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>27.1. Determining Disk Usage</A
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><P
> 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 is also a <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> file associated with the table,
which is used to store values too wide to fit comfortably in the main
table (see <A
HREF="storage-toast.html"
>Section 53.2</A
>). There will be one index on the
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> table, if present. There might also 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 <A
HREF="storage-file-layout.html"
>Section 53.1</A
>.
</P
><P
> You can monitor disk space from three ways: using
SQL functions listed in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-DBSIZE"
>Table 9-57</A
>,
using <TT
CLASS="COMMAND"
>VACUUM</TT
> information, and from the command line
using the tools in <TT
CLASS="FILENAME"
>contrib/oid2name</TT
>. The SQL functions
are the easiest to use and report information about tables, tables with
indexes and long value storage (TOAST), databases, and tablespaces.
</P
><P
> Using <SPAN
CLASS="APPLICATION"
>psql</SPAN
> on a recently vacuumed or analyzed database,
you can issue queries to see the disk usage of any table:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT relfilenode, relpages FROM pg_class WHERE relname = 'customer';
relfilenode | relpages
-------------+----------
16806 | 60
(1 row)</PRE
><P>
Each page is typically 8 kilobytes. (Remember, <TT
CLASS="STRUCTFIELD"
>relpages</TT
>
is only updated by <TT
CLASS="COMMAND"
>VACUUM</TT
>, <TT
CLASS="COMMAND"
>ANALYZE</TT
>, and
a few DDL commands such as <TT
CLASS="COMMAND"
>CREATE INDEX</TT
>.) The
<TT
CLASS="STRUCTFIELD"
>relfilenode</TT
> value is of interest if you want to examine
the table's disk file directly.
</P
><P
> To show the space used by <ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> tables, use a query
like the following:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT relname, relpages
FROM pg_class,
(SELECT reltoastrelid FROM pg_class
WHERE relname = 'customer') 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</PRE
><P>
</P
><P
> You can easily display index sizes, too:
</P><PRE
CLASS="PROGRAMLISTING"
>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</PRE
><P>
</P
><P
> It is easy to find your largest tables and indexes using this
information:
</P><PRE
CLASS="PROGRAMLISTING"
>SELECT relname, relpages FROM pg_class ORDER BY relpages DESC;
relname | relpages
----------------------+----------
bigtable | 3290
customer | 3144</PRE
><P>
</P
><P
> You can also use <TT
CLASS="FILENAME"
>contrib/oid2name</TT
> to show disk usage. See
<TT
CLASS="FILENAME"
>README.oid2name</TT
> in that directory for examples. It includes a script that
shows disk usage for each database.
</P
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