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$PostgreSQL: pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.66 2004/03/09 16:57:47 neilc Exp $
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<refentry id="SQL-ALTERTABLE">
 <refmeta>
  <refentrytitle id="sql-altertable-title">ALTER TABLE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>ALTER TABLE</refname>
  <refpurpose>change the definition of a table</refpurpose>
 </refnamediv>

 <indexterm zone="sql-altertable">
  <primary>ALTER TABLE</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">type</replaceable> [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET DEFAULT <replaceable class="PARAMETER">expression</replaceable> | DROP DEFAULT }
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    SET WITHOUT OIDS
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    RENAME [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> TO <replaceable
    class="PARAMETER">new_column</replaceable>
ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
    RENAME TO <replaceable class="PARAMETER">new_name</replaceable>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    ADD <replaceable class="PARAMETER">table_constraint</replaceable>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    DROP CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
    OWNER TO <replaceable class="PARAMETER">new_owner</replaceable>
ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
    CLUSTER ON <replaceable class="PARAMETER">index_name</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER TABLE</command> changes the definition of an existing table.
   There are several subforms:

  <variablelist>
   <varlistentry>
    <term><literal>ADD COLUMN</literal></term>
    <listitem>
     <para>
      This form adds a new column to the table using the same syntax as
      <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-TITLE">.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DROP COLUMN</literal></term>
    <listitem>
     <para>
      This form drops a column from a table.  Indexes and
      table constraints involving the column will be automatically
      dropped as well.  You will need to say <literal>CASCADE</> if
      anything outside the table depends on the column, for example,
      foreign key references or views.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
    <listitem>
     <para>
      These forms set or remove the default value for a column.
      The default values only apply to subsequent <command>INSERT</command>
      commands; they do not cause rows already in the table to change.
      Defaults may also be created for views, in which case they are
      inserted into <command>INSERT</> statements on the view before
      the view's <literal>ON INSERT</literal> rule is applied.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET</literal>/<literal>DROP NOT NULL</literal></term>
    <listitem>
     <para>
      These forms change whether a column is marked to allow null
      values or to reject null values.  You can only use <literal>SET
      NOT NULL</> when the column contains no null values.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET STATISTICS</literal></term>
    <listitem>
     <para>
      This form
      sets the per-column statistics-gathering target for subsequent
      <xref linkend="sql-analyze" endterm="sql-analyze-title"> operations.
      The target can be set in the range 0 to 1000; alternatively, set it
      to -1 to revert to using the system default statistics
      target. For more information on the use of statistics by the
      <productname>PostgreSQL</productname> query planner, refer to
      <xref linkend="planner-stats">.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <indexterm>
     <primary>TOAST</primary>
     <secondary>per-column storage settings</secondary>
    </indexterm>

    <term><literal>SET STORAGE</literal></term>
    <listitem>
     <para>
      This form sets the storage mode for a column. This controls whether this
      column is held inline or in a supplementary table, and whether the data
      should be compressed or not. <literal>PLAIN</literal> must be used
      for fixed-length values such as <type>integer</type> and is
      inline, uncompressed. <literal>MAIN</literal> is for inline,
      compressible data. <literal>EXTERNAL</literal> is for external,
      uncompressed data, and <literal>EXTENDED</literal> is for external,
      compressed data.  <literal>EXTENDED</literal> is the default for all
      data types that support it.  The use of <literal>EXTERNAL</literal> will, for example,
      make substring operations on a <type>text</type> column faster, at the penalty of
      increased storage space.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET WITHOUT OIDS</literal></term>
    <listitem>
     <para>
      This form removes the <literal>oid</literal> column from the
      table.  Removing OIDs from a table does not occur immediately.
      The space that the OID uses will be reclaimed when the row is
      updated.  Without updating the row, both the space and the value
      of the OID are kept indefinitely.  This is semantically similar
      to the <literal>DROP COLUMN</literal> process.
     </para>

     <para>
      Note that there is no variant of <command>ALTER TABLE</command>
      that allows OIDs to be restored to a table once they have been
      removed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>RENAME</literal></term>
    <listitem>
     <para>
      The <literal>RENAME</literal> forms change the name of a table
      (or an index, sequence, or view) or the name of an individual column in
      a table. There is no effect on the stored data.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ADD <replaceable class="PARAMETER">table_constraint</replaceable></literal></term>
    <listitem>
     <para>
      This form adds a new constraint to a table using the same syntax as
      <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-TITLE">. 
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DROP CONSTRAINT</literal></term>
    <listitem>
     <para>
      This form drops constraints on a table.
      Currently, constraints on tables are not required to have unique
      names, so there may be more than one constraint matching the specified
      name.  All such constraints will be dropped.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>OWNER</literal></term>
    <listitem>
     <para>
      This form changes the owner of the table, index, sequence, or view to the
      specified user.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CLUSTER</literal></term>
    <listitem>
     <para>
      This form marks a table for future <xref linkend="SQL-CLUSTER" endterm="sql-cluster-title">
      operations.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
  </para>

  <para>
   You must own the table to use <command>ALTER TABLE</>; except for
   <command>ALTER TABLE OWNER</>, which may only be executed by a superuser.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>

