Re: Updating histogram_bounds after a delete

"Kevin Grittner" <Kevin.Grittner@wicourts.gov> writes:
> Derrick Rice <derrick.rice@gmail.com> wrote:
>> Kevin Grittner <Kevin.Grittner@wicourts.gov wrote:
>>> there is a feature to probe the end of an index's range in
>>> situations where data skew was often causing less than optimal
>>> plans to be chosen.

>> Was this introduced in 9.0 or was it earlier?

> I don't remember when it was added.  I took a stab at searching for
> it, but didn't get it figured out; if nobody who knows off-hand
> jumps in, I'll try again when I have more time.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Branch: master Release: REL9_0_BR [40608e7f9] 2010-01-04 02:44:40 +0000

    When estimating the selectivity of an inequality "column > constant" or
    "column < constant", and the comparison value is in the first or last
    histogram bin or outside the histogram entirely, try to fetch the actual
    column min or max value using an index scan (if there is an index on the
    column).  If successful, replace the lower or upper histogram bound with
    that value before carrying on with the estimate.  This limits the
    estimation error caused by moving min/max values when the comparison
    value is close to the min or max.  Per a complaint from Josh Berkus.

    It is tempting to consider using this mechanism for mergejoinscansel as well,
    but that would inject index fetches into main-line join estimation not just
    endpoint cases.  I'm refraining from that until we can get a better handle
    on the costs of doing this type of lookup.

            regards, tom lane