num_os_semaphores

num_os_semaphores — Shows the number of semaphores required for the server. Observed in PG18–19 Beta 3; its last measured boot default is 0 in PG19 Beta 3, with internal context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.

Note

Fact — official short description: “Shows the number of semaphores required for the server.”

Identity

Type , Valueinteger: Upstream pg_settings type
Context , Valueinternal: Internal/preset and not user-settable
Unit , Value—: Raw unit
Range , Value0 – 2147483647: Raw limits in the last observed version
Enum values , Value—: — for non-enum types
Category , ValuePreset Options: Upstream classification
Latest boot value , Value0: 0

Lifecycle

Fact	Value
First observed	PG18
Present in	PG18–19 Beta 3
Removed in	No
Introduction commit	`0dcaea569034` — Introduce num_os_semaphores GUC.
Commit date	2024-07-26
Discussion	thread 1

Default history

Measured PG9.0–19 Beta 3 boot defaults
Versions	Raw `boot_val`	Unit	Human value
PG18–19 Beta 3	`0`	—	0

How it works

Shows the number of semaphores required for the server. It is read-only and cannot be changed with SET or configuration-file edits.

It is computed from the startup process and connection configuration so administrators can plan System V semaphore capacity.

Interpret it with shared_memory_size, shared_memory_size_in_huge_pages, huge_pages. Applications and operations tooling may read num_os_semaphores for capability or environment detection, but cannot SET it; cross-cluster comparisons must account for build, initdb, and current-database differences.

Tuning advice

Tip

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

Workload	Guidance
OLTP	Not tunable. Record num_os_semaphores as a deployment/capability fact; if it is unexpected, fix the binary, initdb, or database-creation process rather than trying SET.
OLAP	Not tunable. Use num_os_semaphores to verify format, build, or resource assumptions on analytical nodes and keep every read/write candidate compatible.
Small nodes	Not tunable. Preserve it in inventory and incident reports; do not custom-build or rebuild merely to change it without a demonstrated compatibility need.

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: PG18–19 Beta 3 unmodified; OLAP: PG18–19 Beta 3 unmodified; CRIT: PG18–19 Beta 3 unmodified; TINY: PG18–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

Common pitfalls

Trying to change read-only num_os_semaphores with SET or a configuration file.
Assuming every cluster, database, or vendor build reports the same value.
Ignoring the value in file-format or feature-compatibility checks.
Treating reported metadata as a performance target instead of an environment fact.

shared_memory_size · shared_memory_size_in_huge_pages · huge_pages · huge_pages_status · max_connections · block_size