Which tables in your database are most heavily used? Top 10 tables in terms of numbers of rows read:

SELECT relname, COALESCE(idx_tup_fetch,0) + COALESCE(seq_tup_read,0) AS Total
 FROM pg_stat_all_tables
 WHERE COALESCE(idx_tup_fetch,0) + COALESCE(seq_tup_read,0) <> 0
 AND relname NOT LIKE 'pg_%'
 ORDER BY Total desc
 -- LIMIT 10
 ;

Variations to above:
pg_stat_system_tables = system tables
pg_stat_user_tables = user created tables
pg_stat_all_indexes = indexes
pg_stat_system_indexes = system indexes
pg_stat_user_indexes = user created indexes
pg_stat