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?
cheers
andrew
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