Looking at PostgreSQL Internals with pg_visibility

Ian Parker introduces us to an extension:

When most people think about PostgreSQL internals, they picture tables, indexes, and perhaps the VACUUM process. Tucked away in the contrib extensions, however, is a tool that exposes what really sits on disk: the pg_visibility extension. By querying it, you see how PostgreSQL tracks visibility and freezing at the page level—information that directly affects vacuum efficiency and index-only scans.

Read on to see how this extension works.