create_operator.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_operator.sgml,v 1.29 2002/05/18 15:44:47 petere Exp $
PostgreSQL documentation
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<refentry id="SQL-CREATEOPERATOR">
 <refmeta>
  <refentrytitle id="sql-createoperator-title">CREATE OPERATOR</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>
 <refnamediv>
  <refname>
   CREATE OPERATOR
  </refname>
  <refpurpose>
   define a new operator
  </refpurpose>
  </refnamediv>
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>2000-03-25</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class="parameter">func_name</replaceable>
     [, LEFTARG = <replaceable class="parameter">lefttype</replaceable>
     ] [, RIGHTARG = <replaceable class="parameter">righttype</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 ]
     [, SORT1 = <replaceable class="parameter">left_sort_op</replaceable> ] [, SORT2 = <replaceable class="parameter">right_sort_op</replaceable> ]
     [, LTCMP = <replaceable class="parameter">less_than_op</replaceable> ] [, GTCMP = <replaceable class="parameter">greater_than_op</replaceable> ] )
  </synopsis>
  
  <refsect2 id="R2-SQL-CREATEOPERATOR-1">
   <refsect2info>
    <date>2000-03-25</date>
   </refsect2info>
   <title>
    Inputs
   </title>
   <para>

    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
	The operator to be defined. See below for allowable characters.
	The name may be schema-qualified, for example
	<literal>CREATE OPERATOR myschema.+ (...)</>.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">func_name</replaceable></term>
      <listitem>
       <para>
	The function used to implement this operator.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">lefttype</replaceable></term>
      <listitem>
       <para>
	The type of the left-hand argument of the operator, if any.
	This option would be omitted for a left-unary operator.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">righttype</replaceable></term>
      <listitem>
       <para>
	The type of the right-hand argument of the operator, if any.
	This option would be omitted for a right-unary operator.
       </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>HASHES</term>
      <listitem>
       <para>
       Indicates this operator can support a hash join.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term>MERGES</term>
      <listitem>
       <para>
       Indicates this operator can support a merge join.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">left_sort_op</replaceable></term>
      <listitem>
       <para>
	If this operator can support a merge join, the less-than
	operator that sorts the left-hand data type of this operator.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">right_sort_op</replaceable></term>
      <listitem>
       <para>
	If this operator can support a merge join, the less-than
	operator that sorts the right-hand data type of this operator.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">less_than_op</replaceable></term>
      <listitem>
       <para>
	If this operator can support a merge join, the less-than
	operator that compares the input data types of this operator.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">greater_than_op</replaceable></term>
      <listitem>
       <para>
	If this operator can support a merge join, the greater-than
	operator that compares the input data types of this operator.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATEOPERATOR-2">
   <refsect2info>
    <date>2000-03-25</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>
    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE OPERATOR
       </computeroutput></term>
      <listitem>
       <para>
	Message returned if the operator is successfully created.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>
 </refsynopsisdiv>

 <refsect1 id="R1-SQL-CREATEOPERATOR-1">
  <refsect1info>
   <date>2000-03-25</date>
  </refsect1info>
  <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.
  </para>
  <para>
   If a schema name is given then the operator is created in the
   specified schema.  Otherwise it is created in the current schema (the one
   at the front of the search path; see <literal>CURRENT_SCHEMA()</>).
  </para>
  <para>
   Two operators in the same schema can have the same name if they operate on
   different data types.  This is called <firstterm>overloading</>.  The
   system will attempt to pick the intended operator based on the actual
   input data types when there is ambiguity.
  </para>

  <para>
   The operator <replaceable class="parameter">name</replaceable>
   is a sequence of up to <symbol>NAMEDATALEN</>-1 (31 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> cannot be defined as a single-character operator,
     although it can be part of a multicharacter operator name.
     </para>
    </listitem>
    <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 queries without requiring spaces between tokens.
     </para>
    </listitem>
   </itemizedlist>

