Re: Memory-Bounded Hash Aggregation

On 05/02/2020 21:56, Jeff Davis wrote:
> On Tue, 2020-02-04 at 18:10 +0200, Heikki Linnakangas wrote:
>> I'd love to change the LogicalTape API so that you could allocate
>> and
>> free tapes more freely. I wrote a patch to do that, as part of
>> replacing
>> tuplesort.c's polyphase algorithm with a simpler one (see [1]), but
>> I
>> never got around to committing it. Maybe the time is ripe to do that
>> now?
>
> It's interesting that you wrote a patch to pause the tapes a while ago.
> Did it just fall through the cracks or was there a problem with it?
>
> Is pause/resume functionality required, or is it good enough that
> rewinding a tape frees the buffer, to be lazily allocated later?

It wasn't strictly required for what I was hacking on then. IIRC it
would have saved some memory during sorting, but Peter G felt that it
wasn't worth the trouble, because he made some other changes around the
same time, which made it less important
(https://www.postgresql.org/message-id/CAM3SWZS0nwOPoJQHvxugA9kKPzky2QC2348TTWdSStZOkke5tg%40mail.gmail.com)
I dropped the ball on both patches then, but I still think they would be
worthwhile.

- Heikki