xoper.sgml

关系型数据库 Postgresql 6.5.2

 <Chapter Id="xoper">
  <Title>Extending <Acronym>SQL</Acronym>: Operators</Title>

  <Para>
   <ProductName>Postgres</ProductName> supports left unary,
   right  unary  and  binary
   operators.   Operators  can  be  overloaded; that is,
   the same operator name can be used for different operators
   that have different numbers and types of arguments.   If
   there  is  an ambiguous situation and the system cannot
   determine the correct operator to use, it  will  return
   an  error.  You may have to typecast the left and/or
   right operands to help it understand which operator you
   meant to use.
  </Para>

  <Para>
   Every operator is "syntactic sugar" for a call to an
   underlying function that does the real work; so you must
   first create the underlying function before you can create
   the operator.  However, an operator is <emphasis>not</emphasis>
   merely syntactic sugar, because it carries additional information
   that helps the query planner optimize queries that use the
   operator.  Much of this chapter will be devoted to explaining
   that additional information.
  </Para>

  <Para>
   Here is an example of creating an operator for adding two
   complex numbers.  We assume we've already created the definition
   of type complex.  First we need a function that does the work;
   then we can define the operator:

   <ProgramListing>
CREATE FUNCTION complex_add(complex, complex)
    RETURNS complex
    AS '$PWD/obj/complex.so'
    LANGUAGE 'c';

CREATE OPERATOR + (
    leftarg = complex,
    rightarg = complex,
    procedure = complex_add,
    commutator = +
);
   </ProgramListing>
  </Para>

  <Para>
   Now we can do:
     
   <ProgramListing>
SELECT (a + b) AS c FROM test_complex;

+----------------+
|c               |
+----------------+
|(5.2,6.05)      |
+----------------+
|(133.42,144.95) |
+----------------+
   </ProgramListing>
  </Para>

  <Para>
   We've shown how to create a binary  operator  here.  To
   create  unary  operators, just omit one of leftarg (for
   left unary) or rightarg (for right unary).  The procedure
   clause and the argument clauses are the only required items
   in CREATE OPERATOR.  The COMMUTATOR clause shown in the example
   is an optional hint to the query optimizer.  Further details about
   COMMUTATOR and other optimizer hints appear below.
  </Para>

  <sect1>
   <title>Operator Optimization Information</title>

   <note>
    <title>Author</title>
    <para>
     Written by Tom Lane.
    </para>
   </note>

   <para>
    A <ProductName>Postgres</ProductName> operator definition can include
    several optional clauses that tell the system useful things about how
    the operator behaves.  These clauses should be provided whenever
    appropriate, because they can make for considerable speedups in execution
    of queries that use the operator.  But if you provide them, you must be
    sure that they are right!  Incorrect use of an optimization clause can
    result in backend crashes, subtly wrong output, or other Bad Things.
    You can always leave out an optimization clause if you are not sure
    about it; the only consequence is that queries might run slower than
    they need to.
   </para>

   <para>
    Additional optimization clauses might be added in future versions of
    <ProductName>Postgres</ProductName>.  The ones described here are all
    the ones that release 6.5 understands.
   </para>

   <sect2>
    <title>COMMUTATOR</title>

    <para>
     The COMMUTATOR clause, if provided, names an operator that is the
     commutator of the operator being defined.  We say that operator A is the
     commutator of operator B if (x A y) equals (y B x) for all possible input
     values x,y.  Notice that B is also the commutator of A.  For example,
     operators '<' and '>' for a particular datatype are usually each others'
     commutators, and operator '+' is usually commutative with itself.
     But operator '-' is usually not commutative with anything.
    </para>

    <para>
     The left argument type of a commuted operator is the same as the
     right argument type of its commutator, and vice versa.  So the name of
     the commutator operator is all that <ProductName>Postgres</ProductName>
     needs to be given to look up the commutator, and that's all that need
     be provided in the COMMUTATOR clause.
    </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 two solutions
     to this problem:

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

      <listitem>
       <para>
	The other, more straightforward way is just to include COMMUTATOR clauses
	in both definitions.  When <ProductName>Postgres</ProductName> processes
	the first definition and realizes that COMMUTATOR refers to a non-existent
	operator, the system will make a dummy entry for that operator in the
	system's pg_operator table.  This dummy entry will have valid data only
	for the operator name, left and right argument types, and result type,
	since that's all that <ProductName>Postgres</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.  (Note: this procedure did not work
	reliably in <ProductName>Postgres</ProductName> versions before 6.5,
	but it is now the recommended way to do things.)
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>

   <sect2>
    <title>NEGATOR</title>

    <para>
     The NEGATOR clause, if provided, names an operator that is the
     negator of the operator being defined.  We say that operator A
     is the negator of operator B if both return boolean results and
     (x A y) equals NOT (x B y) for all possible inputs x,y.
     Notice that B is also the negator of A.
     For example, '<' and '>=' are a negator pair for most datatypes.
     An operator can never be validly be its own negator.
    </para>

   <para>
    Unlike COMMUTATOR, a pair of unary operators could validly be marked
    as each others' negators; that would mean (A x) equals NOT (B x)
    for all x, or the equivalent for right-unary operators.
   </para>

   <para>
    An operator's negator must have the same left and/or right argument types
    as the operator itself, so just as with COMMUTATOR, only the operator
    name need be given in the NEGATOR clause.
   </para>

   <para>
    Providing NEGATOR is very helpful to the query optimizer since
    it allows expressions like NOT (x = y) to be simplified into
    x &lt;&gt; y.  This comes up more often than you might think, because
    NOTs can be inserted as a consequence of other rearrangements.
   </para>

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

  </sect2>

  <sect2>
   <title>RESTRICT</title>

   <para>
    The RESTRICT clause, if provided, names a restriction selectivity
    estimation function for the operator (note that this is a function
    name, not an operator name).  RESTRICT clauses only make sense for
    binary operators that return boolean.  The idea behind a restriction
    selectivity estimator is to guess what fraction of the rows in a
    table will satisfy a WHERE-clause condition of the form
   <ProgramListing>
    		field OP constant
   </ProgramListing>
    for the current operator and a particular constant value.
    This assists the optimizer by
    giving it some idea of how many rows will be eliminated by WHERE
    clauses that have this form.  (What happens if the constant is on
    the left, you may be wondering?  Well, that's one of the things that