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<refentry id="SQL-COMMENT">
 <refmeta>
  <refentrytitle id="SQL-COMMENT-TITLE">COMMENT</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>COMMENT</refname>
  <refpurpose>define or change the comment of an object</refpurpose>
 </refnamediv>

 <indexterm zone="sql-comment">
  <primary>COMMENT</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
COMMENT ON
{
  TABLE <replaceable class="PARAMETER">object_name</replaceable> |
  COLUMN <replaceable class="PARAMETER">table_name</replaceable>.<replaceable class="PARAMETER">column_name</replaceable> |
  AGGREGATE <replaceable class="PARAMETER">agg_name</replaceable> (<replaceable class="PARAMETER">agg_type</replaceable>) |
  CAST (<replaceable>sourcetype</replaceable> AS <replaceable>targettype</replaceable>) |
  CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> ON <replaceable class="PARAMETER">table_name</replaceable> |
  CONVERSION <replaceable class="PARAMETER">object_name</replaceable> |
  DATABASE <replaceable class="PARAMETER">object_name</replaceable> |
  DOMAIN <replaceable class="PARAMETER">object_name</replaceable> |
  FUNCTION <replaceable class="PARAMETER">func_name</replaceable> ( [ [ <replaceable class="parameter">argmode</replaceable> ] [ <replaceable class="parameter">argname</replaceable> ] <replaceable class="parameter">argtype</replaceable> [, ...] ] ) |
  INDEX <replaceable class="PARAMETER">object_name</replaceable> |
  LARGE OBJECT <replaceable class="PARAMETER">large_object_oid</replaceable> |
  OPERATOR <replaceable class="PARAMETER">op</replaceable> (<replaceable class="PARAMETER">leftoperand_type</replaceable>, <replaceable class="PARAMETER">rightoperand_type</replaceable>) |
  OPERATOR CLASS <replaceable class="PARAMETER">object_name</replaceable> USING <replaceable class="parameter">index_method</replaceable> |
  [ PROCEDURAL ] LANGUAGE <replaceable class="PARAMETER">object_name</replaceable> |
  ROLE <replaceable class="PARAMETER">object_name</replaceable> |
  RULE <replaceable class="PARAMETER">rule_name</replaceable> ON <replaceable class="PARAMETER">table_name</replaceable> |
  SCHEMA <replaceable class="PARAMETER">object_name</replaceable> |
  SEQUENCE <replaceable class="PARAMETER">object_name</replaceable> |
  TABLESPACE <replaceable class="PARAMETER">object_name</replaceable> |
  TRIGGER <replaceable class="PARAMETER">trigger_name</replaceable> ON <replaceable class="PARAMETER">table_name</replaceable> |
  TYPE <replaceable class="PARAMETER">object_name</replaceable> |
  VIEW <replaceable class="PARAMETER">object_name</replaceable>
} IS <replaceable class="PARAMETER">'text'</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>COMMENT</command> stores a comment about a database object.
  </para>

  <para>
    To modify a comment, issue a new <command>COMMENT</> command for the
    same object.  Only one comment string is stored for each object.
    To remove a comment, write <literal>NULL</literal> in place of the text
    string.
    Comments are automatically dropped when the object is dropped.
  </para>

  <para>
    Comments can be
    easily retrieved with the <application>psql</application> commands
    <command>\dd</command>, <command>\d+</command>, and <command>\l+</command>.
    Other user interfaces to retrieve comments can be built atop
    the same built-in functions that <application>psql</application> uses, namely
    <function>obj_description</>, <function>col_description</>,
    and <function>shobj_description</> 
    (see <xref linkend="functions-info-comment-table">).
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="parameter">object_name</replaceable></term>
    <term><replaceable class="parameter">table_name.column_name</replaceable></term>
    <term><replaceable class="parameter">agg_name</replaceable></term>
    <term><replaceable class="parameter">constraint_name</replaceable></term>
    <term><replaceable class="parameter">func_name</replaceable></term>
    <term><replaceable class="parameter">op</replaceable></term>
    <term><replaceable class="parameter">rule_name</replaceable></term>
    <term><replaceable class="parameter">trigger_name</replaceable></term>
    <listitem>
     <para>
      The name of the object to be commented.  Names of tables,
      aggregates, domains, functions, indexes, operators, operator classes,
      sequences, types, and views may be schema-qualified.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">agg_type</replaceable></term>
    <listitem>
     <para>
      The argument data type of the aggregate function, or
      <literal>*</literal> if the function accepts any data type.
     </para>
    </listitem>
   </varlistentry>
   
