Due to the implementation of convert_ANY_sublink_to_join, we have
limitations below, which has been discussed at [1] [2].
if (contain_vars_of_level((Node *) subselect, 1))
return NULL;
I'm thinking if we can do the ${subject}. If so, the query like
SELECT * FROM t1 WHERE
a IN (SELECT * FROM t2 WHERE t2.b > t1.b);
can be converted to
SELECT * FROM t1 WHERE
EXISTS (SELECT * FROM t2 WHERE t2.b > t1.b AND t1.a = t2.a);
I have coded this and tested my idea, here are some new findings: 1). Not all the
TargetEntry->expr can be used as qual, for example: WindowFunc, AggFunc, SRFs.
2). For simple correlated EXISTS query, the current master code also tries to transform it
to IN format and implement it by hashing (make_subplan). So there is no need to
convert an IN query to EXISTS query if the sublink can be pulled up already,
which means this patch should only take care of !contain_vars_of_level((Node *) subselect, 1).
Note the changes of postgres_fdw.out are expected. The 'a' in foreign_tbl has varlevelsup = 1;
SELECT a FROM base_tbl WHERE a IN (SELECT a FROM foreign_tbl);
Here is some performance testing for this patch:
select * from tenk1 t1
where hundred in (select hundred from tenk2 t2
where t2.odd = t1.odd
and even in (select even from tenk1 t3
where t3.fivethous = t2.fivethous))
and even > 0;
master: 892.902 ms
patched: 56.08 ms
Patch attached, any feedback is welcome.
