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The PostgreSQL 9.0 Reference Manual - Volume 3 - Server Administration Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 274 pages
ISBN 9781906966072
RRP £9.95 ($14.95)

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14.1 Determining Disk Usage

Each table has a primary heap disk file where most of the data is stored. If the table has any columns with potentially-wide values, there also might be a TOAST file associated with the table, which is used to store values too wide to fit comfortably in the main table (see TOAST). There will be one index on the TOAST table, if present. There also might be indexes associated with the base table. Each table and index is stored in a separate disk file--possibly more than one file, if the file would exceed one gigabyte. Naming conventions for these files are described in Database File Layout.

You can monitor disk space in three ways: using the SQL functions listed in Volume 1A: Table 7-58, using the tools in ‘contrib/oid2name’, or using manual inspection of the system catalogs. The SQL functions are the easiest to use and are generally recommended. ‘contrib/oid2name’ is described in oid2name. The remainder of this section shows how to do it by inspection of the system catalogs.

Using psql on a recently vacuumed or analyzed database, you can issue queries to see the disk usage of any table:

SELECT pg_relation_filepath(oid), relpages FROM pg_class 
  WHERE relname = 'customer';

 pg_relation_filepath | relpages
 base/16384/16806     |       60
(1 row)

Each page is typically 8 kilobytes. (Remember, relpages is only updated by VACUUM, ANALYZE, and a few data definition language (DDL) commands such as CREATE INDEX.) The file path name is of interest if you want to examine the table's disk file directly.

To show the space used by TOAST tables, use a query like the following:

SELECT relname, relpages
FROM pg_class,
     (SELECT reltoastrelid
      FROM pg_class
      WHERE relname = 'customer') AS ss
WHERE oid = ss.reltoastrelid OR
      oid = (SELECT reltoastidxid
             FROM pg_class
             WHERE oid = ss.reltoastrelid)
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
ISBN 9781906966072The PostgreSQL 9.0 Reference Manual - Volume 3 - Server Administration GuideSee the print edition