Horton

Re: [PATCH] Push limit to sort through a subquery

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

View thread View on postgr.es
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Douglas Doole <dougdoole@gmail.com>, Konstantin Knizhnik <k.knizhnik@postgrespro.ru>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2017-08-25T18:01:20Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> On another note, here's a second patch applying on top of my earlier
> patch to push down Limit through Gather and Gather Merge.

I looked through this a little, and feel uncomfortable with the division
of typedefs between execnodes.h and tuplesort.h.  I'm inclined to push
struct SortInstrumentation, and maybe also SharedSortInfo, into
tuplesort.h.  Then tuplesort_get_stats could be given the signature

	void tuplesort_get_stats(Tuplesortstate *state,
	                         SortInstrumentation *stats);

which seems less messy.  Thoughts?

I'm also pretty suspicious of the filter code you added to subselect.sql.
What happens when there's more than one worker?

(BTW, would it make sense to number the workers from 1 not 0 in the
EXPLAIN printout?)

			regards, tom lane

Commits

  1. Propagate sort instrumentation from workers back to leader.

  2. Push tuple limits through Gather and Gather Merge.

  3. Code review for pushing LIMIT through subqueries.

  4. Make new regression test case parallel-safe, and improve its output.

  5. Push limit through subqueries to underlying sort, where possible.

  6. Move targetlist SRF handling from expression evaluation to new executor node.