Re: Parallel Aggregates for string_agg and array_agg

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)))

Someday we might need more flexible testing, but that can be solved
some other time.

            regards, tom lane