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Finding Unused Indexes
WITH table_scans as (
SELECT relid,
tables.idx_scan + tables.seq_scan as all_scans,
( tables.n_tup_ins + tables.n_tup_upd + tables.n_tup_del ) as writes,
pg_relation_size(relid) as table_size
FROM pg_stat_user_tables as tables
all_writes as (
SELECT sum(writes) as total_writes
FROM table_scans
indexes as (
SELECT idx_stat.relid, idx_stat.indexrelid,
idx_stat.schemaname, idx_stat.relname as tablename,
idx_stat.indexrelname as indexname,
pg_relation_size(idx_stat.indexrelid) as index_bytes,
indexdef ~* 'USING btree' AS idx_is_btree
FROM pg_stat_user_indexes as idx_stat
JOIN pg_index
USING (indexrelid)
JOIN pg_indexes as indexes
ON idx_stat.schemaname = indexes.schemaname
AND idx_stat.relname = indexes.tablename
AND idx_stat.indexrelname = indexes.indexname
WHERE pg_index.indisunique = FALSE
index_ratios AS (
SELECT schemaname, tablename, indexname,
idx_scan, all_scans,
round(( CASE WHEN all_scans = 0 THEN 0.0::NUMERIC
ELSE idx_scan::NUMERIC/all_scans * 100 END),2) as index_scan_pct,
round((CASE WHEN writes = 0 THEN idx_scan::NUMERIC ELSE idx_scan::NUMERIC/writes END),2)
as scans_per_write,
pg_size_pretty(index_bytes) as index_size,
pg_size_pretty(table_size) as table_size,
idx_is_btree, index_bytes
FROM indexes
JOIN table_scans
USING (relid)
index_groups AS (
SELECT 'Never Used Indexes' as reason, *, 1 as grp
FROM index_ratios
idx_scan = 0
and idx_is_btree
SELECT 'Low Scans, High Writes' as reason, *, 2 as grp
FROM index_ratios
scans_per_write <= 1
and index_scan_pct < 10
and idx_scan > 0
and writes > 100
and idx_is_btree
SELECT 'Seldom Used Large Indexes' as reason, *, 3 as grp
FROM index_ratios
index_scan_pct < 5
and scans_per_write > 1
and idx_scan > 0
and idx_is_btree
and index_bytes > 100000000
SELECT 'High-Write Large Non-Btree' as reason, index_ratios.*, 4 as grp
FROM index_ratios, all_writes
( writes::NUMERIC / ( total_writes + 1 ) ) > 0.02
AND NOT idx_is_btree
AND index_bytes > 100000000
ORDER BY grp, index_bytes DESC )
SELECT reason, schemaname, tablename, indexname,
index_scan_pct, scans_per_write, index_size, table_size
FROM index_groups;

if total_writes is 0 there will ERROR: division by zero
so may use:
all_writes as (
SELECT case when sum(writes) =0 then 1 else sum(writes) end as total_writes
FROM table_scans


jberkus commented May 6, 2014

I fudged the query a different way to avoid div-by-zero

ERROR: cache lookup failed for index 2031405819
********** Error **********

ERROR: cache lookup failed for index 2031405819
SQL state: XX000

I think this comes from indexes on temp tables, if i eliminate indexes that arent in a pg_temp* schema it runs correctly

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