# from_collapse_limit

> from_collapse_limit — Sets the FROM-list size beyond which subqueries are not collapsed. 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 subqueries are not collapsed.”

## 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}

from_collapse_limit bounds how far the planner merges eligible subqueries into the parent FROM list. Flattening exposes more join orders and predicate movement opportunities but enlarges the search space.

If flattening would produce more FROM items than the limit, the subquery boundary is retained. Smaller values can reduce planning time while hiding a better global join order.

The resulting relation count interacts with join_collapse_limit and geqo_threshold; raising this limit can unexpectedly move the query from exhaustive planning into GEQO. 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 from_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 from_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 from_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 from_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}

[`join_collapse_limit`](/parameters/join-collapse-limit/) · [`geqo_threshold`](/parameters/geqo-threshold/) · [`geqo`](/parameters/geqo/) · [`plan_cache_mode`](/parameters/plan-cache-mode/)

## References {#references}

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