# hot_standby

> hot_standby — Allows connections and queries during recovery. Observed in PG9.0–19 Beta 3; its last measured boot default is on in PG19 Beta 3, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Allows connections and queries during recovery.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `bool` | Upstream pg_settings type |
| Context | `postmaster` | Requires a server restart |
| Unit | — | Raw unit |
| Range | — | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Replication / Standby Servers | 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 |
| --- | --- | --- | --- |
| PG9.0–9.6 | `off` | — | off |
| PG10–19 Beta 3 | `on` | — | on |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

Allows connections and queries during recovery. The value is fixed when the server starts, so changing it requires a restart.

When recovery has reached a consistent state, this permits read-only connections and queries. Replay still has priority: WAL records that conflict with snapshots or locks can cancel queries according to the standby-delay settings.

Monitor and change hot_standby together with primary_conninfo, primary_slot_name, restore_command. Validate on the relevant server role and real workload, then use its postmaster 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 | Size hot_standby from topology, failover roles, slot/subscription count, and reconnect headroom. Test worst-case primary latency, standby replay, and disk retention before production. |
| OLAP | Read standbys and logical subscribers often see long queries or large transactions. Put explicit bounds on replay/apply and monitor lag, worker saturation, slot restart_lsn, and conflict cancellations. |
| Small nodes | Configure only replication capacity that is actually used. Even a small topology needs bounded timeouts and slot lifecycle; unlimited retention is not reliability. |
{.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}

- Changing it on the wrong primary, standby, sender, or subscriber role.
- Watching only configured bytes/time instead of actual lag, slot position, and worker state.
- Failing over to a node that lacks the old primary's capacity or prerequisites.
- Using infinite waits or WAL retention to hide a failed consumer.

## Related parameters {#related-parameters}

[`primary_conninfo`](/parameters/primary-conninfo/) · [`primary_slot_name`](/parameters/primary-slot-name/) · [`restore_command`](/parameters/restore-command/) · [`wal_receiver_timeout`](/parameters/wal-receiver-timeout/) · [`wal_retrieve_retry_interval`](/parameters/wal-retrieve-retry-interval/) · [`max_standby_archive_delay`](/parameters/max-standby-archive-delay/)

## References {#references}

- [PostgreSQL 19 Beta 3 — hot_standby](https://www.postgresql.org/docs/19/runtime-config-replication.html#GUC-HOT-STANDBY)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)