PSQL index cache hit ratio

psql_cache_hit_ratio_index.sql

# See Cache Hit Ratio for a given index
SELECT 
  (heap_blks_hit::decimal / (heap_blks_hit + heap_blks_read)) as cache_hit_ratio
FROM 
  pg_statio_user_indexes
WHERE indexrelname = 'some_high_use_index';

# See overall Cache Hit Ratio across all indexes
SELECT 
  sum(idx_blks_hit)/(sum(idx_blks_read) + sum(idx_blks_hit)) as cache_hit_ratio
FROM 
  pg_statio_user_indexes;