Re: Allow sorts to use more available memory

On 9/12/2011 12:57 PM, Shaun Thomas wrote:
> On 09/12/2011 12:47 PM, Andy Colson wrote:
>
>> work_mem is not the total a query can use. I believe each step can
>> use that much, and each backend can use it for multiple bits. So if
>> you had two backends, each doing 2 sorts, you'd use 2*2 = 4 * 2GB =
>> 8GB.
> Exactly. Find a big query somewhere in your system. Use EXPLAIN to
> examine it. Chances are, that one query has one or more sorts. Each one
> of those gets its own work_mem. Each sort. The query have four sorts? It
> may use 4*work_mem. On a whim a while back, I doubled our 8MB setting to
> 16MB on a test system. During a load test, the machine ran out of
> memory, swapped out, and finally crashed after the OOM killer went nuts.
>
> Set this value *at your own risk* and only after *significant* testing.
> Having it too high can have rather unexpected consequences. Setting it
> to 1 or 2GB, unless you have VERY few threads, or a TON of memory, is a
> very, very bad idea.
>
Yep, I know.  But in the context of the data warehouse where *I'm the
only user* and I have a query that does, say 4 large sorts like
http://explain.depesz.com/s/BrAO and I have 32GB RAM I'm not worried
about using 8GB or 16GB in the case of work_mem = 4GB.  I realize the
query above only used 1.9GB for the largest sort but I know I have other
queries with 1 or 2 sorts that I've watched go to disk.

Bob