     <varlistentry>
      <term><replaceable class="PARAMETER">name</replaceable></term>
      <listitem>
       <para>
	The name (possibly schema-qualified) of an existing table to
	alter. If <literal>ONLY</> is specified, only that table is
	altered. If <literal>ONLY</> is not specified, the table and all
	its descendant tables (if any) are updated. <literal>*</> can be
	appended to the table name to indicate that descendant tables are
	to be altered, but in the current version, this is the default
	behavior.  (In releases before 7.1, <literal>ONLY</> was the
	default behavior.  The default can be altered by changing the
	configuration parameter <xref linkend="guc-sql-inheritance">.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">column</replaceable></term>
      <listitem>
       <para>
	Name of a new or existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">type</replaceable></term>
      <listitem>
       <para>
	Data type of the new column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">new_column</replaceable></term>
      <listitem>
       <para>
	New name for an existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">new_name</replaceable></term>
      <listitem>
       <para>
	New name for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">table_constraint</replaceable></term>
      <listitem>
       <para>
	New table constraint for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">constraint_name</replaceable></term>
      <listitem>
       <para>
	Name of an existing constraint to drop.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">new_owner</replaceable></term>
      <listitem>
       <para>
	The user name of the new owner of the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">index_name</replaceable></term>
      <listitem>
       <para>
	The index name on which the table should be marked for clustering.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>CASCADE</literal></term>
      <listitem>
       <para>
        Automatically drop objects that depend on the dropped column
	or constraint (for example, views referencing the column).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>RESTRICT</literal></term>
      <listitem>
       <para>
        Refuse to drop the column or constraint if there are any dependent
	objects. This is the default behavior.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

   <para>
    The key word <literal>COLUMN</literal> is noise and can be omitted.
   </para>

   <para>
    In the current implementation of <literal>ADD COLUMN</literal>,
    default and <literal>NOT NULL</> clauses for the new column are not supported.
    The new column always comes into being with all values null.
    You can use the <literal>SET DEFAULT</literal> form
    of <command>ALTER TABLE</command> to set the default afterward.
    (You may also want to update the already existing rows to the
    new default value, using
    <xref linkend="sql-update" endterm="sql-update-title">.)
    If you want to mark the column non-null, use the <literal>SET NOT NULL</>
    form after you've entered non-null values for the column in all rows.
   </para>

   <para>
    The <literal>DROP COLUMN</literal> form does not physically remove
    the column, but simply makes it invisible to SQL operations.  Subsequent
    insert and update operations in the table will store a null value for the column.
    Thus, dropping a column is quick but it will not immediately reduce the
    on-disk size of your table, as the space occupied 
    by the dropped column is not reclaimed.  The space will be
    reclaimed over time as existing rows are updated.
    To reclaim the space at once, do a dummy <command>UPDATE</> of all rows
    and then vacuum, as in:
<programlisting>
UPDATE table SET col = col;
VACUUM FULL table;
</programlisting>
   </para>

   <para>
    If a table has any descendant tables, it is not permitted to add
    or rename a column in the parent table without doing the same to
    the descendants.  That is, <command>ALTER TABLE ONLY</command>
    will be rejected.  This ensures that the descendants always have
    columns matching the parent.
   </para>

   <para>
    A recursive <literal>DROP COLUMN</literal> operation will remove a
    descendant table's column only if the descendant does not inherit
    that column from any other parents and never had an independent
    definition of the column.  A nonrecursive <literal>DROP
    COLUMN</literal> (i.e., <command>ALTER TABLE ONLY ... DROP
    COLUMN</command>) never removes any descendant columns, but
    instead marks them as independently defined rather than inherited.
   </para>

   <para>
    Changing any part of a system catalog table is not permitted.
   </para>

   <para>
    Refer to <command>CREATE TABLE</command> for a further description
    of valid parameters. <xref linkend="ddl"> has further information on
    inheritance.
   </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To add a column of type <type>varchar</type> to a table:
<programlisting>
ALTER TABLE distributors ADD COLUMN address varchar(30);
</programlisting>
  </para>

  <para>
   To drop a column from a table:
<programlisting>
ALTER TABLE distributors DROP COLUMN address RESTRICT;
</programlisting>
  </para>

  <para>
   To rename an existing column:
<programlisting>
ALTER TABLE distributors RENAME COLUMN address TO city;
</programlisting>
  </para>

  <para>
   To rename an existing table:
<programlisting>
ALTER TABLE distributors RENAME TO suppliers;
</programlisting>
  </para>

  <para>
   To add a not-null constraint to a column:
<programlisting>
ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
</programlisting>
   To remove a not-null constraint from a column:
<programlisting>
ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
</programlisting>
  </para>

  <para> 
   To add a check constraint to a table:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
</programlisting>
  </para>

  <para> 
   To remove a check constraint from a table and all its children:
<programlisting>
ALTER TABLE distributors DROP CONSTRAINT zipchk;
</programlisting>
  </para>

  <para> 
   To add a foreign key constraint to a table:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) MATCH FULL;
</programlisting>
  </para>

  <para> 
   To add a (multicolumn) unique constraint to a table:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
</programlisting>
  </para>

  <para> 
   To add an automatically named primary key constraint to a table, noting
   that a table can only ever have one primary key:
<programlisting>
ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
</programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The <literal>ADD COLUMN</literal> form conforms with the SQL
   standard, with the exception that it does not support defaults and
   not-null constraints, as explained above.  The <literal>ALTER
   COLUMN</literal> form is in full conformance.
  </para>

  <para>
   The clauses to rename tables, columns, indexes, views, and sequences are
   <productname>PostgreSQL</productname> extensions of the SQL standard.
  </para>

  <para>
   <command>ALTER TABLE DROP COLUMN</> can be used to drop the only
   column of a table, leaving a zero-column table.  This is an
   extension of SQL, which disallows zero-column tables.
  </para>
 </refsect1>
</refentry>

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