Re: memory explosion on planning complex query

On Wed, Nov 26, 2014 at 7:24 PM, Andrew Dunstan <andrew@dunslane.net> wrote:
> On 11/26/2014 05:00 PM, Andrew Dunstan wrote:
>> Attached is some anonymized DDL for a fairly complex schema from a
>> PostgreSQL Experts client. Also attached is an explain query that runs
>> against the schema. The client's problem is that in trying to run the
>> explain, Postgres simply runs out of memory. On my untuned 9.3 test rig,
>> (Scientific Linux 6.4 with 24Gb of RAM and 24Gb of swap) vmstat clearly
>> shows the explain chewing up about 7Gb of memory. When it's done the free
>> memory jumps back to where it was. On a similar case on the clients test rig
>> we saw memory use jump lots more.
>>
>> The client's question is whether this is not a bug. It certainly seems
>> like it should be possible to plan a query without chewing up this much
>> memory, or at least to be able to limit the amount of memory that can be
>> grabbed during planning. Going from humming along happily to OOM conditions
>> all through running "explain <somequery>" is not very friendly.
>>
>
> Further data point - thanks to Andrew Gierth (a.k.a. RhodiumToad) for
> pointing this out. The query itself grabs about 600Mb to 700Mb to run,
> whereas the EXPLAIN takes vastly more - on my system 10 times more. Surely
> that's not supposed to happen?

Hmm.  So you can run the query but you can't EXPLAIN it?

That sounds like it could well be a bug, but I'm thinking you might
have to instrument palloc() to find out where all of that space is
being allocated to figure out why it's happening - or maybe connect
gdb to the server while the EXPLAIN is chewing up memory and pull some
backtraces to figure out what section of code it's stuck in.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company