diff options
Diffstat (limited to 'doc/src/sgml/ref/create_operator.sgml')
| -rw-r--r-- | doc/src/sgml/ref/create_operator.sgml | 97 |
1 files changed, 68 insertions, 29 deletions
diff --git a/doc/src/sgml/ref/create_operator.sgml b/doc/src/sgml/ref/create_operator.sgml index e3b41ec5dac..bbe378594e1 100644 --- a/doc/src/sgml/ref/create_operator.sgml +++ b/doc/src/sgml/ref/create_operator.sgml @@ -1,5 +1,5 @@ <!-- -$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_operator.sgml,v 1.25 2002/03/22 19:20:39 petere Exp $ +$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_operator.sgml,v 1.26 2002/04/16 23:08:10 tgl Exp $ PostgreSQL documentation --> @@ -28,7 +28,9 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class ] [, RIGHTARG = <replaceable class="parameter">righttype</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> ] ) + [, HASHES ] [, MERGES ] + [, SORT1 = <replaceable class="parameter">left_sort_op</replaceable> ] [, SORT2 = <replaceable class="parameter">right_sort_op</replaceable> ] + [, LTCMP = <replaceable class="parameter">less_than_op</replaceable> ] [, GTCMP = <replaceable class="parameter">greater_than_op</replaceable> ] ) </synopsis> <refsect2 id="R2-SQL-CREATEOPERATOR-1"> @@ -116,10 +118,18 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class </listitem> </varlistentry> <varlistentry> + <term>MERGES</term> + <listitem> + <para> + Indicates this operator can support a merge join. + </para> + </listitem> + </varlistentry> + <varlistentry> <term><replaceable class="parameter">left_sort_op</replaceable></term> <listitem> <para> - If this operator can support a merge join, the + If this operator can support a merge join, the less-than operator that sorts the left-hand data type of this operator. </para> </listitem> @@ -128,11 +138,29 @@ CREATE OPERATOR <replaceable>name</replaceable> ( PROCEDURE = <replaceable class <term><replaceable class="parameter">right_sort_op</replaceable></term> <listitem> <para> - If this operator can support a merge join, the + If this operator can support a merge join, the less-than operator that sorts the right-hand data type of this operator. </para> </listitem> </varlistentry> + <varlistentry> + <term><replaceable class="parameter">less_than_op</replaceable></term> + <listitem> + <para> + If this operator can support a merge join, the less-than + operator that compares the input data types of this operator. + </para> + </listitem> + </varlistentry> + <varlistentry> + <term><replaceable class="parameter">greater_than_op</replaceable></term> + <listitem> + <para> + If this operator can support a merge join, the greater-than + operator that compares the input data types of this operator. + </para> + </listitem> + </varlistentry> </variablelist> </para> </refsect2> @@ -295,30 +323,39 @@ MYBOXES.description !== box '((0,0), (1,1))' it also works to just have both operators refer to each other.) </para> <para> - The HASHES, SORT1, and SORT2 options are present to support the - query optimizer in performing joins. - <productname>PostgreSQL</productname> can always - evaluate a join (i.e., processing a clause with two tuple - variables separated by an operator that returns a <type>boolean</type>) - by iterative substitution [WONG76]. - In addition, <productname>PostgreSQL</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 data type must mean bitwise equality - of the representation of the type. (For example, a data type that - contains unused bits that don't matter for equality tests could - not be hash-joined.) - The HASHES flag indicates to the query optimizer that a hash join - may safely be used with this operator.</para> + The HASHES, MERGES, SORT1, SORT2, LTCMP, and GTCMP options are present to + support the query optimizer in performing joins. + <productname>PostgreSQL</productname> can always evaluate a join (i.e., + processing a clause with two tuple variables separated by an operator that + returns a <type>boolean</type>) by iterative substitution [WONG76]. In + addition, <productname>PostgreSQL</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 + data type must mean bitwise equality of the representation of the type. + (For example, a data type that contains unused bits that don't matter for + equality tests could not be hash-joined.) 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. + Similarly, the MERGES flag indicates whether merge-sort is a usable join + strategy for this operator. A merge join requires that the two input + datatypes have consistent orderings, and that the mergejoin operator + behave like equality with respect to that ordering. For example, it is + possible to merge-join equality between an integer and a float variable by + sorting both inputs in ordinary + numeric order. Execution of a merge join requires that the system be + able to identify four operators related to the mergejoin equality operator: + less-than comparison for the left input datatype, + less-than comparison for the right input datatype, + less-than comparison between the two datatypes, and + greater-than comparison between the two datatypes. It is possible to + specify these by name, as the SORT1, SORT2, LTCMP, and GTCMP options + respectively. The system will fill in the default names <literal><</>, + <literal><</>, <literal><</>, <literal>></> respectively if + any of these are omitted when MERGES is specified. Also, MERGES will + be assumed to be implied if any of these four operator options appear. </para> <para> If other join strategies are found to be practical, @@ -408,8 +445,10 @@ CREATE OPERATOR === ( RESTRICT = area_restriction_procedure, JOIN = area_join_procedure, HASHES, - SORT1 = <<<, - SORT2 = <<< + SORT1 = <<<, + SORT2 = <<< + -- Since sort operators were given, MERGES is implied. + -- LTCMP and GTCMP are assumed to be < and > respectively ); </programlisting> </refsect1> |