Re: Parallel Aggregates for string_agg and array_agg

> On May 1, 2018, at 2:11 PM, Andres Freund <andres(at)anarazel(dot)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