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$Header: /cvsroot/pgsql/doc/src/sgml/ref/alter_table.sgml,v 1.53 2002/12/16 19:08:25 tgl 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>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</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">table</replaceable> [ * ]
    DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET DEFAULT <replaceable class="PARAMETER">value</replaceable> | DROP DEFAULT }
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    RENAME [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> TO <replaceable
    class="PARAMETER">new_column</replaceable>
ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
    RENAME TO <replaceable class="PARAMETER">new_table</replaceable>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    ADD <replaceable class="PARAMETER">table_constraint</replaceable>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">table</replaceable> [ * ]
    DROP CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
ALTER TABLE <replaceable class="PARAMETER">table</replaceable>
    OWNER TO <replaceable class="PARAMETER">new_owner</replaceable> 
  </synopsis>

  <refsect2 id="R2-SQL-ALTERTABLE-1">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>
    <variablelist>

     <varlistentry>
      <term><replaceable class="PARAMETER"> table </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 scanned, 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
	<option>SQL_INHERITANCE</option> configuration option.
       </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>
	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_table </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>CASCADE</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>RESTRICT</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>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-ALTERTABLE-2">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><computeroutput>ALTER TABLE</computeroutput></term>
      <listitem>
       <para>
	Message returned from column or table renaming.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><computeroutput>ERROR</computeroutput></term>
      <listitem>
       <para>
	Message returned if table or column is not available.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-ALTERTABLE-1">
  <refsect1info>
   <date>1998-04-15</date>
  </refsect1info>
  <title>
   Description
  </title>
  <para>
   <command>ALTER TABLE</command> changes the definition of an existing table.
   There are several sub-forms:
  </para>

  <variablelist>

   <varlistentry>
    <term>ADD COLUMN</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>DROP COLUMN</term>
    <listitem>
     <para>
      This form drops a column from a table.  Note that 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, views, etc.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>SET/DROP DEFAULT</term>
    <listitem>
     <para>
      These forms set or remove the default value for a column. Note
      that defaults 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 ON INSERT rule is applied.
     </para>
    </listitem>
   </varlistentry>

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

   <varlistentry>
    <term>SET STATISTICS</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.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>SET STORAGE</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 <literal>INTEGER</literal> 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
      make substring operations on a TEXT column faster, at the penalty of
      increased storage space.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>RENAME</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>ADD <replaceable class="PARAMETER">table_constraint</replaceable></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>DROP CONSTRAINT</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>OWNER</term>
    <listitem>
     <para>
      This form changes the owner of the table, index, sequence or view to the
      specified user.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>

  <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>

  <refsect2 id="R2-SQL-ALTERTABLE-3">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>
    Notes
</title>
   <para>
    The keyword <literal>COLUMN</literal> is noise and can be omitted.
   </para>

   <para>
    In the current implementation of <literal>ADD COLUMN</literal>,
    default and 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 afterwards.
    (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> command does not physically remove
    the column, but simply makes it invisible to SQL operations.  Subsequent
    inserts and updates of the table will store a NULL 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, ALTER TABLE ONLY will be rejected.  This
    ensures that the descendants always have columns matching the parent.
   </para>

   <para>
    A recursive DROP COLUMN 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 DROP COLUMN (i.e., ALTER TABLE ONLY ... DROP COLUMN)
    never removes any descendant columns, but instead marks them as
    independently defined rather than inherited.
   </para>

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

   <para>
    Refer to <command>CREATE TABLE</command> for a further description
    of valid arguments.
    The <citetitle>PostgreSQL User's Guide</citetitle> has further
    information on inheritance.
   </para>
  </refsect2>
 </refsect1>

 <refsect1 id="R1-SQL-ALTERTABLE-2">
  <title>
   Usage
  </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 id="R1-SQL-ALTERTABLE-3">
  <title>
   Compatibility
  </title>

  <refsect2 id="R2-SQL-ALTERTABLE-4">
   <refsect2info>
    <date>1998-04-15</date>
   </refsect2info>
   <title>SQL92</title>
    <para>
     The <literal>ADD COLUMN</literal> form is compliant 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 compliance.
    </para>

   <para>
    The clauses to rename tables, columns, indexes, and sequences are
    <productname>PostgreSQL</productname> extensions from SQL92.
   </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 from SQL92,
    which disallows zero-column tables.
   </para>
 
  </refsect2>
 </refsect1>
</refentry>

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