4

I'm using the following query to find unused indexes:

SELECT 
PSUI.indexrelid::regclass AS IndexName 
,PSUI.relid::regclass AS TableName 
FROM pg_stat_user_indexes AS PSUI 
JOIN pg_index AS PI 
ON PSUI.IndexRelid = PI.IndexRelid 
WHERE PSUI.idx_scan = 0 
AND PI.indisunique IS FALSE;

Should I run any stats gathering syntax or anything else before running it? Is the above query OK for such purpose? I mean, then all indexes shown in the SQL output should be just deleted?

It's a 8 year old BD, so resulting rows may be actually left overs and, I guess there should be enough stats so tell wherever and is used or not.

4

Seems like a decent approach. Of course, one should apply some human verification to this before automatically dropping everything that seems unused. For example, it's conceivable that the statistics were recently reset and/or an index is only used for some occasional batch tasks.

| improve this answer | |
  • 1
    Is it possible to know when where statistics reset? – Gonzalo Vasquez May 4 '16 at 13:23
  • 3
    You can check in pg_stat_database.stats_reset. This detects not only intentional resets, but also resets due to crashes or unclean shutdowns. Stats can also be reset on individual objects, but I don't think there is any way to see when/if that has happened. – jjanes May 4 '16 at 15:14
  • 1
    and to do that: select stats_reset from pg_stat_database; – John Bachir Nov 21 '19 at 20:38
2

FWIW here's a query I've been using

SELECT
  relname AS table,
  indexrelname AS index,
  pg_size_pretty(pg_relation_size(i.indexrelid)) AS index_size,
  idx_scan as index_scans
FROM pg_stat_user_indexes ui
JOIN pg_index i ON ui.indexrelid = i.indexrelid
WHERE NOT indisunique AND idx_scan =0 AND pg_relation_size(relid) > 5 * 8192
ORDER BY pg_relation_size(i.indexrelid) / nullif(idx_scan, 0) DESC NULLS FIRST,
pg_relation_size(i.indexrelid) DESC;
| improve this answer | |

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