Index I/O

The same basic I/O statistics available for indexes are also available broken down for each index:

pgbench=# SELECT indexrelname,cast(idx_blks_hit as numeric) / (idx_blks_hit + idx_blks_read) AS hit_pct,idx_blks_hit,idx_blks_read FROM pg_statio_user_indexes WHERE (idx_blks_hit + idx_blks_read)>0 ORDER BY hit_pct;     indexrelname          | hit_pct | idx_blks_hit | idx_blks_read 
----------------------+---------+--------------+--------------
pgbench_branches_pkey | 0.33333 |            1 |             2
pgbench_tellers_pkey  | 0.33333 |            1 |             2
pgbench_accounts_pkey | 0.99945 |       405206 |           221

Indexes tend to be better cached than their underlying tables. If they're not, that's another hint that you might not be indexing your data properly.