Hi,
On 12/10/2016 12:51 AM, Tom Lane wrote:
Eric Jiang <eric@xxxxxxxxxxxxx> 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
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