Marten Feldtmann <marten@feki.toppoint.de> writes:
> Ok, indeed it was the wrong statement, therefore I rewrote it to:
> SELECT P3AT.AO,P3AT.AT,P3AT.AV FROM P3AT
> WHERE
> EXISTS
> (SELECT B.AO FROM P3AT AS B WHERE B.AO=P3AT.AO AND ....)
> There're indices on AO,AT,AV. Size of table about 23000 rows.
> He uses indices only for the statement within EXISTS, which seems
> to be pretty fast but for the outer SELECT statement he uses a
> sequential scan ... which brings the the statement down from 23 ms to
> 2000ms.
> Actually I don not understand it. The "B.AO=P3AT.AO" should create
> internally a join (?) and therefore he could use the index on AO in
> the outer select to create the result - but the seq scan seems to
> be wrong.
You are right, this would be better done as a join, but the system is
not currently smart enough to rewrite a sub-SELECT into a join. You
have to do it yourself :-(.
I think preserving the semantics of this exactly would require outer
joins, which we don't yet have, but you could get close with DISTINCT:
SELECT DISTINCT A.AO,A.AT,A.AV FROM P3AT A, P3AT B
WHERE A.AO = B.AO AND ...;
Both of these issues are on the TODO list, and probably will get
addressed in a release or three...
regards, tom lane
