debug_discard_caches

debug_discard_caches — Aggressively flush system caches for debugging purposes. Observed in PG14–19 Beta 3; its last measured boot default is 0 in PG19 Beta 3, with superuser context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.

Note

Fact — official short description: “Aggressively flush system caches for debugging purposes.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valuesuperuser: Settable at runtime by a superuser
Unit , Value—: Raw unit
Range , Value0 – 0: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValueDeveloper Options: Upstream classification
Latest boot value , Value0: 0

Lifecycle

Fact	Value
First observed	PG14
Present in	PG14–19 Beta 3
Removed in	No
Introduction commit	`6201fa3c166f` — Rename debug_invalidate_system_caches_always to debug_discard_caches.
Commit date	2021-07-13
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG14–19 Beta 3	`0`	—	0

How it works

debug_discard_caches invalidates system-catalog cache entries at the earliest opportunity; higher values recurse the behavior. Nonzero values make the server extremely slow to expose cache invalidation bugs.

It is supported only in builds compiled with DISCARD_CACHES_ENABLED, automatically associated with –enable-cassert. Production builds keep zero and reject attempts to change it.

The parameter tests correctness under pathological cache churn. It does not flush shared_buffers, the operating-system page cache, or ordinary query result caches. Its superuser context permits an authorized session change without a server restart.

Tuning advice

Tip

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

Workload	Guidance
OLTP	Do not tune production OLTP with debug_discard_caches. Enable it only for a bounded reproduction with an owner, log/disk budget, rollback condition, and evidence-capture plan; restore the default immediately afterward.
OLAP	Long analytical runs can amplify debug_discard_caches’s debug overhead and artifacts. Prefer standard EXPLAIN and statistics first, and isolate any developer experiment from normal users.
Small nodes	Keep debug_discard_caches at its upstream default. A small host has less spare CPU, disk, connection, and log capacity for developer instrumentation.

Pigsty

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; 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: PG14–19 Beta 3 unmodified; OLAP: PG14–19 Beta 3 unmodified; CRIT: PG14–19 Beta 3 unmodified; TINY: PG14–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Leaving debug_discard_caches enabled after the bounded diagnostic or recovery task.
Running the experiment on the only copy of production data.
Underestimating log, core-file, temporary-file, WAL, CPU, or connection-slot amplification.
Treating a server that merely starts or completes a query as proof that data and behavior are correct.

ignore_system_indexes · shared_buffers · debug_parallel_query · allow_system_table_mods