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@xxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. + -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
