> Is there still a good reason to have the histogram stats so low? Should
> the default be changed to more like 100 at this point?

Low overhead.  This is actually a TODO for me for 8.1.   I need to find some 
test cases to set a differential level of histogram access for indexed 
fields, so like 10 for most fields but 100/150/200 for indexed fields.

However, I got stalled on finding test cases and then ran out of time.

> Also, how extensively does the planner use n_distinct, null_frac,
> reltuples and the histogram to see what the odds are of finding a unique
> value or a low number of values? I've seen cases where it seems the
> planer doesn't think it'll be getting a unique value or a small set of
> values even though stats indicates that it should be.
> One final question... would there be interest in a process that would
> dynamically update the histogram settings for tables based on how
> distinct/unique each field was?

Well, the process by which the analyzer decides that a field is unique could 
probably use some troubleshooting.   And we always, always could use 
suggestions/tests/help with the query planner.


Josh Berkus
Aglio Database Solutions
San Francisco

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