postgres/doc/src/sgml/ref/create_operator.sgml

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doc/src/sgml/ref/create_operator.sgml
PostgreSQL documentation
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<refentry id="sql-createoperator">
 <indexterm zone="sql-createoperator">
  <primary>CREATE OPERATOR</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>CREATE OPERATOR</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>CREATE OPERATOR</refname>
  <refpurpose>define a new operator</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
CREATE OPERATOR <replaceable>name</replaceable> (
    {FUNCTION|PROCEDURE} = <replaceable class="parameter">function_name</replaceable>
    [, LEFTARG = <replaceable class="parameter">left_type</replaceable> ] [, RIGHTARG = <replaceable class="parameter">right_type</replaceable> ]
    [, COMMUTATOR = <replaceable class="parameter">com_op</replaceable> ] [, NEGATOR = <replaceable class="parameter">neg_op</replaceable> ]
    [, RESTRICT = <replaceable class="parameter">res_proc</replaceable> ] [, JOIN = <replaceable class="parameter">join_proc</replaceable> ]
    [, HASHES ] [, MERGES ]
)
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>CREATE OPERATOR</command> defines a new operator,
   <replaceable class="parameter">name</replaceable>.  The user who
   defines an operator becomes its owner.  If a schema name is given
   then the operator is created in the specified schema.  Otherwise it
   is created in the current schema.
  </para>

  <para>
   The operator name is a sequence of up to <symbol>NAMEDATALEN</symbol>-1
   (63 by default) characters from the following list:
<literallayout>
+ - * / &lt; &gt; = ~ ! @ # % ^ &amp; | ` ?
</literallayout>

   There are a few restrictions on your choice of name:
   <itemizedlist>
    <listitem>
     <para>
     <literal>--</literal> and <literal>/*</literal> cannot appear anywhere in an operator name,
     since they will be taken as the start of a comment.
     </para>
    </listitem>
    <listitem>
     <para>
     A multicharacter operator name cannot end in <literal>+</literal> or
     <literal>-</literal>,
     unless the name also contains at least one of these characters:
<literallayout>
~ ! @ # % ^ &amp; | ` ?
</literallayout>
     For example, <literal>@-</literal> is an allowed operator name,
     but <literal>*-</literal> is not.
     This restriction allows <productname>PostgreSQL</productname> to
     parse SQL-compliant commands without requiring spaces between tokens.
     </para>
    </listitem>
    <listitem>
     <para>
     The symbol <literal>=&gt;</literal> is reserved by the SQL grammar,
     so it cannot be used as an operator name.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   The operator <literal>!=</literal> is mapped to
   <literal>&lt;&gt;</literal> on input, so these two names are always
   equivalent.
  </para>

  <para>
   For binary operators, both <literal>LEFTARG</literal> and
   <literal>RIGHTARG</literal> must be defined.  For prefix operators only
   <literal>RIGHTARG</literal> should be defined.
   The <replaceable class="parameter">function_name</replaceable>
   function must have been previously defined using <command>CREATE
   FUNCTION</command> and must be defined to accept the correct number
   of arguments (either one or two) of the indicated types.
  </para>

  <para>
   In the syntax of <literal>CREATE OPERATOR</literal>, the keywords
   <literal>FUNCTION</literal> and <literal>PROCEDURE</literal> are
   equivalent, but the referenced function must in any case be a function, not
   a procedure.  The use of the keyword <literal>PROCEDURE</literal> here is
   historical and deprecated.
  </para>

  <para>
   The other clauses specify optional operator optimization attributes.
   Their meaning is detailed in <xref linkend="xoper-optimization"/>.
  </para>

  <para>
   To be able to create an operator, you must have <literal>USAGE</literal>
   privilege on the argument types and the return type, as well
   as <literal>EXECUTE</literal> privilege on the underlying function.  If a
   commutator or negator operator is specified, you must own those operators.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
        The name of the operator to be defined. See above for allowable
        characters.  The name can be schema-qualified, for example
        <literal>CREATE OPERATOR myschema.+ (...)</literal>.  If not, then
        the operator is created in the current schema.  Two operators
        in the same schema can have the same name if they operate on
        different data types.  This is called
        <firstterm>overloading</firstterm>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">function_name</replaceable></term>
      <listitem>
       <para>
        The function used to implement this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">left_type</replaceable></term>
      <listitem>
       <para>
        The data type of the operator's left operand, if any.
        This option would be omitted for a prefix operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">right_type</replaceable></term>
      <listitem>
       <para>
        The data type of the operator's right operand.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">com_op</replaceable></term>
      <listitem>
       <para>
        The commutator of this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">neg_op</replaceable></term>
      <listitem>
       <para>
        The negator of this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">res_proc</replaceable></term>
      <listitem>
       <para>
        The restriction selectivity estimator function for this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">join_proc</replaceable></term>
      <listitem>
       <para>
        The join selectivity estimator function for this operator.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>HASHES</literal></term>
      <listitem>
       <para>
       Indicates this operator can support a hash join.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>MERGES</literal></term>
      <listitem>
       <para>
       Indicates this operator can support a merge join.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

