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Re: Trying to track down weird query stalls

From: dan(at)sidhe(dot)org
To: "Scott Marlowe" <scott(dot)marlowe(at)gmail(dot)com>
Cc: dan(at)sidhe(dot)org,"Robert Haas" <robertmhaas(at)gmail(dot)com>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Trying to track down weird query stalls
Date: 2009-03-30 20:02:28
Message-ID: 56164.199.172.169.17.1238443348.squirrel@localhost (view raw or flat)
Thread:
Lists: pgsql-performance
> On Mon, Mar 30, 2009 at 1:42 PM,  <dan(at)sidhe(dot)org> wrote:
>>> On Mon, Mar 30, 2009 at 12:42 PM,  <dan(at)sidhe(dot)org> wrote:
>>>> Arguably in this case the actual query should run faster than the
>>>> EXPLAIN
>>>> ANALYZE version, since the cache is hot. (Though that'd only likely
>>>> shave
>>>> a few dozen ms off the runtime)
>>>
>>> Joining a lot of tables together?  Could be GEQO kicking in.
>>
>> Only if I get different query plans for the query depending on whether
>> it's being EXPLAIN ANALYZEd or not. That seems unlikely...
>
> Yes, you can.  In fact you often will.  Not because it's being
> explained or not, just because that's how GEQO works.

Ouch. I did *not* know that was possible -- I assumed that the plan was
deterministic and independent of explain analyze. The query has seven
tables (one of them a temp table) and my geqo_threshold is set to 12. If
I'm reading the docs right GEQO shouldn't kick in.

-Dan

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