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Diffstat (limited to 'doc/src/sgml/xindex.sgml')
| -rw-r--r-- | doc/src/sgml/xindex.sgml | 69 |
1 files changed, 67 insertions, 2 deletions
diff --git a/doc/src/sgml/xindex.sgml b/doc/src/sgml/xindex.sgml index 85aba8abe7..877709cee6 100644 --- a/doc/src/sgml/xindex.sgml +++ b/doc/src/sgml/xindex.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.34 2003/11/01 01:56:29 petere Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.35 2003/11/12 21:15:45 tgl Exp $ --> <sect1 id="xindex"> @@ -80,7 +80,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/xindex.sgml,v 1.34 2003/11/01 01:56:29 pete The same operator class name can be used for several different index methods (for example, both B-tree and hash index methods have operator classes named - <literal>oid_ops</literal>), but each such class is an independent + <literal>int4_ops</literal>), but each such class is an independent entity and must be defined separately. </para> </sect2> @@ -589,6 +589,71 @@ CREATE OPERATOR CLASS complex_abs_ops </para> </sect2> + <sect2 id="xindex-opclass-crosstype"> + <title>Cross-Data-Type Operator Classes</title> + + <para> + So far we have implicitly assumed that an operator class deals with + only one data type. While there certainly can be only one data type in + a particular index column, it is often useful to index operations that + compare an indexed column to a value of a different data type. This is + presently supported by the B-tree and GiST index methods. + </para> + + <para> + B-trees require the left-hand operand of each operator to be the indexed + data type, but the right-hand operand can be of a different type. There + must be a support function having a matching signature. For example, + the built-in operator class for type <type>bigint</> (<type>int8</>) + allows cross-type comparisons to <type>int4</> and <type>int2</>. It + could be duplicated by this definition: + +<programlisting> +CREATE OPERATOR CLASS int8_ops +DEFAULT FOR TYPE int8 USING btree AS + -- standard int8 comparisons + OPERATOR 1 < , + OPERATOR 2 <= , + OPERATOR 3 = , + OPERATOR 4 >= , + OPERATOR 5 > , + FUNCTION 1 btint8cmp(int8, int8) , + + -- cross-type comparisons to int2 (smallint) + OPERATOR 1 < (int8, int2) , + OPERATOR 2 <= (int8, int2) , + OPERATOR 3 = (int8, int2) , + OPERATOR 4 >= (int8, int2) , + OPERATOR 5 > (int8, int2) , + FUNCTION 1 btint82cmp(int8, int2) , + + -- cross-type comparisons to int4 (integer) + OPERATOR 1 < (int8, int4) , + OPERATOR 2 <= (int8, int4) , + OPERATOR 3 = (int8, int4) , + OPERATOR 4 >= (int8, int4) , + OPERATOR 5 > (int8, int4) , + FUNCTION 1 btint84cmp(int8, int4) ; +</programlisting> + + Notice that this definition <quote>overloads</> the operator strategy and + support function numbers. This is allowed (for B-tree operator classes + only) so long as each instance of a particular number has a different + right-hand data type. The instances that are not cross-type are the + default or primary operators of the operator class. + </para> + + <para> + GiST indexes do not allow overloading of strategy or support function + numbers, but it is still possible to get the effect of supporting + multiple right-hand data types, by assigning a distinct strategy number + to each operator that needs to be supported. The <literal>consistent</> + support function must determine what it needs to do based on the strategy + number, and must be prepared to accept comparison values of the appropriate + data types. + </para> + </sect2> + <sect2 id="xindex-opclass-dependencies"> <title>System Dependencies on Operator Classes</title> |