24.1. Determining Disk Usage

Each table has a primary heap disk file where most of the data is stored. To store long column values, there is also a TOAST file associated with the table, named based on the table's OID (actually pg_class.relfilenode), and an index on the TOAST table. There also may be indexes associated with the base table.

You can monitor disk space from three places: from psql using VACUUM information, from psql using the tools in contrib/dbsize, and from the command line using the tools in contrib/oid2name. Using psql on a recently vacuumed or analyzed database, you can issue queries to see the disk usage of any table:

SELECT relfilenode, relpages FROM pg_class WHERE relname = 'customer';

 relfilenode | relpages 
-------------+----------
       16806 |       60
(1 row)

Each page is typically 8 kilobytes. (Remember, relpages is only updated by VACUUM and ANALYZE.)

To show the space used by TOAST tables, use a query like the following, substituting the relfilenode number of the heap (determined by the query above):

SELECT relname, relpages
    FROM pg_class
    WHERE relname = 'pg_toast_16806' OR relname = 'pg_toast_16806_index'
    ORDER BY relname;

       relname        | relpages 
----------------------+----------
 pg_toast_16806       |        0
 pg_toast_16806_index |        1

You can easily display index sizes, too:

SELECT c2.relname, c2.relpages
    FROM pg_class c, pg_class c2, pg_index i
    WHERE c.relname = 'customer'
        AND c.oid = i.indrelid
        AND c2.oid = i.indexrelid
        ORDER BY c2.relname;

       relname        | relpages 
----------------------+----------
 customer_id_indexdex |       26

It is easy to find your largest tables and indexes using this information:

SELECT relname, relpages FROM pg_class ORDER BY relpages DESC;

       relname        | relpages 
----------------------+----------
 bigtable             |     3290
 customer             |     3144

contrib/dbsize loads functions into your database that allow you to find the size of a table or database from inside psql without the need for VACUUM or ANALYZE.

You can also use contrib/oid2name to show disk usage. See README.oid2name in that directory for examples. It includes a script that shows disk usage for each database.