Horton

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Commits

  1. Skip allocating hash table in EXPLAIN-only mode.

  1. don't allocate HashAgg hash tables when running explain only

    Alexey Bashtanov <bashtanov@imap.cc> — 2020-11-13T16:10:30Z

    Hi,

I got somewhat scared when my explain took a few seconds to complete and 
used a few gigs of RAM.
To reproduce try the following:

discard temp;
create temp table a as select to_timestamp(generate_series(1, 7000)) i;
analyze a;
set work_mem to '3GB';
explain select distinct a1.i - a2.i from a a1, a a2;

I would appreciate if someone could have a look at the patch attached, 
which makes executor skip initializing hash tables when doing explain only.

Best, Alex

  2. Re: don't allocate HashAgg hash tables when running explain only

    Heikki Linnakangas <hlinnaka@iki.fi> — 2020-11-18T10:40:24Z

    On 13/11/2020 18:10, Alexey Bashtanov wrote:
> Hi,
> 
> I got somewhat scared when my explain took a few seconds to complete and
> used a few gigs of RAM.
> To reproduce try the following:
> 
> discard temp;
> create temp table a as select to_timestamp(generate_series(1, 7000)) i;
> analyze a;
> set work_mem to '3GB';
> explain select distinct a1.i - a2.i from a a1, a a2;
> 
> I would appreciate if someone could have a look at the patch attached,
> which makes executor skip initializing hash tables when doing explain only.

Makes sense. Committed, thanks for the patch!

- Heikki

  3. Re: don't allocate HashAgg hash tables when running explain only

    Greg Stark <stark@mit.edu> — 2020-11-19T05:20:15Z

    On Wed, 18 Nov 2020 at 05:40, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>
> On 13/11/2020 18:10, Alexey Bashtanov wrote:
>> > I would appreciate if someone could have a look at the patch attached,
> > which makes executor skip initializing hash tables when doing explain only.
>
> Makes sense. Committed, thanks for the patch!


Egads. That seems like a backpatchable bug fix to me. Have we been
doing this all along?!

-- 
greg

  4. Re: don't allocate HashAgg hash tables when running explain only

    Heikki Linnakangas <hlinnaka@iki.fi> — 2020-11-19T06:47:51Z

    On 19/11/2020 07:20, Greg Stark wrote:
> On Wed, 18 Nov 2020 at 05:40, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>>
>> On 13/11/2020 18:10, Alexey Bashtanov wrote:
>>>> I would appreciate if someone could have a look at the patch attached,
>>> which makes executor skip initializing hash tables when doing explain only.
>>
>> Makes sense. Committed, thanks for the patch!
> 
> Egads. That seems like a backpatchable bug fix to me. Have we been
> doing this all along?!

Yeah, I believe it's always been like that. Yeah, arguably it should be 
backpatched. I felt conservative and didn't backpatch, but feel free to 
do it if you think it should be.

- Heikki

  5. Re: don't allocate HashAgg hash tables when running explain only

    Michael Paquier <michael@paquier.xyz> — 2020-11-20T06:31:22Z

    On Thu, Nov 19, 2020 at 08:47:51AM +0200, Heikki Linnakangas wrote:
> Yeah, I believe it's always been like that. Yeah, arguably it should be
> backpatched. I felt conservative and didn't backpatch, but feel free to do
> it if you think it should be.

+1 for a backpatch.  The difference in runtime for EXPLAIN in this case
is quite something.
--
Michael

  6. Re: don't allocate HashAgg hash tables when running explain only

    Heikki Linnakangas <hlinnaka@iki.fi> — 2020-11-20T13:30:53Z

    On 20/11/2020 08:31, Michael Paquier wrote:
> On Thu, Nov 19, 2020 at 08:47:51AM +0200, Heikki Linnakangas wrote:
>> Yeah, I believe it's always been like that. Yeah, arguably it should be
>> backpatched. I felt conservative and didn't backpatch, but feel free to do
>> it if you think it should be.
> 
> +1 for a backpatch.  The difference in runtime for EXPLAIN in this case
> is quite something.

That's two votes for backpatching. Ok, I'll go do it.

- Heikki