Re: get_actual_variable_range vs idx_scan/idx_tup_fetch

On 10/18/14, 8:58 AM, Bruce Momjian wrote:
> On Fri, Oct 17, 2014 at 11:03:04PM -0400, Tom Lane wrote:
>> Bruce Momjian <bruce(at)momjian(dot)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.
>
> Oh, I had forgotten we did that. It is confusing that there is no way
> via EXPLAIN to see the access, making the method of consulting pg_stat_*
> and using EXPLAIN unreliable. Should we document this somewhere?

I think we should. The common (mis)conception is that pg_stats shows *user-driven* access, not access because of stuff the system is doing.

This is actually a huge problem for anyone who's trying to figure out how useful indexes are; they see usage and thing they have queries that are using the index when in reality they don't.
--
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com