Comment by madelfio

I think the ratio expressions are slightly off. Using the queries from the article can result in negative ratios, which shouldn't be possible if we're measuring the percent of accesses that hit the buffer. Each access is either a cache hit or a read, not both. So instead of subtracting in the numerator, they should be added in the denominator:

  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 ratio
  FROM 
    pg_statio_user_tables;