Re: [SQL] uncorrelated subqueries

Bruce Lowery <bruce(dot)lowery(at)edventions(dot)com> writes:
> In v6.4.2 do uncorrelated subqueries get run on each iteration of the
> outer loop?

It looks like the particular example you give is done that way, but the
system does know about uncorrelated subqueries. For example, using 6.5:

explain SELECT a FROM table1 WHERE table1.b=33 AND
table1.c = ( SELECT d FROM table2 WHERE table2.e=44);
NOTICE: QUERY PLAN:
Seq Scan on table1 (cost=43.00 rows=1 width=4)
InitPlan
-> Seq Scan on table2 (cost=43.00 rows=1 width=4)

explain SELECT a FROM table1 WHERE table1.b=33 AND
table1.c = ( SELECT d FROM table2 WHERE table2.e=table1.a);
NOTICE: QUERY PLAN:
Seq Scan on table1 (cost=43.00 rows=1 width=4)
SubPlan
-> Seq Scan on table2 (cost=43.00 rows=1 width=4)

You can see that we get an "InitPlan" (ie, run once) for an uncorrelated
subquery but a "SubPlan" (repeat each time) for a correlated one.
Unfortunately, the case you care about is:

explain SELECT a FROM table1 WHERE table1.b=33 AND
table1.c IN ( SELECT d FROM table2 WHERE table2.e=44);
NOTICE: QUERY PLAN:
Seq Scan on table1 (cost=43.00 rows=1 width=4)
SubPlan
-> Seq Scan on table2 (cost=43.00 rows=1 width=4)

The main problem that would have to be solved to convert this to
an InitPlan is what to do if the subselect returns a huge number
of tuples ... with the current implementation, since we scan the
tuples one at a time, there's no problem, but if we try to store
all the tuples we could run out of memory.

regards, tom lane