Re: Memory consumed by child SpecialJoinInfo in partitionwise join planning

The attached patch (0002) fixes this issue by
1. declaring child SpecialJoinInfo as a local variable, thus
allocating memory on the stack instead of CurrentMemoryContext. The
memory on the stack is freed automatically.
2. Freeing the Relids in SpecialJoinInfo explicitly after
SpecialJoinInfo has been used.

It doesn't seem too expensive to translate SpecialJoinInfos, so I think
it's OK to construct and free the SpecialJoinInfo for a child join on
the fly.  So the approach in 0002 looks reasonable to me.  But there is
something that needs to be fixed in 0002.

+   bms_free(child_sjinfo->commute_above_l);
+   bms_free(child_sjinfo->commute_above_r);
+   bms_free(child_sjinfo->commute_below_l);
+   bms_free(child_sjinfo->commute_below_r);

These four members in SpecialJoinInfo only contain outer join relids.
They do not need to be translated.  So they would reference the same
memory areas in child_sjinfo as in parent_sjinfo.  We should not free
them, otherwise they would become dangling pointers in parent sjinfo.

Thanks