Folks,
Early on in the default_stats thread, I made a proposal that got dropped
without discussion. I'd like to revisit it, because I still think it's a
good idea.
The Issue: The low default_stats_target of 10 is not sufficient for many
complex queries involving multi-column correlation or oddly distributed data.
Yet modestly increasing the stats target for *all* columns, as demonstrated,
substantially increases the time required for Analyze, without gain on most
queries.
If only there were a way to automatically increas the default stats on only
"important" columns, and not on other columns! Yet if we burden the DBA with
flagging important colummns all over the database, we haven't saved him/her
any work.
Ah, but there is a way! Most "important" columns are already indicated ...
because they are indexed. If we implemented a system where indexed columns
would have a significantly higher stats_target than non-indexed columns, this
might improve our default behavior without overburdening Analyze.
Proposal: That we consider:
-- adding a new GUC default_stats_indexed
-- that this GUC be set initially to 100 if stats_target is 10
-- that the system be adjusted to that indexed columns take their
stats_target from default_stats_indexed and not default_stats_target
-- that expressional indexes be ignored for this purpose, as implementation
would be too complex, and they have their own stats anyway
If this proposal is worth considering, I will spend some time building up a
test case to demonstrate the cost and utility of the plan. With Neil's help,
of course!
--
-Josh Berkus
Aglio Database Solutions
San Francisco
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