| From: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Justin Pryzby <pryzby(at)telsasoft(dot)com>, Gunther <raj(at)gusw(dot)net>, pgsql-performance(at)lists(dot)postgresql(dot)org, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> |
| Subject: | Re: Out of Memory errors are frustrating as heck! |
| Date: | 2019-04-20 20:36:50 |
| Message-ID: | 20190420203650.7kiausfmiv3hixg6@development |
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| Lists: | pgsql-performance |
On Sat, Apr 20, 2019 at 04:26:34PM -0400, Tom Lane wrote:
>Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
>> Considering how rare this issue likely is, we need to be looking for a
>> solution that does not break the common case.
>
>Agreed. What I think we need to focus on next is why the code keeps
>increasing the number of batches. It seems like there must be an undue
>amount of data all falling into the same bucket ... but if it were simply
>a matter of a lot of duplicate hash keys, the growEnabled shutoff
>heuristic ought to trigger.
>
I think it's really a matter of underestimate, which convinces the planner
to hash the larger table. In this case, the table is 42GB, so it's
possible it actually works as expected. With work_mem = 4MB I've seen 32k
batches, and that's not that far off, I'd day. Maybe there are more common
values, but it does not seem like a very contrived data set.
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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