Re: d25ea01275 and partitionwise join

Amit Langote <amitlangote09(at)gmail(dot)com> writes:
> Updated patches attached.

I looked through these and committed 0001+0002, with some further
comment-polishing. However, I have no faith at all in 0003. It is
blithely digging through COALESCE expressions with no concern for
whether they came from full joins or not, or whether the other values
being coalesced to might completely change the semantics. Digging
through PlaceHolderVars scares me even more; what's that got to do
with the problem, anyway? So while this might fix the complained-of
issue of failing to use a partitionwise join, I think it wouldn't be
hard to create examples that it would incorrectly turn into
partitionwise joins.

I wonder whether it'd be feasible to fix the problem by going in the
other direction; that is, while constructing the nullable_partexprs
lists for a full join, add synthesized COALESCE() expressions for the
output columns (by wrapping COALESCE around copies of the input rels'
partition expressions), and then not need to do anything special in
match_expr_to_partition_keys. We'd still need to convince ourselves
that this did the right thing and not any of the wrong things, but
I think it might be easier to prove it that way.

regards, tom lane