is not used at all >> select * from a join b using(id) >> where a.time >= '2011-08-15' OR b.time >= '2011-08-01' >> >> -- This can be optimized by using CTEs >> with am as ( >> select * from a where time >= '2011-08-15' >> ) >> , bm as ( >> select * from b where time >= '2011-08-15' >> ) >> select * from am join bm using(id) >> >> -- end >> >> I'm just wondering why the optimizer does things the way it does - and >> if the CTE version is the best way to go... >> >> The actual case is slightly more complex and uses more tables - this is >> mostly a way to find updated data. >> >> -- >> Eld på åren og sol på eng gjer mannen fegen og fjåg. [Jøtul] >> <demo> 2011 Tore Halvorsen || +052 0553034554 > > > > On PG 9, after I ANALYZED the tables, it used indexes: > > > QUERY PLAN > --------------------------------------------------------------------------------------------------------------- > Merge Join (cost=1.59..82778.35 rows=13171 width=20) (actual > time=0.066..1076.616 rows=12966 loops=1) > Merge Cond: (a.id = b.id) > Join Filter: ((a."time" >= '2011-08-15 00:00:00'::timestamp without > time zone) OR (b."time" >= '2011-08-01 0 > -> Index Scan using a_pkey on a (cost=0.00..31389.36 rows=1000000 > width=12) (actual time=0.007..204.856 ro > -> Index Scan using b_pkey on b (cost=0.00..31389.36 rows=1000000 > width=12) (actual time=0.006..224.189 ro > > > ANALYZE is the magic. Yes ;-) Who says we don't have a magical fairy dust? Anyway you could try to postpone the join a bit - determine the IDs first and then join. Something like this WITH t AS ( SELECT id FROM a WHERE time >= '2011-08-15' UNION SELECT id FROM b WHERE time >= '2011-08-15' ) SELECT * FROM a JOIN b ON (a.id = b.id) WHERE id IN (SELECT id FROM t); or something like that. It's not as clean as your query, but in some cases it's faster. Tomas -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
