Re: [PERFORM] Querying with multicolumn index

Hi,

On 12/10/2016 12:51 AM, Tom Lane wrote:
> Eric Jiang <eric@doublemap.com> writes:
>> I have a query that I *think* should use a multicolumn index, but
>> sometimes isn't, resulting in slow queries.
>
> I tried to duplicate this behavior, without success.  Are you running
> with nondefault planner parameters?
>

My guess is this is a case of LIMIT the matching rows are uniformly
distributed in the input data. The planner likely concludes that for a
driver with a lot of data we'll find the first row using ix_updates_time
very quickly, and that it will be cheaper than inspecting the larger
multi-column index. But imagine a driver with a lots of data long time
ago. That breaks the LIMIT fairly quickly.

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services