| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Mark Dilger <hornschnorter(at)gmail(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Stephen Frost <sfrost(at)snowman(dot)net>, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Parallel Aggregates for string_agg and array_agg |
| Date: | 2018-05-01 21:16:16 |
| Message-ID: | 16902.1525209376@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Andres Freund <andres(at)anarazel(dot)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
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