   <note>
    <para>
     When working with non-SQL-standard operator names, you will usually
     need to separate adjacent operators with spaces to avoid ambiguity.
     For example, if you have defined a left-unary operator named <literal>@</literal>,
     you cannot write <literal>X*@Y</literal>; you must write
     <literal>X* @Y</literal> to ensure that
     <productname>PostgreSQL</productname> reads it as two operator names
     not one.
     </para>
   </note>   
  </para>
  <para>
   The operator <literal>!=</literal> is mapped to <literal>&lt;&gt;</literal> on input, so these two names
   are always equivalent.
  </para>
  <para>
   At least one of LEFTARG and RIGHTARG must be defined.  For
   binary operators, both should be defined. For right  unary
   operators,  only  LEFTARG  should  be defined, while for left
   unary operators only RIGHTARG should be defined.
  </para>
  <para>
   The
   <replaceable class="parameter">func_name</replaceable> procedure 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>
   The commutator operator should be identified if one exists,
   so that <productname>PostgreSQL</productname> can
   reverse the order of the operands if it wishes.
   For example, the operator area-less-than, &lt;&lt;&lt;,
   would probably have a commutator
   operator, area-greater-than, &gt;&gt;&gt;.
   Hence, the query optimizer could freely  convert:

   <programlisting>
box '((0,0), (1,1))'  &gt;&gt;&gt; MYBOXES.description
   </programlisting>

   to

   <programlisting>
MYBOXES.description &lt;&lt;&lt; box '((0,0), (1,1))'
   </programlisting>
  </para>
  <para>
   This  allows  the  execution code to always use the latter
   representation and simplifies the  query  optimizer  somewhat.
  </para>
  <para>
   Similarly, if there is a negator operator then it should be
   identified.
   Suppose  that  an
   operator,  area-equal, ===, exists, as well as an area not
   equal, !==.
   The negator link allows the query optimizer to simplify
   <programlisting>
NOT MYBOXES.description === box '((0,0), (1,1))'
   </programlisting>
   to
   <programlisting>
MYBOXES.description !== box '((0,0), (1,1))'
   </programlisting>
  </para>
  <para>
   If  a  commutator  operator  name  is  supplied,  
   <productname>PostgreSQL</productname>
   searches  for  it  in  the catalog.  If it is found and it
   does not yet have a commutator itself, then the commutator's
   entry is updated to have the newly created operator as its
   commutator.  This applies to the negator, as well.
   This  is to allow the definition of two operators that are
   the commutators or the negators of each other.  The  first
   operator should be defined without a commutator or negator
   (as appropriate).  When the second  operator  is  defined,
   name  the  first  as the commutator or negator.  The first
   will be updated as a side effect.  (As of
   <application>PostgreSQL</application> <literal>6.5</literal>, 
   it also works to just have both operators refer to each other.)
  </para>
  <para>
   The HASHES, MERGES, SORT1, SORT2, LTCMP, and GTCMP options are present to
   support the query optimizer in performing joins.
   <productname>PostgreSQL</productname> can always evaluate a join (i.e.,
   processing a clause with two tuple variables separated by an operator that
   returns a <type>boolean</type>) by iterative substitution [WONG76].  In
   addition, <productname>PostgreSQL</productname> can use a hash-join
   algorithm along the lines of [SHAP86]; however, it must know whether this
   strategy is applicable.  The current hash-join algorithm is only correct
   for operators that represent equality tests; furthermore, equality of the
   data type must mean bitwise equality of the representation of the type.
   (For example, a data type that contains unused bits that don't matter for
   equality tests could not be hash-joined.)  The HASHES flag indicates to the
   query optimizer that a hash join may safely be used with this
   operator.
  </para>
  <para>
   Similarly, the MERGES flag indicates whether merge-sort is a usable join
   strategy for this operator.  A merge join requires that the two input
   datatypes have consistent orderings, and that the mergejoin operator
   behave like equality with respect to that ordering.  For example, it is
   possible to merge-join equality between an integer and a float variable by
   sorting both inputs in ordinary 
   numeric order.  Execution of a merge join requires that the system be
   able to identify four operators related to the mergejoin equality operator:
   less-than comparison for the left input datatype,
   less-than comparison for the right input datatype,
   less-than comparison between the two datatypes, and
   greater-than comparison between the two datatypes.  It is possible to
   specify these by name, as the SORT1, SORT2, LTCMP, and GTCMP options
   respectively.  The system will fill in the default names <literal>&lt;</>,
   <literal>&lt;</>, <literal>&lt;</>, <literal>&gt;</> respectively if
   any of these are omitted when MERGES is specified.  Also, MERGES will
   be assumed to be implied if any of these four operator options appear.
  </para>
  <para>
   If  other join strategies are found to be practical,
   <productname>PostgreSQL</productname>
   will change the optimizer and run-time system to  use
   them  and  will  require  additional specification when an
   operator is defined.  Fortunately, the research  community
   invents  new  join  strategies infrequently, and the added
   generality of user-defined join strategies was not felt to
   be worth the complexity involved.
  </para>
  <para>
   The RESTRICT and JOIN options assist the query optimizer in estimating
   result sizes.  If a clause of the form:
   <programlisting>
MYBOXES.description &lt;&lt;&lt; box '((0,0), (1,1))'
   </programlisting>
   is present in the qualification,
   then <productname>PostgreSQL</productname> may have to
   estimate the fraction of the  instances  in  MYBOXES  that
   satisfy  the clause.  The function
   <replaceable class="parameter">res_proc</replaceable>
   must be a registered function (meaning  it  is  already  defined  using
   <command>CREATE FUNCTION</command>) which accepts arguments of the correct
   data types and returns a floating-point number.   The
   query  optimizer  simply  calls this function, passing the
   parameter <literal>((0,0), (1,1))</literal> and multiplies the result by the relation
   size to get the expected number of instances.
  </para>
  <para>
   Similarly, when the operands of the operator both  contain
   instance  variables, the query optimizer must estimate the
   size of the resulting join.  The function  join_proc  will
   return  another floating-point number which will be multiplied
   by the cardinalities of the two tables involved  to
   compute the expected result size.
  </para>
  <para>
   The difference between the function
   <programlisting>
my_procedure_1 (MYBOXES.description, box '((0,0), (1,1))')
   </programlisting>
   and the operator
   <programlisting>
MYBOXES.description === box '((0,0), (1,1))'
   </programlisting>
   is  that  <productname>PostgreSQL</productname>
   attempts to optimize operators and can
   decide to use an index to restrict the search  space  when
   operators  are  involved.  However, there is no attempt to
   optimize functions, and they are performed by brute force.
   Moreover, functions can have any number of arguments while
   operators are restricted to one or two.
  </para>
  
