# summarize_wal

> summarize_wal starts the WAL summarizer process to enable incremental backup. It is a sighup setting present in PG17–18; the latest recorded boot default is off.
---

> [!NOTE]
> **Fact — official short description:** “Starts the WAL summarizer process to enable incremental backup.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `bool` | Upstream pg_settings type |
| Context | `sighup` | Takes effect after configuration reload |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Write-Ahead Log / Summarization | Upstream classification |
| Latest boot value | `off` | off |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG17 |
| Present in | PG17–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`174c480508ac`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=174c480508ac25568561443e6d4a82d5c1103487) — Add a new WAL summarizer process. |
| Commit date | 2023-12-20 |
| Discussion | [thread 1](https://postgr.es/m/CA+TgmoYOYZfMCyOXFyC-P+-mdrZqm5pP2N7S-r0z3_402h9rsA@mail.gmail.com) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG17–19 Beta 3 | `off` | — | off |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Starts the WAL summarizer process to enable incremental backup. A configuration reload applies a new value; existing work already in flight is not retroactively changed.

The WAL summarizer records which blocks changed over WAL ranges so pg_basebackup can construct incremental backups. It can run on a primary or standby, but cannot summarize WAL generated at wal_level=minimal.

Monitor and change summarize_wal together with wal_summary_keep_time, wal_level, archive_mode. Validate on the relevant server role and real workload, then use its sighup 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 | Enable or tune summarize_wal only when using PostgreSQL incremental backup. Monitor summarizer progress and retain summaries across the complete WAL range between every dependent backup pair. |
| OLAP | Bulk writes increase summarizer work. Capacity-test backup interval, WAL peak, and backup window together; a summary gap makes an incremental backup fail. |
| Small nodes | Keep summarize_wal off when incremental backup is not used. If enabled, derive retention from the real backup chain and alert on summary-directory capacity. |
{.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: PG17–19 Beta 3 unmodified; OLAP: PG17–19 Beta 3 unmodified; CRIT: PG17–19 Beta 3 unmodified; TINY: PG17–19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Enabling it with wal_level=minimal and expecting usable summary files.
- Treating WAL summaries as a WAL archive or a backup by themselves.
- Turning the summarizer off and expecting wal_summary_keep_time cleanup to continue.
- Taking an incremental backup without checking summarizer progress and summary coverage.

## Related parameters {#related-parameters}

[`wal_summary_keep_time`](/parameters/wal-summary-keep-time/) · [`wal_level`](/parameters/wal-level/) · [`archive_mode`](/parameters/archive-mode/) · [`max_wal_size`](/parameters/max-wal-size/) · [`archive_cleanup_command`](/parameters/archive-cleanup-command/) · [`archive_command`](/parameters/archive-command/)

## References {#references}

- [PostgreSQL 19 Beta 3 — summarize_wal](https://www.postgresql.org/docs/19/runtime-config-wal.html#GUC-SUMMARIZE-WAL)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)