> Even though we all agree default_statistics_target = 10 is too low,
> proposing a 40X increase in the default value requires more evidence
> than this. In particular, the prospect of a 1600-fold increase in
> the typical cost of eqjoinsel() is a mite scary.
I just did some very quick testing of a couple values for
default_statistics_target on a database for work whose contents I am
not at liberty to share. Suffice it to say it is a database with
small tables, but a lot of the queries involve many joins.
ANALYZE with default_statistics_target set to 10 takes 13 s. With
100, 92 s. With 1000, 289 s.
I tried several representative queries and there was no measurable
change in run-time. I compared a couple of the plans and the plans
didn't change either.
So then I said - gee, how much can I reduce this? I looked at two of
the queries in more detail. The lowest default_statistics_target that
produced the same plan for both queries was 5. Reducing the
default_statistics_target all the way down to 1 changed the plans, but
the new plans were just as fast as the old plans.
Given the amount of clamor for a higher value for
default_statistics_target, I'm a little surprised by these results.
It may be that the queries I'm running are not ones for which more
statistics generate better plans, but I think they really are
representative of what we run. Maybe someone could suggest some types
of query that would be likely to helped by better statistics?
A random thought: maybe the reason I'm not seeing any benefit is
because my tables are just too small - most contain at most a few
thousand rows, and some are much smaller. Maybe
default_statistics_target should vary with the table size? Something
like, 0.1% of the rows to a maximum of 100... and then 0.01% of the
rows after that to some higher maximum.... but always a minimum of at
least 10. I'm guessing that people with really big tables are less
likely to mind longer planning times and more likely to benefit from
finding better plans...
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