Calculate the database cache hit ratio:

14 Nov 2015

select sum(blks_hit) * 100 / sum(blks_hit + blks_read) as hit_ratio
  from pg_stat_database;

A newer way (11 Aug 2020):

select sum(heap_blks_read)                                             as heap_read,
       sum(heap_blks_hit)                                              as heap_hit,
       sum(heap_blks_hit) / (sum(heap_blks_hit) + sum(heap_blks_read)) as hit_ratio
  from pg_statio_user_tables;