Re: [HACKERS] Aggregate transition state merging vs. hypothetical set functions

On 13 October 2017 at 12:08, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Therefore, I think we need to bite the bullet and provide an aggregate
> property (CREATE AGGREGATE argument / pg_aggregate column) that tells
> whether the aggregate supports transition state merging.  Likely this
> should have been in the state-merging patch to begin with, but better
> late than never.
>
> The main thing that probably has to be hashed out before we can write
> that patch is what the default should be for user-created aggregates.
> I am inclined to think that we should err on the side of safety and
> default it to false (no merge support).  You could argue that the
> lack of complaints since 9.6 came out is sufficient evidence that
> defaulting to true would be all right, but I'm not sure.

Are you considering that this is an option only for ordered-set
aggregates or for all?

If the user defines their normal aggregate as not safe for merging,
then surely it'll not be suitable to be used as a window function
either, since the final function will also be called there multiple
times per state.


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


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