postgresql/doc/src/sgml/ref/create_operator.sgml

<REFENTRY ID="SQL-CREATEOPERATOR">
 <REFMETA>
  <REFENTRYTITLE>
   CREATE OPERATOR
  </REFENTRYTITLE>
  <REFMISCINFO>SQL - Language Statements</REFMISCINFO>
 </REFMETA>
 <REFNAMEDIV>
  <REFNAME>
   CREATE OPERATOR
  </REFNAME>
  <REFPURPOSE>
   Defines a new user operator
  </REFPURPOSE>
  </refnamediv>
 <REFSYNOPSISDIV>
  <REFSYNOPSISDIVINFO>
   <DATE>1998-09-09</DATE>
  </REFSYNOPSISDIVINFO>
  <SYNOPSIS>
CREATE OPERATOR <replaceable>name</replaceable> (
        PROCEDURE  = <replaceable class="parameter">func_name</replaceable>
     [, LEFTARG    = <replaceable class="parameter">type1</replaceable> ]
     [, RIGHTARG   = <replaceable class="parameter">type2</replaceable> ]
     [, COMMUTATOR = <replaceable class="parameter">com_op</replaceable> ]
     [, NEGATOR    = <replaceable class="parameter">neg_op</replaceable> ]
     [, RESTRICT   = <replaceable class="parameter">res_proc</replaceable> ]
     [, HASHES ]
     [, JOIN       = <replaceable class="parameter">join_proc</replaceable> ]
     [, SORT       = <replaceable class="parameter">sort_op</replaceable> [, ...] ]
    )
  </SYNOPSIS>
  
  <REFSECT2 ID="R2-SQL-CREATEOPERATOR-1">
   <REFSECT2INFO>
    <DATE>1998-09-09</DATE>
   </REFSECT2INFO>
   <TITLE>
    Inputs
   </TITLE>
   <PARA>
   </PARA>
       <VARIABLELIST>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">name</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
	   The operator to be defined. See below for allowable characters.
	  </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">type1</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
The type for the left-hand side of the operator, if any. This option would be
omitted for a right-unary operator.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">type2</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
The type for the right-hand side of the operator, if any. This option would be
omitted for a left-unary operator.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">com_op</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
The corresponding commutative operator.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">neg_op</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
The corresponding negation operator.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">res_proc</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
The corresponding restriction operator.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
HASHES
	 </TERM>
	 <LISTITEM>
	  <PARA>
This operator can support a hash-join algorithm.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">join_proc</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
Procedure supporting table joins.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
	<VARLISTENTRY>
	 <TERM>
	  <replaceable class="parameter">sort_op</replaceable>
	 </TERM>
	 <LISTITEM>
	  <PARA>
Operator to use for sorting.
	  </PARA>
	 </LISTITEM>
	</VARLISTENTRY>
       </variablelist>

