# enable_distinct_reordering

> enable_distinct_reordering — Enables reordering of DISTINCT keys. Observed in PG18–19 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 DISTINCT 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 | PG18 |
| Present in | PG18–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`a8ccf4e93a7e`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=a8ccf4e93a7eeaae66007bbf78cf9183ceb1b371) — Reordering DISTINCT keys to match input path's pathkeys |
| Commit date | 2024-11-26 |
| Discussion | [thread 1](https://postgr.es/m/CAMbWs48dR26cCcX0f=8bja2JKQPcU64136kHk=xekHT9xschiQ@mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG18–19 Beta 3 | `on` | — | on |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

enable_distinct_reordering controls whether the planner may choose reordering DISTINCT keys to match useful input pathkeys and avoid or reduce sorting.

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 PG18 default on. For a suspected regression, compare session-local plans and verify whether reordered DISTINCT keys actually match useful input pathkeys or merely trade one sort for another. |
| OLAP | Large DISTINCT operations can benefit when reordering exploits index, merge, or existing sort order. Measure sort memory, spill volume, planning time, and total execution rather than disabling the optimization globally after one plan. |
| Small nodes | Keep on unless a reproducible plan regression is demonstrated. If sorts spill, correct work_mem and plan inputs before treating this planner switch as a permanent hint. |
{.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: 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 {#common-pitfalls}

- Treating enable_distinct_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_presorted_aggregate`](/parameters/enable-presorted-aggregate/) · [`work_mem`](/parameters/work-mem/)

## References {#references}

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