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;