</refnamediv>
<REFSYNOPSISDIV>
<REFSYNOPSISDIVINFO>
- <DATE>1998-09-09</DATE>
+ <DATE>1999-04-14</DATE>
</REFSYNOPSISDIVINFO>
<SYNOPSIS>
CREATE OPERATOR <replaceable>name</replaceable> (
[, COMMUTATOR = <replaceable class="parameter">com_op</replaceable> ]
[, NEGATOR = <replaceable class="parameter">neg_op</replaceable> ]
[, RESTRICT = <replaceable class="parameter">res_proc</replaceable> ]
- [, HASHES ]
[, JOIN = <replaceable class="parameter">join_proc</replaceable> ]
- [, SORT = <replaceable class="parameter">sort_op</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>1998-09-09</DATE>
+ <DATE>1999-04-14</DATE>
</REFSECT2INFO>
<TITLE>
Inputs
</TERM>
<LISTITEM>
<PARA>
-The corresponding commutative operator.
+The commutator for this operator.
</PARA>
</LISTITEM>
</VARLISTENTRY>
</TERM>
<LISTITEM>
<PARA>
-The corresponding negation operator.
+The negator of this operator.
</PARA>
</LISTITEM>
</VARLISTENTRY>
</TERM>
<LISTITEM>
<PARA>
-The corresponding restriction operator.
+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>
</TERM>
<LISTITEM>
<PARA>
-This operator can support a hash-join algorithm.
+Indicates this operator can support a hash-join algorithm.
</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>
- <replaceable class="parameter">join_proc</replaceable>
+ <replaceable class="parameter">left_sort_op</replaceable>
</TERM>
<LISTITEM>
<PARA>
-Procedure supporting table joins.
+Operator that sorts the left-hand data type of this operator.
</PARA>
</LISTITEM>
</VARLISTENTRY>
<VARLISTENTRY>
<TERM>
- <replaceable class="parameter">sort_op</replaceable>
+ <replaceable class="parameter">right_sort_op</replaceable>
</TERM>
<LISTITEM>
<PARA>
-Operator to use for sorting.
+Operator that sorts the right-hand data type of this operator.
</PARA>
</LISTITEM>
</VARLISTENTRY>
<REFSECT2 ID="R2-SQL-CREATEOPERATOR-2">
<REFSECT2INFO>
- <DATE>1998-09-09</DATE>
+ <DATE>1999-04-14</DATE>
</REFSECT2INFO>
<TITLE>
Outputs
<REFSECT1 ID="R1-SQL-CREATEOPERATOR-1">
<REFSECT1INFO>
- <DATE>1998-09-09</DATE>
+ <DATE>1999-04-14</DATE>
</REFSECT1INFO>
<TITLE>
Description
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 current (new) operator
- as its commutator. This applies to the negator, as well.
+ 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
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.
+ 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 two specifications are present to support the
+ 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>
- is planning on implementing a hash-join algorithm along
+ can use a hash-join algorithm along
the lines of [SHAP86]; however, it must know whether this
- strategy is applicable.
- For example, a hash-join
- algorithm is usable for a clause of the form:
- <programlisting>
- MYBOXES.description === MYBOXES2.description
- </programlisting>
- but not for a clause of the form:
- <programlisting>
- MYBOXES.description <<< MYBOXES2.description.
- </programlisting>
- The HASHES flag gives the needed information to the query
- optimizer concerning whether a hash join strategy is
- usable for the operator in question.</para>
+ 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
- what operators should be used to sort the two operand
- classes. For the === clause above, the optimizer must
- sort both relations using the operator, <<<. On the other
- hand, merge-sort is not usable with the clause:
- <programlisting>
- MYBOXES.description <<< MYBOXES2.description
- </programlisting>
+ 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,
<REFSECT2 ID="R2-SQL-CREATEOPERATOR-3">
<REFSECT2INFO>
- <DATE>1998-09-09</DATE>
+ <DATE>1999-04-14</DATE>
</REFSECT2INFO>
<TITLE>
Notes
COMMUTATOR = ===,
NEGATOR = !==,
RESTRICT = area_restriction_procedure,
+ JOIN = area_join_procedure,
HASHES,
- JOIN = area-join-procedure,
- SORT = <<<, <<<)
+ SORT1 = <<<,
+ SORT2 = <<<)
</ProgramListing>
</REFSECT1>
<REFSECT2 ID="R2-SQL-CREATEOPERATOR-4">
<REFSECT2INFO>
- <DATE>1998-09-09</DATE>
+ <DATE>1999-04-14</DATE>
</REFSECT2INFO>
<TITLE>
SQL92
.\" This is -*-nroff-*-
.\" XXX standard disclaimer belongs here....
