# enable_group_by_reordering

> enable_group_by_reordering — Enables reordering of GROUP BY keys. Observed in PG17–19 Beta 3; its last measured boot default is on 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:** “Enables reordering of GROUP BY keys.”

## Identity {#identity}

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

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG17 |
| Present in | PG17–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`db0d67db2401`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=db0d67db2401eb6238ccc04c6407a4fd4f985832) — Optimize order of GROUP BY keys |
| Commit date | 2022-03-31 |
| Discussion | [thread 1](https://postgr.es/m/7c79e6a5-8597-74e8-0671-1c39d124c9d6%40sigaev.ru) · [thread 2](https://postgr.es/m/CA%2Bq6zcW_4o2NC0zutLkOJPsFt80megSpX_dVRo6GK9PC-Jx_Ag%40mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG17–19 Beta 3 | `on` | — | on |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

enable_group_by_reordering controls whether the planner may choose reordering GROUP BY keys to match a child's pathkeys and exploit preordered input.

It is consulted while a plan is built. A session-level change is useful for comparing EXPLAIN alternatives, but an already cached plan is not retroactively rewritten; invalidation or replanning is required to observe a different choice.

The switch changes which candidate paths the planner may cost; it does not make the chosen method faster by itself. 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 the default on. If a plan regresses, compare session-local on/off plans and check whether reordered GROUP BY keys exploit a child's existing pathkeys or add planning work without avoiding a sort. |
| OLAP | Large groupings can benefit from index or presorted input order. Evaluate sort and aggregate nodes, spill, planning time, and result ordering requirements before changing the switch for a reporting role. |
| Small nodes | Keep on unless a repeatable regression is isolated. Resolve stale statistics, missing ordering paths, and work_mem pressure before using off as a scoped workaround. |
{.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}

- Treating enable_group_by_reordering as a query hint even though it affects every plan built in its scope.
- Testing an already cached prepared plan and concluding that the setting has no effect.
- Masking stale statistics or cardinality errors by disabling a plan method globally.
- The switch changes which candidate paths the planner may cost; it does not make the chosen method faster by itself.

## Related parameters {#related-parameters}

[`enable_sort`](/parameters/enable-sort/) · [`enable_incremental_sort`](/parameters/enable-incremental-sort/) · [`enable_hashagg`](/parameters/enable-hashagg/) · [`enable_presorted_aggregate`](/parameters/enable-presorted-aggregate/)

## References {#references}

- [PostgreSQL 19 Beta 3 — enable_group_by_reordering](https://www.postgresql.org/docs/19/runtime-config-query.html#GUC-ENABLE-GROUP-BY-REORDERING)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)