Re: Querying with multicolumn index

Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> writes:
> On 12/10/2016 12:51 AM, Tom Lane wrote:
>> 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.

The fact that it's slow enough to be a problem is doubtless related to
that effect. But AFAICS, the planner should never prefer that index
for this query, because even with a uniform-density assumption, the
index that really matches the query ought to look better.

regards, tom lane