Horton

Re: Parallel Aggregates for string_agg and array_agg

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

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
Cc: David Rowley <david.rowley@2ndquadrant.com>, Stephen Frost <sfrost@snowman.net>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Andres Freund <andres@anarazel.de>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2018-05-01T20:59:35Z
Lists: pgsql-hackers 
Robert Haas <robertmhaas@gmail.com> writes:
> On Mon, Mar 26, 2018 at 4:27 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> I fear that what will happen, if we commit this, is that something like
>> 0.01% of the users of array_agg and string_agg will be pleased, another
>> maybe 20% will be unaffected because they wrote ORDER BY which prevents
>> parallel aggregation, and the remaining 80% will scream because we broke
>> their queries.  Telling them they should've written ORDER BY isn't going
>> to cut it, IMO, when the benefit of that breakage will accrue only to some
>> very tiny fraction of use-cases.

> I think your estimated percentages here are wildly inaccurate.

My estimate for the number of people positively impacted could be off
by a factor of a thousand, and it still wouldn't change the conclusion
that this will hurt more people than it helps.

I see that I'm in the minority on this, so I'm prepared to accept defeat,
but I stand by that conclusion.

			regards, tom lane

Commits

  1. Fix unstable aggregate regression test

  2. Allow parallel aggregate on string_agg and array_agg

  3. Improve performance of ORDER BY / DISTINCT aggregates

  4. Support ORDER BY within aggregate function calls, at long last providing a

  5. Fix broken markup.