On Fri, Apr 23, 2010 at 6:53 PM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote: > Robert Haas <robertmhaas@xxxxxxxxx> 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 -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
