Re: [PATCH] Resolve Parallel Hash Join Performance Issue

(Replies to both Gang and Tom below).

On Fri, Jan 10, 2020 at 1:52 PM Deng, Gang <gang.deng@intel.com> wrote:
> Thank you for the comment. Yes, I agree the alternative of using '(!parallel)', so that no need to test the bit. Will someone submit patch to for it accordingly?

Here's a patch like that.

On Fri, Jan 10, 2020 at 3:43 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Thomas Munro <thomas.munro@gmail.com> writes:
> > Right, I see.  The funny thing is that the match bit is not even used
> > in this query (it's used for right and full hash join, and those
> > aren't supported for parallel joins yet).  Hmm.  So, instead of the
> > test you proposed, an alternative would be to use if (!parallel).
> > That's a value that will be constant-folded, so that there will be no
> > branch in the generated code (see the pg_attribute_always_inline
> > trick).  If, in a future release, we need the match bit for parallel
> > hash join because we add parallel right/full hash join support, we
> > could do it the way you showed, but only if it's one of those join
> > types, using another constant parameter.
>
> Can we base the test off the match type today, and avoid leaving
> something that will need to be fixed later?

I agree that it'd be nicer to use the logically correct thing, namely
a test of HJ_FILL_INNER(node), but that'd be a run-time check.  I'd
like to back-patch this and figured that we don't want to add new
branches too casually.

I have an experimental patch where "fill_inner" and "fill_outer" are
compile-time constants and you can skip various bits of code without
branching (part of a larger experiment to figure out which of many
parameters are worth specialising at a cost of a couple of KB of text
per combination, including the ability to use wider hashes so that
monster sized joins work better).  Then I could test the logically
correct thing explicitly without branches.