-.\" $Header: /cvsroot/pgsql/src/man/Attic/create_operator.l,v 1.7 1998/07/25 00:17:30 momjian Exp $
+.\" $Header: /cvsroot/pgsql/src/man/Attic/create_operator.l,v 1.8 1999/04/15 00:09:00 tgl Exp $
.TH "CREATE OPERATOR" SQL 11/05/95 PostgreSQL PostgreSQL
.SH NAME
create operator - define a new user operator
[\fB, commutator =\fR com_op ]
[\fB, negator =\fR neg_op ]
[\fB, restrict =\fR res_proc ]
- [\fB, hashes\fR]
[\fB, join =\fR join_proc ]
- [\fB, sort =\fR sor_op1 {\fB,\fR sor_op2 } ]
+ [\fB, hashes\fR]
+ [\fB, sort1 =\fR left_sort_op ]
+ [\fB, sort2 =\fR right_sort_op ]
\fB)\fR
.\" \fB"arg is ("
.\" type [
The commutator operator is present so that Postgres can reverse the order
of the operands if it wishes. For example, the operator
area-less-than, >>>, would have a commutator operator,
-area-greater-than, <<<. Suppose that an operator, area-equal, ===,
-exists, as well as an area not equal, !==. Hence, the query optimizer
+area-greater-than, <<<. Hence, the query optimizer
could freely convert:
.nf
This allows the execution code to always use the latter representation
and simplifies the query optimizer somewhat.
.PP
+Suppose that an operator, area-equal, ===,
+exists, as well as an area not equal, !==.
The negator operator allows the query optimizer to convert
.nf
.fi
If a commutator operator name is supplied, Postgres 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 current
-(new) operator as its commutator. This applies to the negator, as
-well.
+itself, then the commutator's entry is updated to have the newly created
+operator as its commutator. This applies to the negator, as well.
.PP
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.
+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.)
.PP
-The next two specifications are present to support the query optimizer
+The next three specifications are present to support the query optimizer
in performing joins. Postgres 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, Postgres is planning on implementing a hash-join algorithm
+addition, Postgres can use a hash-join algorithm
along the lines of [SHAP86]; however, it must know whether this
-strategy is applicable. For example, a hash-join algorithm is usable
-for a clause of the form:
-.nf
-
-.ce 1
-MYBOXES.description === MYBOXES2.description
-
-.fi
-but not for a clause of the form:
-.nf
-
-.ce 1
-MYBOXES.description <<< MYBOXES2.description.
-
-.fi
+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
.BR hashes
-flag gives the needed information to the query optimizer concerning
-whether a hash join strategy is usable for the operator in question.
+flag indicates to the query optimizer that a hash join may safely be
+used with this operator.
.PP
Similarly, the two sort operators indicate to the query optimizer
-whether merge-sort is a usable join strategy and what operators should
-be used to sort the two operand classes. For the === clause above,
-the optimizer must sort both relations using the operator, <<<. On
-the other hand, merge-sort is not usable with the clause:
-.nf
-
-.ce 1
-MYBOXES.description <<< MYBOXES2.description
-
-.fi
+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.
+.PP
If other join strategies are found to be practical, Postgres will change
the optimizer and run-time system to use them and will require
additional specification when an operator is defined. Fortunately,
commutator = ===,
negator = !==,
restrict = area_restriction_procedure,
+ join = area_join_procedure,
hashes,
- join = area-join-procedure,
- sort = <<<, <<<)
+ sort1 = <<<,
+ sort2 = <<<)
.\" arg is (box, box)
.fi
.SH "SEE ALSO"
Operator names cannot be composed of alphabetic characters in
Postgres.
.PP
-If an operator is defined before its commuting operator has been defined
-(a case specifically warned against above), a dummy operator with invalid
-fields will be placed in the system catalogs. This may interfere with
-the definition of later operators.
+If an operator is defined before its commuting operator has been defined,
+a dummy entry for the commutator (with invalid oprproc field) will be placed
+in the system catalogs. This entry will be overridden when the commutator
+is eventually defined.
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