Re: Optimization idea

On Fri, Apr 23, 2010 at 6:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> Hmm.  We currently have a heuristic that we don't record a value as an
>> MCV unless it's more frequent than the average frequency.  When the
>> number of MCVs is substantially smaller than the number of distinct
>> values in the table this is probably a good heuristic, since it
>> prevents us from bothering with the recording of some values that are
>> probably only marginally more interesting than other values we don't
>> have space to record.  But if ndistinct is less than the stats target
>> we could in theory record every value we find in the MCVs table and
>> leave the histogram empty.
>
> Which, in fact, is exactly what we do.  Cf analyze.c lines 2414ff
> (as of CVS HEAD).  The heuristic you mention only gets applied after
> we determine that a complete MCV list won't fit.

Oh, hrmm.  I guess I need to go try to understand this example again, then.

...Robert