pg_freespacemap

PostgreSQL 9.0.17 Documentation
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# F.26. pg_freespacemap

The `pg_freespacemap` module provides a means for examining the free space map (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 public access is revoked from the functions, just in case there are security issues lurking.

## F.26.1. Functions

`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 full or empty.

NOTE: The interface was changed in version 8.4, to reflect the new FSM implementation introduced in the same version.

## F.26.2. 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.26.3. 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>`

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