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_visibilityextension. 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.