# join_collapse_limit

> join_collapse_limit — Sets the FROM-list size beyond which JOIN constructs are not flattened. Observed in PG9.0–19 Beta 3; its last measured boot default is 8 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:** “Sets the FROM-list size beyond which JOIN constructs are not flattened.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `user` | Settable by an ordinary user |
| Unit | — | Raw unit |
| Range | `1` – `2147483647` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Query Tuning / Other Planner Options | Upstream classification |
| Latest boot value | `8` | 8 |
{.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 | `8` | — | 8 |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

join_collapse_limit controls when explicit JOIN constructs, except FULL JOIN, are flattened into a reorderable FROM list. A value of 1 preserves the written explicit join order.

Flattening enlarges the set of join orders the planner can explore, improving opportunities at the cost of planning CPU and memory. Outer-join semantics still constrain legal reorderings.

The exposed item count interacts with from_collapse_limit and geqo_threshold. Using 1 as a manual join-order tool transfers responsibility to SQL authors and is not a general plan-stability guarantee. 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 join_collapse_limit 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 join_collapse_limit 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 join_collapse_limit 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: 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}

- Treating join_collapse_limit 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}

[`from_collapse_limit`](/parameters/from-collapse-limit/) · [`geqo_threshold`](/parameters/geqo-threshold/) · [`geqo`](/parameters/geqo/) · [`enable_hashjoin`](/parameters/enable-hashjoin/) · [`enable_nestloop`](/parameters/enable-nestloop/)

## References {#references}

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