Postgres uses a visibility bitmap [1] to avoid hitting the heap merely to determine visibility of stable data. [1] https://www.postgresql.org/docs/9.6/static/i

Postgres uses a visibility bitmap [1] to avoid hitting the heap merely to determine visibility of stable data. [1] https://www.postgresql.org/docs/9.6/static/indexes-index-only-scans.html https://www.postgresql.org/docs/9.6/static/indexes-index-onl...