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Subject: Re: Parallel Aggregates for string_agg and array_agg
From: Mark Dilger <hornschnorter@gmail.com>
In-Reply-To: <20180501211147.3balprbggw3pdpxk@alap3.anarazel.de>
Date: Tue, 1 May 2018 14:35:46 -0700
Cc: Tom Lane <tgl@sss.pgh.pa.us>,
 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>
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To: Andres Freund <andres@anarazel.de>
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> On May 1, 2018, at 2:11 PM, Andres Freund <andres@anarazel.de> wrote:
> 
> Hi,
> 
> 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?

I was responding in part to Tom's upthread statement:

  Your own example of assuming that separate aggregates are computed
  in the same order reinforces my point, I think.  In principle, anybody
  who's doing that should write

      array_agg(e order by x),
      array_agg(f order by x),
      string_agg(g order by x)

  because otherwise they shouldn't assume that;

It seems Tom is saying that you can't assume separate aggregates will be
computed in the same order.  Hence my response.  What am I missing here?

mark