# subtransaction_buffers

> subtransaction_buffers — Sets the size of the dedicated buffer pool used for the subtransaction cache. Observed in PG17–19 Beta 3; its last measured boot default is 0 B (0 × 8kB) in PG19 Beta 3, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Sets the size of the dedicated buffer pool used for the subtransaction cache.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `postmaster` | Requires a server restart |
| Unit | `8kB` | Raw unit |
| Range | `0` – `131072` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Resource Usage / Memory | Upstream classification |
| Latest boot value | `0` | 0 B (0 × 8kB) |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG17 |
| Present in | PG17–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`53c2a97a9266`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=53c2a97a92665be6bd7d70bd62ae6158fe4db96e) — Improve performance of subsystems on top of SLRU |
| Commit date | 2024-02-28 |
| Discussion | [thread 1](https://postgr.es/m/2BEC2B3F-9B61-4C1D-9FB5-5FAB0F05EF86@yandex-team.ru) · [thread 2](https://postgr.es/m/CAFiTN-vzDvNz=ExGXz6gdyjtzGixKSqs0mKHMmaQ8sOSEFZ33A@mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG17–19 Beta 3 | `0` | `8kB` | 0 B (0 × 8kB) |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

subtransaction_buffers allocates a dedicated shared buffer pool for pg_subtrans, which caches subtransaction parent mappings. This is real startup shared memory, not a planner estimate.

Zero is an automatic-sizing request: PostgreSQL derives shared_buffers/512 and clamps it between 16 and 1024 blocks. The unit is BLCKSZ blocks, normally 8kB.

A larger cache can reduce SLRU read/write churn for unusually heavy use of the underlying feature, but it permanently consumes shared memory and does not increase the feature's logical capacity. Its postmaster context fixes the value at server start; changing it requires a restart.

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Leave subtransaction_buffers at automatic or upstream sizing unless SLRU-specific I/O and contention prove this cache is undersized. A larger number consumes shared memory for the entire server lifetime. |
| OLAP | Analytical workload labels alone do not justify changing subtransaction_buffers; tune only when the underlying transaction-state facility, not table scans, is the measured bottleneck. |
| Small nodes | Keep subtransaction_buffers at its default on a small host. Moving scarce shared memory into an internal cache without direct evidence can reduce room for more valuable caches and processes. |
{.full-width}

## Pigsty {#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 | — | — |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG17–19 Beta 3 unmodified; OLAP: PG17–19 Beta 3 unmodified; CRIT: PG17–19 Beta 3 unmodified; TINY: PG17–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Changing subtransaction_buffers 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.

## Related parameters {#related-parameters}

[`transaction_buffers`](/parameters/transaction-buffers/) · [`shared_buffers`](/parameters/shared-buffers/) · [`max_locks_per_transaction`](/parameters/max-locks-per-transaction/)

## References {#references}

- [PostgreSQL 19 Beta 3 — subtransaction_buffers](https://www.postgresql.org/docs/19/runtime-config-resource.html#GUC-SUBTRANSACTION-BUFFERS)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)