  <refsect2 id="R2-SQL-CREATEOPERATOR-3">
   <refsect2info>
    <date>2000-03-25</date>
   </refsect2info>
   <title>
    Notes
   </title>
   <para>
    Refer to the chapter on operators in the
    <citetitle>PostgreSQL User's Guide</citetitle>
    for further information.
    Refer to <command>DROP OPERATOR</command> to delete
    user-defined operators from a database.
   </para>
  </refsect2>
 </refsect1>
  
 <refsect1 id="R1-SQL-CREATEOPERATOR-2">
  <title>
   Usage
  </title>
  <para>The following command defines a new operator,
   area-equality, for the BOX data type:
  </para>
  <programlisting>
CREATE OPERATOR === (
   LEFTARG = box,
   RIGHTARG = box,
   PROCEDURE = area_equal_procedure,
   COMMUTATOR = ===,
   NEGATOR = !==,
   RESTRICT = area_restriction_procedure,
   JOIN = area_join_procedure,
   HASHES,
   SORT1 = &lt;&lt;&lt;,
   SORT2 = &lt;&lt;&lt;
   -- Since sort operators were given, MERGES is implied.
   -- LTCMP and GTCMP are assumed to be &lt; and &gt; respectively
);
  </programlisting>  
 </refsect1>
 
 <refsect1 id="R1-SQL-CREATEOPERATOR-3">
  <title>
   Compatibility
  </title>
  
  <refsect2 id="R2-SQL-CREATEOPERATOR-4">
   <refsect2info>
    <date>2000-03-25</date>
   </refsect2info>
   <title>
    SQL92
   </title>

   <para>
    <command>CREATE OPERATOR</command>
    is a <productname>PostgreSQL</productname> extension.
    There is no <command>CREATE OPERATOR</command>
    statement in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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