# allow_system_table_mods

> allow_system_table_mods — Allows modifications of the structure of system tables. Observed in PG9.0–19 Beta 3; its last measured boot default is off 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:** “Allows modifications of the structure of system tables.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `bool` | Upstream pg_settings type |
| Context | `superuser` | Settable at runtime by a superuser |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Developer Options | Upstream classification |
| Latest boot value | `off` | off |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.0 (research boundary) |
| Present in | PG9.0–19 Beta 3 |
| Removed in | No |
| Introduction commit | Not asserted: predates the PG9.0 research boundary |
| Commit date | — |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.0–19 Beta 3 | `off` | — | off |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

allow_system_table_mods removes protections against changing system-catalog structure and permits other normally forbidden catalog actions. Superuser status alone does not normally grant these operations.

Catalog definitions are coupled to PostgreSQL code, cache descriptors, bootstrap data, WAL, and upgrade assumptions. A superficially valid ALTER can create irreversible corruption or data loss.

The parameter exists for PostgreSQL development, bootstrap, and tightly controlled tooling. It does not provide a supported extension mechanism; extensions must use published catalog and hook interfaces. Its superuser context permits an authorized session change without a server restart.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Never use allow_system_table_mods for performance tuning. Restrict it to PostgreSQL development or a documented, disposable maintenance procedure with a verified backup and exact rollback path. |
| OLAP | Workload type does not reduce the catalog-corruption risk from allow_system_table_mods; keep it off on analytical systems too. |
| Small nodes | Keep allow_system_table_mods=off. Lack of a staging system is not permission to experiment on the only copy of the data. |
{.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: PG9.0–19 Beta 3 unmodified; OLAP: PG9.0–19 Beta 3 unmodified; CRIT: PG9.0–19 Beta 3 unmodified; TINY: PG9.0–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Leaving allow_system_table_mods 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.

## Related parameters {#related-parameters}

[`allow_in_place_tablespaces`](/parameters/allow-in-place-tablespaces/) · [`ignore_system_indexes`](/parameters/ignore-system-indexes/) · [`zero_damaged_pages`](/parameters/zero-damaged-pages/) · [`wal_consistency_checking`](/parameters/wal-consistency-checking/)

## References {#references}

- [PostgreSQL 19 Beta 3 — allow_system_table_mods](https://www.postgresql.org/docs/19/runtime-config-developer.html#GUC-ALLOW-SYSTEM-TABLE-MODS)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)