Finding Unused Indexes

2018-02-04 By Ruohang Feng In PGSQL 373 words 2 min

Ruohang FengPigsty founder, PostgreSQL mage, database guru, cloud-exit evangelist

Author: Vonng

Indexes are useful, but they’re not free. Unused indexes are a waste. Use the following SQL to identify unused indexes:

First, exclude indexes used to implement constraints (can’t be dropped)
Expression indexes (containing field 0 in pg_index.indkey)
Then find indexes with zero index scans (you can also use a more lenient condition, such as fewer than 1000 scans)

Finding Unused Indexes

View name: monitor.v_bloat_indexes
Calculation time: 1 second, suitable for daily/manual checks, not suitable for frequent polling
Verified versions: 9.3 ~ 10
Function: Shows current database index bloat situation

Works well on versions 9.3 and 10.4. View definition:

-- CREATE SCHEMA IF NOT EXISTS monitor;
-- DROP VIEW IF EXISTS monitor.pg_stat_dummy_indexes;

CREATE OR REPLACE VIEW monitor.pg_stat_dummy_indexes AS
SELECT s.schemaname,
       s.relname AS tablename,
       s.indexrelname AS indexname,
       pg_relation_size(s.indexrelid) AS index_size
FROM pg_catalog.pg_stat_user_indexes s
   JOIN pg_catalog.pg_index i ON s.indexrelid = i.indexrelid
WHERE s.idx_scan = 0      -- has never been scanned
  AND 0 <>ALL (i.indkey)  -- no index column is an expression
  AND NOT EXISTS          -- does not enforce a constraint
         (SELECT 1 FROM pg_catalog.pg_constraint c
          WHERE c.conindid = s.indexrelid)
ORDER BY pg_relation_size(s.indexrelid) DESC;

COMMENT ON VIEW monitor.pg_stat_dummy_indexes IS 'monitor unused indexes'

-- Human-readable manual query
SELECT s.schemaname,
       s.relname AS tablename,
       s.indexrelname AS indexname,
       pg_size_pretty(pg_relation_size(s.indexrelid)) AS index_size
FROM pg_catalog.pg_stat_user_indexes s
   JOIN pg_catalog.pg_index i ON s.indexrelid = i.indexrelid
WHERE s.idx_scan = 0      -- has never been scanned
  AND 0 <>ALL (i.indkey)  -- no index column is an expression
  AND NOT EXISTS          -- does not enforce a constraint
         (SELECT 1 FROM pg_catalog.pg_constraint c
          WHERE c.conindid = s.indexrelid)
ORDER BY pg_relation_size(s.indexrelid) DESC;

Batch Generate Index Drop Commands

SELECT 'DROP INDEX CONCURRENTLY IF EXISTS "' 
	|| s.schemaname || '"."' || s.indexrelname || '";'
FROM pg_catalog.pg_stat_user_indexes s
   JOIN pg_catalog.pg_index i ON s.indexrelid = i.indexrelid
WHERE s.idx_scan = 0      -- has never been scanned
  AND 0 <>ALL (i.indkey)  -- no index column is an expression
  AND NOT EXISTS          -- does not enforce a constraint
         (SELECT 1 FROM pg_catalog.pg_constraint c
          WHERE c.conindid = s.indexrelid)
ORDER BY pg_relation_size(s.indexrelid) DESC;

Finding Duplicate Indexes

Check if there are indexes working on the same columns of the same table, but be careful with partial indexes.

SELECT
  indrelid :: regclass              AS table_name,
  array_agg(indexrelid :: regclass) AS indexes
FROM pg_index
GROUP BY
  indrelid, indkey
HAVING COUNT(*) > 1;