   <varlistentry>
     <term><replaceable>sourcetype</replaceable></term>
     <listitem>
      <para>
       The name of the source data type of the cast.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><replaceable>targettype</replaceable></term>
     <listitem>
      <para>
       The name of the target data type of the cast.
      </para>
     </listitem>
    </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">argmode</replaceable></term>

    <listitem>
     <para>
      The mode of a function argument: either <literal>IN</>, <literal>OUT</>,
      or <literal>INOUT</>.  If omitted, the default is <literal>IN</>.
      Note that <command>COMMENT ON FUNCTION</command> does not actually pay
      any attention to <literal>OUT</> arguments, since only the input
      arguments are needed to determine the function's identity.
      So it is sufficient to list the <literal>IN</> and <literal>INOUT</>
      arguments.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">argname</replaceable></term>

    <listitem>
     <para>
      The name of a function argument.
      Note that <command>COMMENT ON FUNCTION</command> does not actually pay
      any attention to argument names, since only the argument data
      types are needed to determine the function's identity.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">argtype</replaceable></term>

    <listitem>
     <para>
      The data type(s) of the function's arguments (optionally 
      schema-qualified), if any.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">large_object_oid</replaceable></term>
    <listitem>
     <para>
      The OID of the large object.
     </para>
    </listitem>
   </varlistentry>

    <varlistentry>
     <term><literal>PROCEDURAL</literal></term>

     <listitem>
      <para>
       This is a noise word.
      </para>
     </listitem>
    </varlistentry>

   <varlistentry>
    <term><replaceable class="parameter">text</replaceable></term>
    <listitem>
     <para>
      The new comment, written as a string literal; or <literal>NULL</>
      to drop the comment.
     </para>
    </listitem>
   </varlistentry>
    
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   There is presently no security mechanism for comments: any user
   connected to a database can see all the comments for objects in
   that database (although only superusers can change comments for
   objects that they don't own).  For shared objects such as
   databases, roles, and tablespaces comments are stored gloablly
   and any user connected to any database can see all the comments
   for shared objects.  Therefore, don't put security-critical
   information in comments.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Attach a comment to the table <literal>mytable</literal>:

<programlisting>
COMMENT ON TABLE mytable IS 'This is my table.';
</programlisting>

   Remove it again:

<programlisting>
COMMENT ON TABLE mytable IS NULL;
</programlisting>
  </para>

  <para>
   Some more examples:

<programlisting>
COMMENT ON AGGREGATE my_aggregate (double precision) IS 'Computes sample variance';
COMMENT ON CAST (text AS int4) IS 'Allow casts from text to int4';
COMMENT ON COLUMN my_table.my_column IS 'Employee ID number';
COMMENT ON CONVERSION my_conv IS 'Conversion to UTF8';
COMMENT ON DATABASE my_database IS 'Development Database';
COMMENT ON DOMAIN my_domain IS 'Email Address Domain';
COMMENT ON FUNCTION my_function (timestamp) IS 'Returns Roman Numeral';
COMMENT ON INDEX my_index IS 'Enforces uniqueness on employee ID';
COMMENT ON LANGUAGE plpython IS 'Python support for stored procedures';
COMMENT ON LARGE OBJECT 346344 IS 'Planning document';
COMMENT ON OPERATOR ^ (text, text) IS 'Performs intersection of two texts';
COMMENT ON OPERATOR - (NONE, text) IS 'This is a prefix operator on text';
COMMENT ON OPERATOR CLASS int4ops USING btree IS '4 byte integer operators for btrees';
COMMENT ON ROLE my_role IS 'Administration group for finance tables';
COMMENT ON RULE my_rule ON my_table IS 'Logs updates of employee records';
COMMENT ON SCHEMA my_schema IS 'Departmental data';
COMMENT ON SEQUENCE my_sequence IS 'Used to generate primary keys';
COMMENT ON TABLE my_schema.my_table IS 'Employee Information';
COMMENT ON TABLESPACE my_tablespace IS 'Tablespace for indexes';
COMMENT ON TRIGGER my_trigger ON my_table IS 'Used for RI';
COMMENT ON TYPE complex IS 'Complex number data type';
COMMENT ON VIEW my_view IS 'View of departmental costs';
</programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   There is no <command>COMMENT</command> command in the SQL standard.
  </para>
 </refsect1>
</refentry>

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