Overview

Cache hits show the ratio of queries hitting PostgreSQL's cache (i.e. shared buffer cache) compared to the total number of queries for all tables and indexes. In general, most databases should be serving 99% of all queries from the cache. When too many queries are served from disk, overall query performance suffers.

If cache hits are below 99%, it often means that too many queries are not using an index or that the most commonly queried data no longer fits within PostgreSQL's cache and the server instance should be upgraded to one with more memory.

Recommendation

When overall table or index cache hit ratios are below 99%, a recommendation is created for your PostgreSQL server. Your slowest queries (including table scans) should be investigated to see if there are any missing indexes that could be added. Queries that load large amounts of data are good candidates for improvement.

Tune your queries, add any missing indexes or upgrade the server's instance size to one with more memory to improve cache hit rates. You may also ignore the recommendation to remove it from your recommendation dashboard.