Re: Combining Aggregates
| От | David Rowley |
|---|---|
| Тема | Re: Combining Aggregates |
| Дата | |
| Msg-id | CAApHDvoPZFuzvvOQXOM2oDxyuUC7Y=g+5Oyz3jMtKpsGT0fU-A@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Combining Aggregates (Peter Eisentraut <peter_e@gmx.net>) |
| Список | pgsql-hackers |
On 25 February 2015 at 08:15, Peter Eisentraut <peter_e@gmx.net> wrote:
David Rowley
On 2/20/15 3:32 PM, Tomas Vondra wrote:
> Also, there are aggregate functions like array_agg() or string_agg()
> that make this impossible, just like for many custom aggregates (like
> hyperloglog for example). Again, I might not understand the idea
> correctly ...
How would a combining function work for something like array_agg()? I
don't think it would, at least if you want to preserve the ordering
option for the user.
They just wouldn't work in that case. We'd simply not have a combine function for that aggregate.
The yet to be written code, (say parallel hash aggregate), the planner would have to ensure that each aggregate function being used had a combine function, if any aggregate in the current query level didn't have one then it would not parallelise the query.
Regards
David Rowley
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