old_snapshot_threshold

old_snapshot_threshold — Time before a snapshot is too old to read pages changed after the snapshot was taken. Observed in PG9.6–16; its last measured boot default is -1 min in PG16, with postmaster context. It was removed in PG17.

Note

Fact — official short description: “Time before a snapshot is too old to read pages changed after the snapshot was taken.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valuepostmaster: Requires a server restart
Unit , Valuemin: Raw unit
Range , Value-1 – 86400: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueResource Usage / Asynchronous Behavior: Upstream classification
Latest boot value , Value-1: -1 min

Lifecycle

Fact	Value
First observed	PG9.6
Present in	PG9.6–16
Removed in	PG17
Introduction commit	`848ef42bb8c7` — Add the “snapshot too old” feature
Commit date	2016-04-08
Discussion	—

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG9.6–16	`-1`	`min`	-1 min

How it works

old_snapshot_threshold, available through PG16 and removed in PG17, marked snapshots as too old after a configured time so page pruning could proceed more aggressively. A later read could fail with snapshot-too-old rather than return a historical page image.

It was not a transaction timeout: the transaction could continue until it touched data whose old versions had been removed. The feature required startup-time tracking overhead even before a failure appeared.

It did not replace vacuum discipline or prevent all bloat. Because the feature was removed, migration must not carry the parameter into PG17+ and applications must not depend on its error behavior. Its postmaster context fixes the value at server start; changing it requires a restart.

Tuning advice

Tip

Advice. These are workload-specific starting points and must be validated with measurements.

Workload	Guidance
OLTP	Do not tune old_snapshot_threshold on current PostgreSQL: remove it from upgrade targets and use the current replacement behavior described above. Retain it only when reproducing the historical version.
OLAP	Do not carry old_snapshot_threshold into a modern analytical cluster. Benchmark the supported current mechanisms instead of trying to emulate a removed implementation detail.
Small nodes	Delete old_snapshot_threshold during version migration; an unknown-parameter startup failure is more likely than a benefit. Historical test instances should keep the old upstream default.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG16; this does not assert current Pigsty support for that historical or beta release.

Template	Effective value	Versus upstream boot	Source expression
OLTP	Unmodified	—	—
OLAP	Unmodified	—	—
CRIT	Unmodified	—	—
TINY	Unmodified	—	—

Caution

Advice — pending human review. Fact from the current Pigsty template projection: OLTP: PG9.6–16 unmodified; OLAP: PG9.6–16 unmodified; CRIT: PG9.6–16 unmodified; TINY: PG9.6–16 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Changing old_snapshot_threshold without applying its documented unit and configuration context.
Optimizing an isolated benchmark while ignoring concurrent aggregate resource use.
Assuming a configured value guarantees operating-system or storage behavior.
Failing to retest startup, failover, and workload latency after the change.

vacuum_defer_cleanup_age · autovacuum · hot_standby_feedback · idle_in_transaction_session_timeout