On Thu, Jul 9, 2015 at 09:45:25AM -0700, Jeff Janes wrote:
>
> On Thu, Jul 9, 2015 at 5:20 AM, Nicolas Paris <niparisco@gmail.com> wrote:
>
> Hello,
>
> I wonder how understanding pg_stat_all_indexes working
>
> When I run an explain, some index are not used, but
> pg_stat_all_indexes.idx_scan is incremented for those indexes.
>
>
> When the planner considers using a merge join on a indexed column, it uses an
> index to check the endpoints of the column (the min and the max) to make sure
> it has the latest values to get the most accurate estimate. This causes the
> usage counts to get incremented. Even when it doesn't end up using the merge
> join.
And it will be documented in 9.5:
commit 7e9ed623d9988fcb1497a2a8ca7f676a5bfa136f
Author: Bruce Momjian <bruce@momjian.us>
Date: Thu Mar 19 22:38:12 2015 -0400
docs: mention the optimizer can increase the index usage count
Report by Marko Tiikkaja
+ The optimizer also accesses indexes to check for supplied constants
+ whose values are outside the recorded range of the optimizer statistics
+ because the optimizer statistics might be stale.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ Everyone has their own god. +
