Re: count(*), EXISTS, indexes

Itai Zukerman <zukerman(at)math-hat(dot)com> writes:
> Define:
> CREATE TABLE A (x int PRIMARY KEY, real v);
> CREATE TABLE B (x int);

> I'd like to calculate:
> SELECT sum(v) FROM A WHERE EXISTS (SELECT 1 FROM B WHERE A.x=B.x);
> ...but then it won't use the primary key index on A.x.

In CVS tip (7.4-to-be) I think

SELECT sum(v) FROM A WHERE A.x IN (SELECT B.x FROM B);

would probably work well. In current releases I think all you can do is
add an index to B.x and settle for the EXISTS() approach.

> PS. B is relatively small, a few thousand rows, while A has well over
> 500,000 rows. The DISTINCT A.x should be about 10,000-50,000.

BTW, how can a PRIMARY KEY column have fewer DISTINCT values than there
are rows?

regards, tom lane