PostgreSQL Cache Hit Ratio

The cache hit ratio in PostgreSQL is the number of buffer cache hits divided by the sum of the number of buffer cache hits and the number of disk blocks read.¹ It can be calculated with a query to a pg_statio_ view.

select
  sum(heap_blks_hit) / (sum(heap_blks_hit) + sum(heap_blks_read))
    as cache_hit_ratio
from
  pg_statio_user_tables;

As an example, if your buffer hits total 1,000 and disk blocks read total 5 your cache hit ratio is ≈0.995 or ≈99.5%.