On 12/09/13 04:55, Giuseppe Broccolo wrote:
Il 11/09/2013 13:16, Mikkel Lauritsen ha scritto:
Hi all,
I have a number of Postgres 9.2.4 databases with the same schema but
with
slightly different contents, running on small servers that are basically
alike (8-16 GB ram).
I think that your answer can be found in your statement "slightly
different contents". Planner choices query execution plans basing on
statistics obtained during ANALYSE operations, including the
autovacuum. In this way, Planner can decide which execution plan is
the most suitable. Different content of values in your table could
correspond to different statistical distribution of values in your
columns and of rows in your tables, bringing to different choices of
the Planner. Execution times can be very different, also by factor
10-100.
There is a parameter (stat_target) which set the "selectivity" of
statistical samples of a table. Maybe, but it's not necessarily true,
you could obtain more comparable execution times for the two execution
plans changing it, probably increasing them.
Giuseppe.
Even identical content could lead to different plans, as the sampling is
done randomly (or at least 'randomly' according to the documentation).
