> For the complete documentation index, see [llms.txt](https://docs.postgresql.tw/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.postgresql.tw/appendixes/contrib/pgfreespacemap.md).

# F.27. pg\_freespacemap — examine the free space map \#

[F.27.1. Functions](#PGFREESPACEMAP-FUNCS)

[F.27.2. Sample Output](#PGFREESPACEMAP-SAMPLE-OUTPUT)

[F.27.3. Author](#PGFREESPACEMAP-AUTHOR)

The `pg_freespacemap` module provides a means for examining the [free space map](/internals/storage/storage-fsm.md) (FSM). It provides a function called `pg_freespace`, or two overloaded functions, to be precise. The functions show the value recorded in the free space map for a given page, or for all pages in the relation.

By default use is restricted to superusers and roles with privileges of the `pg_stat_scan_tables` role. Access may be granted to others using `GRANT`.

## F.27.1. Functions [#](#PGFREESPACEMAP-FUNCS)

`pg_freespace(rel regclass IN, blkno bigint IN) returns int2`: Returns the amount of free space on the page of the relation, specified by `blkno`, according to the FSM.

`pg_freespace(rel regclass IN, blkno OUT bigint, avail OUT int2)` : Displays the amount of free space on each page of the relation, according to the FSM. A set of `(blkno bigint, avail int2)` tuples is returned, one tuple for each page in the relation.

The values stored in the free space map are not exact. They're rounded to precision of 1/256th of `BLCKSZ` (32 bytes with default `BLCKSZ`), and they're not kept fully up-to-date as tuples are inserted and updated.

For indexes, what is tracked is entirely-unused pages, rather than free space within pages. Therefore, the values are not meaningful, just whether a page is in-use or empty.

## F.27.2. Sample Output [#](#PGFREESPACEMAP-SAMPLE-OUTPUT)

```

postgres=# SELECT * FROM pg_freespace('foo');
 blkno | avail
-------+-------
     0 |     0
     1 |     0
     2 |     0
     3 |    32
     4 |   704
     5 |   704
     6 |   704
     7 |  1216
     8 |   704
     9 |   704
    10 |   704
    11 |   704
    12 |   704
    13 |   704
    14 |   704
    15 |   704
    16 |   704
    17 |   704
    18 |   704
    19 |  3648
(20 rows)

postgres=# SELECT * FROM pg_freespace('foo', 7);
 pg_freespace
--------------
         1216
(1 row)
```

## F.27.3. Author [#](#PGFREESPACEMAP-AUTHOR)

Original version by Mark Kirkwood `<markir@paradise.net.nz>`. Rewritten in version 8.4 to suit new FSM implementation by Heikki Linnakangas `<heikki@enterprisedb.com>`

***

原文：[PostgreSQL 18.6 Documentation](https://www.postgresql.org/docs/18/pgfreespacemap.html)（英文原文，待翻譯）


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://docs.postgresql.tw/appendixes/contrib/pgfreespacemap.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.