class="parameter">collation</replaceable> ] [ <replaceable class="parameter">opclass</replaceable> [ ( <replaceable class="parameter">opclass_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] ) ] ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]

<phrase><replaceable class="parameter">referential_action</replaceable> in a <literal>FOREIGN KEY</literal>/<literal>REFERENCES</literal> constraint is:</phrase>

{ NO ACTION | RESTRICT | CASCADE | SET NULL [ ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ] | SET DEFAULT [ ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ] }
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER TABLE</command> changes the definition of an existing table.
   There are several subforms described below. Note that the lock level required
   may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is
   acquired unless explicitly noted. When multiple subcommands are given, the
   lock acquired will be the strictest one required by any subcommand.

  <variablelist>
   <varlistentry id="sql-altertable-desc-add-column">
    <term><literal>ADD COLUMN [ IF NOT EXISTS ]</literal></term>
    <listitem>
     <para>
      This form adds a new column to the table, using the same syntax as
      <link linkend="sql-createtable"><command>CREATE TABLE</command></link>. If <literal>IF NOT EXISTS</literal>
      is specified and a column already exists with this name,
      no error is thrown.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-drop-column">
    <term><literal>DROP COLUMN [ IF EXISTS ]</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.
      Multivariate statistics referencing the dropped column will also be
      removed if the removal of the column would cause the statistics to
      contain data for only a single column.
      You will need to say <literal>CASCADE</literal> if anything outside the table
      depends on the column, for example, foreign key references or views.
      If <literal>IF EXISTS</literal> is specified and the column
      does not exist, no error is thrown. In this case a notice
      is issued instead.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-data-type">
    <term><literal>SET DATA TYPE</literal></term>
    <listitem>
     <para>
      This form changes the type of a column of a table. Indexes and
      simple table constraints involving the column will be automatically
      converted to use the new column type by reparsing the originally
      supplied expression.
      The optional <literal>COLLATE</literal> clause specifies a collation
      for the new column; if omitted, the collation is the default for the
      new column type.
      The optional <literal>USING</literal>
      clause specifies how to compute the new column value from the old;
      if omitted, the default conversion is the same as an assignment
      cast from old data type to new.  A  <literal>USING</literal>
      clause must be provided if there is no implicit or assignment
      cast from old to new type.
     </para>

     <para>
      When this form is used, the column's statistics are removed,
      so running <link linkend="sql-analyze"><command>ANALYZE</command></link>
      on the table afterwards is recommended.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-drop-default">
    <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
    <listitem>
     <para>
      These forms set or remove the default value for a column (where
      removal is equivalent to setting the default value to NULL).  The new
      default value will only apply in subsequent <command>INSERT</command>