re: performance decrease after reboot

pgsql performance gurus,

I sent the following message earlier this week.  I have continued
attempting to find something on the net that would explain this
strange change of query plans, but nothing seems to apply.

Are there any thoughts, such as possibly tweaking the database
somehow to see if I can get this to repeat consistently?

Please let me know if any of you have any pointers as to
the cause of the different query plans.

Thank you very much in advance for any pointers you can provide.

JohnM

On Tue, 19 Jul 2005, John Mendenhall wrote:

> I tuned a query last week to obtain acceptable performance.
> Here is my recorded explain analyze results:
>
> LOG:  duration: 826.505 ms  statement: explain analyze
> [cut for brevity]
>
> I rebooted the database machine later that night.
> Now, when I run the same query, I get the following
> results:
>
> LOG:  duration: 6931.701 ms  statement: explain analyze
> [cut for brevity]

I just ran my query again, no changes from yesterday
and it is back to normal:

LOG:  duration: 795.839 ms  statement: explain analyze

What could have been the problem?

The major differences in the query plan are as follows:

(1) The one that runs faster uses a Hash Join at the
very top of the query plan.  It does a Hash Cond on
the country and code fields.

(2) The one that runs slower uses a Materialize with
the subplan, with no Hash items.  The Materialize does
Seq Scan of the countries table, and above it, a Join
Filter is run.

(3) The partners_pkey index on the partners table is
in a different place in the query.

Does anyone know what would cause the query plan to be
different like this, for the same server, same query?
I run vacuum analyze every night.  Is this perhaps the
problem?

What setting do I need to tweak to make sure the faster
plan is always found?

Thanks for any pointers in this dilemma.

JohnM

--
John Mendenhall
john@surfutopia.net
surf utopia
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