pg_prewarm

prewarm relation data

https://www.postgresql.org/docs/current/pgprewarm.html

Overview

Package	Version	Category	License	Language
`pg_prewarm`	`1.2`	ADMIN	PostgreSQL	C

ID	Extension	Bin	Lib	Load	Create	Trust	Reloc	Schema
5890	`pg_prewarm`	No	Yes	Yes	Yes	No	No	-

Related	`pgfincore` `pg_cooldown` `pgcozy` `pg_buffercache` `pg_repack` `pg_rewrite` `pg_squeeze` `old_snapshot`

Version

PG18	PG17	PG16	PG15	PG14
1.2	1.2	1.2	1.2	1.2

Install

Note: This is a built-in contrib extension of PostgreSQL

CREATE EXTENSION pg_prewarm;

Usage

pg_prewarm: prewarm relation data

The pg_prewarm extension provides functions to load relation data into the operating system buffer cache or PostgreSQL buffer cache, reducing I/O latency for subsequent queries.

Prewarm a Relation

-- Load entire table into PostgreSQL buffer cache (default mode)
SELECT pg_prewarm('my_table');

-- Load with specific mode
SELECT pg_prewarm('my_table', 'prefetch');  -- async OS prefetch
SELECT pg_prewarm('my_table', 'read');      -- sync read into OS cache
SELECT pg_prewarm('my_table', 'buffer');    -- load into PG buffer cache

-- Load specific block range
SELECT pg_prewarm('my_table', 'buffer', 'main', 0, 999);

-- Prewarm an index
SELECT pg_prewarm('my_table_pkey');

Function Signature

pg_prewarm(regclass,
           mode text DEFAULT 'buffer',
           fork text DEFAULT 'main',
           first_block int8 DEFAULT NULL,
           last_block int8 DEFAULT NULL
) RETURNS int8

Returns the number of blocks prewarmed.

Parameter	Description
`mode`	`prefetch` (async OS), `read` (sync OS), or `buffer` (PG cache)
`fork`	Relation fork to prewarm (e.g., `main`, `fsm`, `vm`)
`first_block`	First block number (default: 0)
`last_block`	Last block number (default: last block of relation)

Autoprewarm

When loaded via shared_preload_libraries, autoprewarm periodically saves the list of buffers in the shared buffer cache to disk and restores them on restart.

-- Manually launch autoprewarm worker
SELECT autoprewarm_start_worker();

-- Force immediate dump of buffer state
SELECT autoprewarm_dump_now();  -- returns number of records written

GUC Parameters

Parameter	Default	Description
`pg_prewarm.autoprewarm`	`true`	Enable autoprewarm background worker
`pg_prewarm.autoprewarm_interval`	`300s`	Interval between `autoprewarm.blocks` file updates (0 = dump only at shutdown)

Buffer state is saved to autoprewarm.blocks in the data directory. After restart, two background workers reload the saved buffers.

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Last Modified 2026-03-12: add pg extension catalog (95749bf)