Re: 9.5: Better memory accounting, towards memory-bounded HashAgg

On 23.12.2014 10:16, Jeff Davis wrote:
> It seems that these two patches are being reviewed together. Should
> I just combine them into one? My understanding was that some wanted
> to review the memory accounting patch separately.

I think we should keep the patches separate. Applying two patches is
trivial, splitting them not so much.

> On Sun, 2014-12-21 at 20:19 +0100, Tomas Vondra wrote:
>> That's the only conflict, and after fixing it it compiles OK.
>> However, I got a segfault on the very first query I tried :-(
>
> If lookup_hash_entry doesn't find the group, and there's not enough
> memory to create it, then it returns NULL; but the caller wasn't
> checking for NULL. My apologies for such a trivial mistake, I was
> doing most of my testing using DISTINCT. My fix here was done
> quickly, so I'll take a closer look later to make sure I didn't miss
> something else.
>
> New patch attached (rebased, as well).
>
> I also see your other message about adding regression testing. I'm
> hesitant to slow down the tests for everyone to run through this
> code path though. Should I add regression tests, and then remove them
> later after we're more comfortable that it works?

I think when done right, the additional time will be negligible. By
setting the work_mem low, we don't need that much data.

regards
Tomas