# plan_cache_mode

> plan_cache_mode — Controls the planner's selection of custom or generic plan. Observed in PG12–19 Beta 3; its last measured boot default is auto in PG19 Beta 3, with user context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Controls the planner's selection of custom or generic plan.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `enum` | Upstream pg_settings type |
| Context | `user` | Settable by an ordinary user |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | `auto`, `force_generic_plan`, `force_custom_plan` | — for non-enum types |
| Category | Query Tuning / Other Planner Options | Upstream classification |
| Latest boot value | `auto` | auto |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG12 |
| Present in | PG12–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`f7cb2842bf47`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=f7cb2842bf47715133b40e4a503f35dbe60d1b72) — Add plan_cache_mode setting |
| Commit date | 2018-07-16 |
| Discussion | [thread 1](https://www.postgresql.org/message-id/flat/CAFj8pRAGLaiEm8ur5DWEBo7qHRWTk9HxkuUAz00CZZtJj-LkCA%40mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG12–19 Beta 3 | `auto` | — | auto |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Prepared statements can use a custom plan built with current parameter values or a generic plan independent of those values. plan_cache_mode overrides PostgreSQL's normal cost-based choice with auto, force_custom_plan, or force_generic_plan.

The setting is checked when a cached statement is executed, not when PREPARE is issued. A custom plan pays planning cost repeatedly but can adapt to skew; a generic plan saves planning work but can be poor for parameter-sensitive predicates.

PL/pgSQL and protocol-level prepared statements are both affected. Forcing a mode is diagnostic or workload-specific policy, not a cure for bad statistics or missing indexes. Its user context permits session- or transaction-local changes; newly performed or newly planned work sees the value.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Keep plan_cache_mode at its upstream default unless representative plans show a repeatable, workload-wide problem. Test a local override first and include planning latency as well as execution latency. |
| OLAP | Analytical SQL can make plan_cache_mode more visible because joins, cursors, or recursion are larger. Benchmark the full statement family and inspect estimates rather than copying a single successful value. |
| Small nodes | On a small host, avoid increasing planning search or memory pressure through plan_cache_mode without a measured benefit. Prefer query-local structure or a scoped role setting over a cluster-wide override. |
{.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: PG12–19 Beta 3 unmodified; OLAP: PG12–19 Beta 3 unmodified; CRIT: PG12–19 Beta 3 unmodified; TINY: PG12–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Treating plan_cache_mode as an executor resource limit rather than a planning assumption or policy.
- Testing only one parameter set or one data distribution.
- Expecting an already cached plan to be rewritten automatically.
- Using a global override to hide stale statistics or fragile SQL structure.

## Related parameters {#related-parameters}

[`default_statistics_target`](/parameters/default-statistics-target/) · [`cursor_tuple_fraction`](/parameters/cursor-tuple-fraction/) · [`join_collapse_limit`](/parameters/join-collapse-limit/) · [`compute_query_id`](/parameters/compute-query-id/) · [`jit_above_cost`](/parameters/jit-above-cost/)

## References {#references}

- [PostgreSQL 19 Beta 3 — plan_cache_mode](https://www.postgresql.org/docs/19/runtime-config-query.html#GUC-PLAN-CACHE-MODE)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)