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>28.1. 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 also might be 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 56.2</A
>). There will be one index on the
<ACRONYM
CLASS="ACRONYM"
>TOAST</ACRONYM
> 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 <A
HREF="storage-file-layout.html"
>Section 56.1</A
>.
</P
><P
> You can monitor disk space in three ways:
using the SQL functions listed in <A
HREF="functions-admin.html#FUNCTIONS-ADMIN-DBSIZE"
>Table 9-64</A
>,
using the <A
HREF="oid2name.html"
>oid2name</A
> module, or
using manual inspection of the system catalogs.
The SQL functions are the easiest to use and are generally recommended.
The remainder of this section shows how to do it by inspection of the
system catalogs.
</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 pg_relation_filepath(oid), relpages FROM pg_class WHERE relname = 'customer';
pg_relation_filepath | relpages
----------------------+----------
base/16384/16806 | 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 file path name
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') 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</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
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