Am 27.09.2012, 02:04 Uhr, schrieb Jeff Janes <jeff.janes@xxxxxxxxx>:
On Wed, Sep 26, 2012 at 1:21 PM, hubert depesz lubaczewski
<depesz@xxxxxxxxxx> wrote:
On Wed, Sep 26, 2012 at 02:38:09PM -0400, Robert Sosinski wrote:
The first query shows a cost of 190,169.55 and runs in 199,806.951 ms.
When I disable nested loop, I get a cost of 2,535,992.34 which runs in
only 133,447.790 ms. We have run queries on our database with a cost
of 200K cost before and they ran less then a few seconds, which makes
me wonder if the first query plan is inaccurate. The other issue is
understanding why a query plan with a much higher cost is taking less
time to run.
Are you under impression that cost should be somehow related to actual
time?
I am certainly under that impression. If the estimated cost has
nothing to do with run time, then what is it that the cost-based
optimizer is trying to optimize?
See http://www.postgresql.org/docs/9.2/static/runtime-config-query.html
section "18.7.2. Planner Cost Constants".
-Matthias
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