# parallel_leader_participation

> parallel_leader_participation — Controls whether Gather and Gather Merge also run subplans. Observed in PG11–19 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:** “Controls whether Gather and Gather Merge also run subplans.”

## 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 | Resource Usage / Worker Processes | Upstream classification |
| Latest boot value | `on` | on |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG11 |
| Present in | PG11–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`e5253fdc4f5f`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=e5253fdc4f5fe2f38aec47e08c6aee93f934183d) — Add parallel_leader_participation GUC. |
| Commit date | 2017-11-15 |
| Discussion | [thread 1](https://postgr.es/m/CAEepm=2U++Lp3bNTv2Bv_kkr5NE2pOyHhxU=G0YTa4ZhSYhHiw@mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG11–19 Beta 3 | `on` | — | on |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

parallel_leader_participation controls whether the process above Gather or Gather Merge also runs the parallel subplan while coordinating workers. With it off, the leader focuses on reading worker output.

Leader participation can add useful CPU when result production is expensive, but a leader busy executing the subplan may be slower to consume a large worker result stream.

The value is a planning/execution policy for eligible parallel plans, not an extra worker slot. Its effect depends on tuple volume, Gather versus Gather Merge, worker availability, and where the bottleneck lies. 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 unless a representative Gather plan shows that the leader is too busy executing the subplan to consume worker tuples promptly. Test off only at session scope and compare first-row latency, total latency, and worker blocking. |
| OLAP | For CPU-heavy subplans, leader participation often adds useful execution capacity; for very large result streams, disabling it can let the leader drain workers sooner. Compare both boolean states on the same plan and concurrency level. |
| Small nodes | This boolean does not add or remove worker slots. Leave it on unless measurements show a result-consumption bottleneck; size worker counts separately with max_parallel_workers_per_gather and the shared pools. |
{.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: PG11–19 Beta 3 unmodified; OLAP: PG11–19 Beta 3 unmodified; CRIT: PG11–19 Beta 3 unmodified; TINY: PG11–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Treating the boolean as a degree-of-parallelism or worker-count setting.
- Speaking of a 'larger' value when the only choices are on and off.
- Disabling participation without measuring the extra wait before workers produce the first tuples.
- Enabling participation without checking whether the leader then drains a large worker result stream too slowly.

## Related parameters {#related-parameters}

[`max_parallel_workers_per_gather`](/parameters/max-parallel-workers-per-gather/) · [`max_parallel_workers`](/parameters/max-parallel-workers/) · [`enable_gathermerge`](/parameters/enable-gathermerge/) · [`parallel_tuple_cost`](/parameters/parallel-tuple-cost/) · [`parallel_setup_cost`](/parameters/parallel-setup-cost/)

## References {#references}

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