  <para>
   To give a schema-qualified operator name in <replaceable
   class="parameter">com_op</replaceable> or the other optional
   arguments, use the <literal>OPERATOR()</literal> syntax, for example:
<programlisting>
COMMUTATOR = OPERATOR(myschema.===) ,
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   Refer to <xref linkend="xoper"/> and <xref linkend="xoper-optimization"/>
   for further information.
  </para>

  <para>
   When you are defining a self-commutative operator, you just do it.
   When you are defining a pair of commutative operators, things are
   a little trickier: how can the first one to be defined refer to the
   other one, which you haven't defined yet?  There are three solutions
   to this problem:

   <itemizedlist>
    <listitem>
     <para>
      One way is to omit the <literal>COMMUTATOR</literal> clause in the
      first operator that you define, and then provide one in the second
      operator's definition.  Since <productname>PostgreSQL</productname>
      knows that commutative operators come in pairs, when it sees the
      second definition it will automatically go back and fill in the
      missing <literal>COMMUTATOR</literal> clause in the first
      definition.
     </para>
    </listitem>

    <listitem>
     <para>
      Another, more straightforward way is just to
      include <literal>COMMUTATOR</literal> clauses in both definitions.
      When <productname>PostgreSQL</productname> processes the first
      definition and realizes that <literal>COMMUTATOR</literal> refers to
      a nonexistent operator, the system will make a dummy entry for that
      operator in the system catalog.  This dummy entry will have valid
      data only for the operator name, left and right operand types, and
      owner, since that's all that <productname>PostgreSQL</productname>
      can deduce at this point.  The first operator's catalog entry will
      link to this dummy entry.  Later, when you define the second
      operator, the system updates the dummy entry with the additional
      information from the second definition.  If you try to use the dummy
      operator before it's been filled in, you'll just get an error
      message.
     </para>
    </listitem>

    <listitem>
     <para>
      Alternatively, both operators can be defined
      without <literal>COMMUTATOR</literal> clauses
      and then <command>ALTER OPERATOR</command> can be used to set their
      commutator links.  It's sufficient to <command>ALTER</command>
      either one of the pair.
     </para>
    </listitem>
   </itemizedlist>

   In all three cases, you must own both operators in order to mark
   them as commutators.
  </para>

  <para>
   Pairs of negator operators can be defined using the same methods
   as for commutator pairs.
  </para>

  <para>
   It is not possible to specify an operator's lexical precedence in
   <command>CREATE OPERATOR</command>, because the parser's precedence behavior
   is hard-wired.  See <xref linkend="sql-precedence"/> for precedence details.
  </para>

  <para>
   The obsolete options <literal>SORT1</literal>, <literal>SORT2</literal>,
   <literal>LTCMP</literal>, and <literal>GTCMP</literal> were formerly used to
   specify the names of sort operators associated with a merge-joinable
   operator.  This is no longer necessary, since information about
   associated operators is found by looking at B-tree operator families
   instead.  If one of these options is given, it is ignored except
   for implicitly setting <literal>MERGES</literal> true.
  </para>

  <para>
   Use <link linkend="sql-dropoperator"><command>DROP OPERATOR</command></link> to delete user-defined operators
   from a database.  Use <link linkend="sql-alteroperator"><command>ALTER OPERATOR</command></link> to modify operators in a
   database.
  </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   The following command defines a new operator, area-equality, for
   the data type <type>box</type>:
<programlisting>
CREATE OPERATOR === (
    LEFTARG = box,
    RIGHTARG = box,
    FUNCTION = area_equal_function,
    COMMUTATOR = ===,
    NEGATOR = !==,
    RESTRICT = area_restriction_function,
    JOIN = area_join_function,
    HASHES, MERGES
);
</programlisting></para>
 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   <command>CREATE OPERATOR</command> is a
   <productname>PostgreSQL</productname> extension.  There are no
   provisions for user-defined operators in the SQL standard.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-alteroperator"/></member>
   <member><xref linkend="sql-createopclass"/></member>
   <member><xref linkend="sql-dropoperator"/></member>
  </simplelist>
 </refsect1>
</refentry>