Horton

Re: Allowing parallel-safe initplans

Robert Haas <robertmhaas@gmail.com>

View thread View on postgr.es
From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@lists.postgresql.org
Date: 2023-04-12T18:06:05Z
Lists: pgsql-hackers 
On Wed, Apr 12, 2023 at 12:44 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Pursuant to the discussion at [1], here's a patch that removes our
> old restriction that a plan node having initPlans can't be marked
> parallel-safe (dating to commit ab77a5a45).  That was really a special
> case of the fact that we couldn't transmit subplans to parallel
> workers at all.  We fixed that in commit 5e6d8d2bb and follow-ons,
> but this case never got addressed.

Nice.

> Along the way, this also takes care of some sloppiness about updating
> path costs to match when we move initplans from one place to another
> during createplan.c and setrefs.c.  Since all the planning decisions are
> already made by that point, this is just cosmetic; but it seems good
> to keep EXPLAIN output consistent with where the initplans are.

OK. It would be nicer if we had a more principled approach here, but
that's a job for another day.

> There's only one existing test case that visibly changes plan with
> these changes.  The new plan is clearly saner-looking than before,
> and testing with some data loaded into the table confirms that it
> is faster.  I'm not sure if it's worth devising more test cases.

It seems like it would be nice to see one or two additional scenarios
where these changes bring a benefit, with different kinds of plan
shapes.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. Account for optimized MinMax aggregates during SS_finalize_plan.

  2. Add test case showing that commit d0d44049d fixed a live bug.

  3. Allow plan nodes with initPlans to be considered parallel-safe.

  4. Allow parallel workers to execute subplans.

  5. Mark a query's topmost Paths parallel-unsafe if they will have initPlans.