Re: Combining Aggregates
| От | David Rowley |
|---|---|
| Тема | Re: Combining Aggregates |
| Дата | |
| Msg-id | CAKJS1f8=RzEE+3Lc_oR5+9gY759xtVtFCvCRNH+PZfjuLeHnkg@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: Combining Aggregates (Tomas Vondra <tomas.vondra@2ndquadrant.com>) |
| Список | pgsql-hackers |
On 19 January 2016 at 18:04, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote:
Hi,
On 01/19/2016 05:00 AM, David Rowley wrote:On 19 January 2016 at 06:03, Pavel Stehule <pavel.stehule@gmail.com...
<mailto:pavel.stehule@gmail.com>> wrote:
It is strange, why hashaggregate is too slow?
Good question. I looked at this and found my VM was swapping like crazy.
Upon investigation it appears that's because, since the patch creates a
memory context per aggregated group, and in this case I've got 1 million
of them, it means we create 1 million context, which are
ALLOCSET_SMALL_INITSIZE (1KB) in size, which means about 1GB of memory,
which is more than my VM likes.
Really? Where do we create the memory context? IIRC string_agg uses the aggcontext directly, and indeed that's what I see in string_agg_transfn and makeStringAggState.
Yeah, all this is talk is relating to Robert's expandedstring-v1.patch which changes string_agg to use text and expanded-objects. This also means that a memory context is created per group, which is rather a big overhead.
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