From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Mark Dilger <hornschnorter@gmail.com>,
        David Rowley <david.rowley@2ndquadrant.com>,
        Alvaro Herrera <alvherre@alvh.no-ip.org>,
        Stephen Frost <sfrost@snowman.net>,
        Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Parallel Aggregates for string_agg and array_agg
In-reply-to: <20180501211147.3balprbggw3pdpxk@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Tue, 01 May 2018 14:11:47 -0700"
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Date: Tue, 01 May 2018 17:16:16 -0400
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Andres Freund <andres@anarazel.de> writes:
> On 2018-05-01 14:09:39 -0700, Mark Dilger wrote:
>> I don't care which order the data is in, as long as x[i] and y[i] are
>> matched correctly.  It sounds like this patch would force me to write
>> that as, for example:
>> 
>> select array_agg(a order by a, b) AS x, array_agg(b order by a, b) AS y
>> from generate_a_b_func(foo);
>> 
>> which I did not need to do before.

> Why would it require that? Rows are still processed row-by-row even if
> there's parallelism, no?

Yeah, as long as we distribute all the aggregates in the same way,
it seems like they'd all see the same random-ish input ordering.
I can vaguely conceive of future optimizations that might break
that, but not what we have today.

			regards, tom lane