  </REFSECT2>
  
  <REFSECT2 ID="R2-SQL-CREATEOPERATOR-2">
   <REFSECT2INFO>
    <DATE>1998-09-09</DATE>
   </REFSECT2INFO>
   <TITLE>
    Outputs
   </TITLE>
   <PARA>
    <VARIABLELIST>
     <VARLISTENTRY>
      <TERM>
       <ReturnValue>CREATE</ReturnValue>
      </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>1998-09-09</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>
   The operator <replaceable class="parameter">name</replaceable>
   is a sequence of up to thirty two (32) characters in any combination
   from the following:
   <literallayout>
    + - * / &lt; &gt; = ~ ! @ # % ^ & | ` ? $ : 
   </literallayout>
   <note>
    <para>
     No  alphabetic characters are allowed in an operator name.
     This enables <productname>Postgres</productname> to parse SQL input
     into tokens without requiring spaces between each token.
     </para>
   </note>   
  </para>
  <para>
   The operator "!=" is mapped to "&lt;&gt;" on input, so they are
   therefore 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>
   Also, 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).
  </para>
  <para>
   The  commutator  operator  is present so that
   <productname>Postgres</productname> can
   reverse the order of the operands if it wishes.
   For example, the operator area-less-than, &lt;&lt;&lt;,
   would have a commutator
   operator, area-greater-than, &gt;&gt;&gt;.
   Hence, the query optimizer could freely  convert:
   <programlisting>
    "0,0,1,1"::box  &gt;&gt;&gt; MYBOXES.description
   </programlisting>
   to
   <programlisting>
    MYBOXES.description &lt;&lt;&lt; "0,0,1,1"::box</programlisting>
  </para>
  <para>
   This  allows  the  execution code to always use the latter
   representation and simplifies the  query  optimizer  some
   what.
  </para>
  <para>
   Suppose  that  an
   operator,  area-equal, ===, exists, as well as an area not
   equal, !==.
   The negator operator allows the query optimizer to convert
   <programlisting>
    NOT MYBOXES.description === "0,0,1,1"::box
   </programlisting>
   to
   <programlisting>
    MYBOXES.description !== "0,0,1,1"::box
   </programlisting>
  </para>
  <para>
   If  a  commutator  operator  name  is  supplied,  
   <productname>Postgres</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 current (new) operator
   as its commutator.  This applies to the negator, as  well.
  </para>
  <para>
   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.
  </para>
  <para>
   The next two specifications are  present  to  support  the
   query  optimizer in performing joins.  
   <productname>Postgres</productname> can always
   evaluate a join (i.e., processing a clause with two  tuple
   variables separated by an operator that returns a boolean)
   by iterative substitution [WONG76].  
   In addition, <productname>Postgres</productname>
   is  planning  on  implementing a hash-join algorithm along
   the lines of [SHAP86]; however, it must know whether  this
   strategy  is  applicable.   
   For example, a hash-join
   algorithm is usable for a clause of the form:
   <programlisting>
    MYBOXES.description === MYBOXES2.description
   </programlisting>
   but not for a clause of the form:
   <programlisting>
    MYBOXES.description &lt;&lt;&lt; MYBOXES2.description.
   </programlisting>
   The HASHES flag gives the needed information to the  query
   optimizer  concerning  whether  a  hash  join  strategy is
   usable for the operator in question.</para>
  <para>
   Similarly, the two sort operators indicate  to  the  query
   optimizer whether merge-sort is a usable join strategy and
   what operators should be used  to  sort  the  two  operand
   classes.   For  the  ===  clause above, the optimizer must
   sort both relations using the operator, &lt;&lt;&lt;.  On the other
   hand, merge-sort is not usable with the clause:
   <programlisting>
    MYBOXES.description &lt;&lt;&lt; MYBOXES2.description
   </programlisting>
  </para>
  <para>
   If  other join strategies are found to be practical,
   <productname>Postgres</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  last  two  pieces of the specification are present so
   the query optimizer  can  estimate  result  sizes.   If  a
   clause of the form:
   <programlisting>
    MYBOXES.description &lt;&lt;&lt; "0,0,1,1"::box
   </programlisting>
   is present in the qualification,
   then <productname>Postgres</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
   define function(l)) which accepts one argument of the correct
   data type and returns a floating point  number.   The
   query  optimizer  simply  calls this function, passing the
   parameter "0,0,1,1" and multiplies the result by the relation
   size to get the desired 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 classes involved  to
   compute the desired expected result size.
  </para>
  <para>
   The difference between the function
   <programlisting>
    my_procedure_1 (MYBOXES.description, "0,0,1,1"::box)
   </programlisting>
   and the operator
   <programlisting>
    MYBOXES.description === "0,0,1,1"::box
   </programlisting>
   is  that  <productname>Postgres</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>1998-09-09</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,
   HASHES,
   JOIN = area-join-procedure,
   SORT = <<<, <<<)
  </ProgramListing>  
 </REFSECT1>
 
 <REFSECT1 ID="R1-SQL-CREATEOPERATOR-3">
  <TITLE>
   Compatibility
  </TITLE>
  <PARA>
   CREATE OPERATOR is a <productname>Postgres</productname> extension.
  </PARA>
  
  <REFSECT2 ID="R2-SQL-CREATEOPERATOR-4">
   <REFSECT2INFO>
    <DATE>1998-09-09</DATE>
   </REFSECT2INFO>
   <TITLE>
    SQL92
   </TITLE>
   <PARA>
    There is no CREATE OPERATOR statement in <acronym>SQL92</acronym>.
   </PARA>
  </refsect2>
 </refsect1>
</REFENTRY>

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