Re: get_actual_variable_range vs idx_scan/idx_tup_fetch - Mailing list pgsql-hackers

Bruce Momjian <bruce@momjian.us> writes:
> On Fri, Oct 17, 2014 at 06:15:37PM -0400, Tom Lane wrote:
>> Those stats were perfectly valid: what the planner is looking for is
>> accurate minimum and maximum values for the index's leading column, and
>> that's what it got.  You're correct that a narrower index could have given
>> the same results with a smaller disk footprint, but the planner got the
>> results it needed from the index you provided for it to work with.

> Uh, why is the optimizer looking at the index on a,b,c and not just the
> stats on column a, for example?  I am missing something here.

Because it needs up-to-date min/max values in order to avoid being
seriously misled about selectivities of values near the endpoints.
See commit 40608e7f949fb7e4025c0ddd5be01939adc79eec.
        regards, tom lane