Re: Parallel Aggregates for string_agg and array_agg

On 27 March 2018 at 12:49, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> I wrote:
>> The main thing that remains undone is to get some test coverage ---
>> AFAICS, none of these new functions get exercised in the standard
>> regression tests.
>
> Oh, I thought of another thing that would need to get done, if we decide
> to commit this.  array_agg_serialize/deserialize only work if the array
> element type has send/receive functions.  The planner's logic to decide
> whether partial aggregation is possible doesn't look any deeper than
> whether the aggregate has serialize/deserialize, but I think we have to
> teach it that if it's these particular serialize/deserialize functions,
> it needs to check the array element type.  Otherwise we'll get runtime
> failures if partial aggregation is attempted on array_agg().
>
> This would not matter for any core datatypes, since they all have
> send/receive functions, but I imagine it's still pretty common for
> extension types not to.
>
> For my money it'd be sufficient to hard-code the test like
>
>     if ((aggserialfn == F_ARRAY_AGG_SERIALIZE ||
>          aggdeserialfn == F_ARRAY_AGG_DESERIALIZE) &&
>         !array_element_type_has_send_and_recv(exprType(aggregate input)))

I'd failed to consider this.

Would it not be cleaner to just reject trans types without a
send/receive function? Or do you think that would exclude other valid
cases?


-- 
 David Rowley                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services