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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).
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