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><A
NAME="XOPER"
>35.12. User-defined Operators</A
></H1
><P
> Every operator is <SPAN
CLASS="QUOTE"
>"syntactic sugar"</SPAN
> 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 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not merely</I
></SPAN
>
syntactic sugar, because it carries additional information
that helps the query planner optimize queries that use the
operator. The next section will be devoted to explaining
that additional information.
</P
><P
> <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> 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 operands. When a query is
executed, the system determines the operator to call from the
number and types of the provided operands.
</P
><P
> Here is an example of creating an operator for adding two complex
numbers. We assume we've already created the definition of type
<TT
CLASS="TYPE"
>complex</TT
> (see <A
HREF="xtypes.html"
>Section 35.11</A
>). First we need a
function that does the work, then we can define the operator:
</P><PRE
CLASS="PROGRAMLISTING"
>CREATE FUNCTION complex_add(complex, complex)
RETURNS complex
AS '<TT
CLASS="REPLACEABLE"
><I
>filename</I
></TT
>', 'complex_add'
LANGUAGE C IMMUTABLE STRICT;
CREATE OPERATOR + (
leftarg = complex,
rightarg = complex,
procedure = complex_add,
commutator = +
);</PRE
><P>
</P
><P
> Now we could execute a query like this:
</P><PRE
CLASS="SCREEN"
>SELECT (a + b) AS c FROM test_complex;
c
-----------------
(5.2,6.05)
(133.42,144.95)</PRE
><P>
</P
><P
> We've shown how to create a binary operator here. To create unary
operators, just omit one of <TT
CLASS="LITERAL"
>leftarg</TT
> (for left unary) or
<TT
CLASS="LITERAL"
>rightarg</TT
> (for right unary). The <TT
CLASS="LITERAL"
>procedure</TT
>
clause and the argument clauses are the only required items in
<TT
CLASS="COMMAND"
>CREATE OPERATOR</TT
>. The <TT
CLASS="LITERAL"
>commutator</TT
>
clause shown in the example is an optional hint to the query
optimizer. Further details about <TT
CLASS="LITERAL"
>commutator</TT
> and other
optimizer hints appear in the next section.
</P
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