# synchronize_seqscans

> synchronize_seqscans — Enables synchronized sequential scans. Observed in PG9.0–19 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 synchronized sequential scans.”

## 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 | Version and Platform Compatibility / Previous PostgreSQL Versions | Upstream classification |
| Latest boot value | `on` | on |
{.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 | `on` | — | on |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Enables synchronized sequential scans. It can be changed at session scope, so different sessions may observe different behavior.

Concurrent sequential scans of the same large relation may join a shared scan position, improving cache reuse while returning rows in a less predictable physical order. SQL without ORDER BY has no ordering guarantee regardless of this switch.

Monitor and change synchronize_seqscans together with default_with_oids, lo_compat_privileges, operator_precedence_warning. Validate on the relevant server role and real workload, then use its user context to choose session change, reload, or restart; a historical boot default is not the current effective value.

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Normally keep it on so concurrent large-table scans can share cache footprint. Queries that require deterministic order must use ORDER BY; disabling this setting is not an ordering contract. |
| OLAP | Analytical concurrency is the most likely beneficiary. Experiment at session scope only when repeatable benchmarks show the shared scan position harms locality. |
| Small nodes | Keep the default. When data fits in cache or concurrent scans are rare, this setting usually needs no attention and is not a bottleneck. |
{.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}

- Keeping a compatibility switch permanently instead of fixing the client.
- Testing in one session and deploying globally to unrelated applications.
- Confusing parsing compatibility with data or security compatibility.
- Forgetting to remove an override after the upgrade migration is complete.

## Related parameters {#related-parameters}

[`default_with_oids`](/parameters/default-with-oids/) · [`lo_compat_privileges`](/parameters/lo-compat-privileges/) · [`operator_precedence_warning`](/parameters/operator-precedence-warning/) · [`standard_conforming_strings`](/parameters/standard-conforming-strings/) · [`array_nulls`](/parameters/array-nulls/) · [`backslash_quote`](/parameters/backslash-quote/)

## References {#references}

- [PostgreSQL 19 Beta 3 — synchronize_seqscans](https://www.postgresql.org/docs/19/runtime-config-compatible.html#GUC-SYNCHRONIZE-SEQSCANS)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)