A statistic target of 400 fir a specific column may make sense but
even then I would recommend monitoring performance to ensure it
doesn't cause problems. As a global setting it's, IMHO, ridiculous.
Even for the smaller data types (except boolean and "char") and array
of 400 will be large enough to be toasted. Planning queries will
involve many more disk I/Os than some of those queries end up taking
themselves. Even for stats which are already cached there are some
algorithms in the planner known to be inefficient for large arrays.
It may make sense for specific skewed columns with indexes on them,
but keep in mind postgres needs to consult the statistics on any
column referenced in a qual even if there are no indexes and for most
data distributions do fine with a target of 10.
I think we all agree the default may need to be raised but until there
is some data we have little basis to recommend anything specific.
I would suggest starting from the basis that "mixed" (with a
conservative memory setting) is the same as "Postgres default".
Perhaps (probably) the defaults should be changed but we shouldn't
have two different tools with different (drastically different!) ideas
for the same situation.
greg
On 13 Nov 2008, at 07:46 PM, Josh Berkus <josh@agliodbs.com> wrote:
> Gregory Stark wrote:
>> Josh Berkus <josh@agliodbs.com> writes:
>>> DW:
>>> default_statistics_target = 400
>>> Mixed:
>>> default_statistics_target = 100
>> You, my friend, are certifiably insane.
>
> Hmmm? Why? I've used those settings in the field, fairly
> frequently. I was actually wondering if we should raise the default
> for web as well, but decided to let it alone.
>
> Actually, I think a DW should begin at 400; often it needs to go up
> to 1000, but I don't think a script should do that.
>
> --Josh
>