# cursor_tuple_fraction

> cursor_tuple_fraction — Sets the planner's estimate of the fraction of a cursor's rows that will be retrieved. Observed in PG9.0–19 Beta 3; its last measured boot default is 0.1 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 planner's estimate of the fraction of a cursor's rows that will be retrieved.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `real` | Upstream pg_settings type |
| Context | `user` | Settable by an ordinary user |
| Unit | — | Raw unit |
| Range | `0` – `1` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Query Tuning / Other Planner Options | Upstream classification |
| Latest boot value | `0.1` | 0.1 |
{.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 | `0.1` | — | 0.1 |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

cursor_tuple_fraction tells the planner what fraction of a cursor's result is expected to be fetched. Lower fractions give more weight to startup cost; 1.0 plans like an ordinary query that consumes the full result.

The estimate can change join order and access paths by preferring a fast first row even when total execution would be slower. It does not stop FETCH after that fraction and is not a row limit.

It is consulted at planning time for cursor plans. Application behavior matters: cursors used for pagination, early exit, or full export have very different appropriate fractions. 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 cursor_tuple_fraction 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 cursor_tuple_fraction 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 cursor_tuple_fraction 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 cursor_tuple_fraction 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}

[`plan_cache_mode`](/parameters/plan-cache-mode/) · [`random_page_cost`](/parameters/random-page-cost/) · [`enable_nestloop`](/parameters/enable-nestloop/) · [`enable_indexscan`](/parameters/enable-indexscan/)

## References {#references}

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