Over the years I've wrote many scripts and queries to track the database status. Recently I've had to convince a client who thought it was a good idea to create indexes for every column on every table that it is really a bad idea. To do so, I wrote useless_indexes2.sql, which shows every index that has never been scanned. They still didn't believe me, so I wrote wasted_index_space.sql. That shows how much space is wasted by all the unused indexes.
I'd like to share those queries with the community, as I know there must be others out there with the same problem.
/* useless_indexes.sql */
SELECT
idstat.schemaname AS schema,
idstat.relname AS table_name,
indexrelname AS index_name,
idstat.idx_scan AS times_used,
pg_size_pretty(pg_relation_size(quote_ident(idstat.schemaname) || '.' || quote_ident(idstat.relname))) AS table_size,
pg_size_pretty(pg_relation_size(quote_ident(idstat.schemaname) || '.' || quote_ident(indexrelname))) AS index_size,
n_tup_upd + n_tup_ins + n_tup_del as num_writes,
indexdef AS definition
FROM pg_stat_user_indexes AS idstat
JOIN pg_indexes ON indexrelname = indexname
JOIN pg_stat_user_tables AS tabstat ON idstat.relname = tabstat.relname
WHERE idstat.idx_scan < 200
AND indexdef !~* 'unique'
ORDER BY idstat.schemaname,
idstat.relname,
indexrelname;
/*wasted_index_space.sql
Requires PostgreSQL 8.4 or greater */
WITH s AS(
SELECT SUM(pg_relation_size(quote_ident(n.nspname) || '.' || quote_ident(c.relname))::bigint) AS table_size,
pg_size_pretty(SUM(pg_relation_size(quote_ident(n.nspname) || '.' || quote_ident(c.relname))::bigint)::bigint) AS table_size_pretty
FROM pg_class c
JOIN pg_namespace n ON (n.oid = c.relnamespace)
WHERE c.relkind = 'r'
AND c.relname NOT LIKE 'pg_%'
AND c.relname NOT LIKE 'sql%'
)
SELECT s.table_size,
s.table_size_pretty,
SUM(pg_relation_size(quote_ident(n.nspname) || '.' || quote_ident(i.indexrelname))::bigint) AS unused_idx_size,
pg_size_pretty(SUM(pg_relation_size(quote_ident(n.nspname) || '.' || quote_ident(i.indexrelname))::bigint)::bigint) AS unused_idx_size_pretty,
pg_database_size(current_database()) as db_size,
pg_size_pretty(pg_database_size(current_database()))as db_size_pretty,
pg_size_pretty(pg_database_size(current_database()) - SUM(pg_relation_size(quote_ident(n.nspname) || '.' || quote_ident(i.indexrelname))::bigint)::bigint) as db_minus_wasted_space
FROM s, pg_stat_all_indexes i
JOIN pg_class c ON (c.oid = i.relid)
JOIN pg_namespace n ON (n.oid = c.relnamespace)
JOIN pg_index idx ON (idx.indexrelid = i.indexrelid )
WHERE i.idx_scan = 0
AND NOT idx.indisprimary
AND NOT idx.indisunique
GROUP BY table_size, table_size_pretty;
Melvin Davidson
