don't allocate HashAgg hash tables when running explain only - Mailing list pgsql-hackers

From Alexey Bashtanov
Subject don't allocate HashAgg hash tables when running explain only
Date
Msg-id 36823f65-050d-ae24-aa4d-a37726998240@imap.cc
Whole thread Raw
Responses Re: don't allocate HashAgg hash tables when running explain only  (Heikki Linnakangas <hlinnaka@iki.fi>)
List pgsql-hackers
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

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