create_operator.sgml

关系型数据库 Postgresql 6.5.2

<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>1999-04-14</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> ]
     [, JOIN       = <replaceable class="parameter">join_proc</replaceable> ]
     [, HASHES ]
     [, SORT1      = <replaceable class="parameter">left_sort_op</replaceable> ]
     [, SORT2      = <replaceable class="parameter">right_sort_op</replaceable> ]
    )
  </synopsis>
  
  <refsect2 id="R2-SQL-CREATEOPERATOR-1">
   <refsect2info>
    <date>1999-04-14</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.
       </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 commutator for 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 algorithm.
       </para>
      </listitem>
     </varlistentry>
     <varlistentry>
      <term><replaceable class="parameter">left_sort_op</replaceable></term>
      <listitem>
       <para>
	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>
	Operator that sorts the right-hand data type of this operator.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect2>

  <refsect2 id="R2-SQL-CREATEOPERATOR-2">
   <refsect2info>
    <date>1999-04-14</date>
   </refsect2info>
   <title>
    Outputs
   </title>
   <para>
    <variablelist>
     <varlistentry>
      <term><computeroutput>
CREATE
       </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>1999-04-14</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 should be identified if one exists,
   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 probably 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  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 === "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 newly created 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.  (As of Postgres 6.5,
   it also works to just have both operators refer to each other.)
  </para>
  <para>
   The next three 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>
   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 datatype must mean bitwise equality
   of the representation of the type.  (For example, a datatype that
   contains unused bits that don't matter for equality tests could
   not be hashjoined.)
   The HASHES flag indicates to the query optimizer that a hash join
   may safely be used with this operator.</para>
  <para>
   Similarly, the two sort operators indicate  to  the  query
   optimizer whether merge-sort is a usable join strategy and
   which operators should be used  to  sort  the  two  operand
   classes.  Sort operators should only be provided for an equality
   operator, and they should refer to less-than operators for the
   left and right side data types respectively.
  </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
   <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 "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>1999-04-14</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 = <<<,
   SORT2 = <<<
);
  </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>1999-04-14</date>
   </refsect2info>
   <title>
    SQL92
   </title>
   <para>
    There is no CREATE OPERATOR statement in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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