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Short answer

PostgreSQL uses MVCC (multiversion concurrency control): an UPDATE or DELETE leaves the old row version in place so other transactions can still see it. VACUUM cleans them. [S1][S2]

What VACUUM does

• Recovers or reuses space from updated or deleted rows. [S1]
• Updates planner statistics and the visibility map (which speeds up index-only scans). [S1]
• To protect against loss of very old data due to transaction ID wraparound or multixact ID wraparound. (S1)

If you have a table whose entire contents are deleted on a periodic basis, consider doing it with TRUNCATE rather than using DELETE followed by VACUUM. (S1)

Claims

• id: C1 claim: VACUUM recovers or reuses disk space from updated or deleted rows. source: S1 evidence: "To recover or reuse disk space occupied by updated or deleted rows."
• id: C2 claim: VACUUM updates the visibility map for index-only scans. source: S1 evidence: "To update the visibility map, which speeds up index-only scans."
• id: C3 claim: VACUUM protects against transaction ID wraparound data loss. source: S1 evidence: "To protect against loss of very old data due to transaction ID wraparound"
• id: C4 claim: Under MVCC reading never blocks writing. source: S2 evidence: "reading never blocks writing and writing never blocks"

Sources

• [S1] PostgreSQL: Documentation: 18: 24.1. Routine Vacuuming. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/routine-vacuuming.html . License: PostgreSQL License. Accessed 2026-11-04.
• [S2] PostgreSQL: Documentation: 18: 13.1. Introduction. PostgreSQL Global Development Group. https://www.postgresql.org/docs/current/mvcc-intro.html . License: PostgreSQL License. Accessed 2026-11-04.

License: this note is CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Summary by iinventedlight (AI-assisted).

@misc{iil_note_ewke3cyc,
  author = {{iinventedlight}},
  title = {{PostgreSQL VACUUM and MVCC: why dead rows pile up}},
  howpublished = {iinventedlight Library},
  year = {2026},
  date = {2026-11-04},
  url = {https://iinventedlight.com/library/notes/postgres-vacuum-mvcc/},
  note = {Permanent ID iil:note:ewke3cyc. License: CC-BY-4.0.},
}

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