dear pgers -- consider the following toy example (using pg 8.4.3) :
create temporary table foo (
ts timestamp not null,
id integer not null,
val double precision not null,
primary key (ts, id)
);
i might want to return the vals, minus the averages at each timestamp. the
obvious self-join results in a sequential scan over foo -- we aggregate the
average val for EVERY timestamp, then join against the timestamps we want.
us_quotedb=# explain select ts, id, val - aval from foo join (select ts,
avg(val) as aval from foo group by ts) as a using (ts) where ts > '2010-11-11'
and ts < '2010-11-13';
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Hash Join (cost=49.06..54.41 rows=8 width=28)
Hash Cond: (pg_temp_2.foo.ts = pg_temp_2.foo.ts)
-> HashAggregate (cost=34.45..36.95 rows=200 width=16)
-> Seq Scan on foo (cost=0.00..26.30 rows=1630 width=16)
-> Hash (cost=14.51..14.51 rows=8 width=20)
-> Bitmap Heap Scan on foo (cost=4.33..14.51 rows=8 width=20)
Recheck Cond: ((ts > '2010-11-11 00:00:00'::timestamp without
time zone) AND (ts < '2010-11-13 00:00:00'::timestamp without time zone))
-> Bitmap Index Scan on foo_pkey (cost=0.00..4.33 rows=8
width=0)
Index Cond: ((ts > '2010-11-11 00:00:00'::timestamp
without time zone) AND (ts < '2010-11-13 00:00:00'::timestamp without time
zone))
on the other hand, if i specify "which" timestamp i'm restricting, it appears
to do the right thing:
us_quotedb=# explain select ts, id, val - aval from foo join (select ts,
avg(val) as aval from foo group by ts) as a using (ts) where a.ts >
'2010-11-11' and a.ts < '2010-11-13';
QUERY
PLAN
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Nested Loop (cost=18.86..29.14 rows=8 width=28)
-> HashAggregate (cost=14.55..14.56 rows=1 width=16)
-> Bitmap Heap Scan on foo (cost=4.33..14.51 rows=8 width=16)
Recheck Cond: ((ts > '2010-11-11 00:00:00'::timestamp without
time zone) AND (ts < '2010-11-13 00:00:00'::timestamp without time zone))
-> Bitmap Index Scan on foo_pkey (cost=0.00..4.33 rows=8
width=0)
Index Cond: ((ts > '2010-11-11 00:00:00'::timestamp
without time zone) AND (ts < '2010-11-13 00:00:00'::timestamp without time
zone))
-> Bitmap Heap Scan on foo (cost=4.31..14.45 rows=8 width=20)
Recheck Cond: (pg_temp_2.foo.ts = pg_temp_2.foo.ts)
-> Bitmap Index Scan on foo_pkey (cost=0.00..4.31 rows=8 width=0)
Index Cond: (pg_temp_2.foo.ts = pg_temp_2.foo.ts)
i find this behavior curious. my understanding is that both queries are
equivalent, and i would expect that the query planner would be able to choose
either of those plans. this is important -- with the real data i'm working
with, the table is very large, and the sequential scan is a killer.
are these queries equivalent, or am i mistaken? if the planner distinguishes
between these plans, how do i ensure that where clause restrictions propagate
(correctly) to subqueries?
best regards, ben
--
Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-performance
