Horton

Re: Parallel Full Hash Join

Tom Lane <tgl@sss.pgh.pa.us>

View thread View on postgr.es
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>, Ian Lawrence Barwick <barwick@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>, vignesh C <vignesh21@gmail.com>, Greg Nancarrow <gregn4422@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2023-04-04T19:37:52Z
Lists: pgsql-hackers 
Thomas Munro <thomas.munro@gmail.com> writes:
> I committed the main patch.

This left the following code in hash_inner_and_outer (joinpath.c):

        /*
         * If the joinrel is parallel-safe, we may be able to consider a
         * partial hash join.  However, we can't handle JOIN_UNIQUE_OUTER,
         * because the outer path will be partial, and therefore we won't be
         * able to properly guarantee uniqueness.  Similarly, we can't handle
         * JOIN_FULL and JOIN_RIGHT, because they can produce false null
         * extended rows.  Also, the resulting path must not be parameterized.
         */
        if (joinrel->consider_parallel &&
            save_jointype != JOIN_UNIQUE_OUTER &&
            outerrel->partial_pathlist != NIL &&
            bms_is_empty(joinrel->lateral_relids))
        {

The comment is no longer in sync with the code: this if-test used to
reject JOIN_FULL and JOIN_RIGHT, and no longer does so, but the comment
still claims it should.  Shouldn't we drop the sentence beginning
"Similarly"?  (I see that there's now one sub-section that still rejects
such cases, but it no longer seems correct to claim that they're rejected
overall.)

			regards, tom lane

Commits

  1. Remove overzealous assertion from PHJ.

  2. Remove comment obsoleted by 11c2d6fd.

  3. Parallel Hash Full Join.

  4. Improve the naming of Parallel Hash Join phases.

  5. Update the names of Parallel Hash Join phases.

  6. Add BarrierArriveAndDetachExceptLast().

  7. Mop-up for wait event naming issues.

  8. Push tuple limits through Gather and Gather Merge.