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220 lines
7.2 KiB
Plaintext
220 lines
7.2 KiB
Plaintext
.\" This is -*-nroff-*-
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.\" XXX standard disclaimer belongs here....
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.\" $Header: /cvsroot/pgsql/doc/man/Attic/create_operator.l,v 1.1.1.1 1996/08/18 22:14:21 scrappy Exp $
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.TH "CREATE OPERATOR" SQL 11/05/95 Postgres95 Postgres95
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.SH NAME
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create operator \(em define a new user operator
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.SH SYNOPSIS
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.nf
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\fBcreate operator\fR operator_name
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\fB(\fR[ \fBleftarg\fR \fB=\fR type-1 ]
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[ \fB,\fR \fBrightarg\fR \fB=\fR type-2 ]
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, \fBprocedure =\fR func_name
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[\fB, commutator =\fR com_op ]
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[\fB, negator =\fR neg_op ]
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[\fB, restrict =\fR res_proc ]
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[\fB, hashes\fR]
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[\fB, join =\fR join_proc ]
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[\fB, sort =\fR sor_op1 {\fB,\fR sor_op2 } ]
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\fB)\fR
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.\" \fB"arg is ("
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.\" type [
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.\" \fB,
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.\" type ]
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.\" \fB)
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.fi
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.SH DESCRIPTION
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This command defines a new user operator,
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.IR "operator_name" .
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The user who defines an operator becomes its owner.
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.PP
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The
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.IR "operator_name"
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is a sequence of up to sixteen punctuation characters. The following
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characters are valid for single-character operator names:
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.nf
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~ ! @ # % ^ & ` ?
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.fi
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If the operator name is more than one character long, it may consist
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of any combination of the above characters or the following additional
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characters:
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.nf
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| $ : + - * / < > =
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.fi
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.PP
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At least one of
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.IR leftarg
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and
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.IR rightarg
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must be defined. For binary operators, both should be defined. For
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right unary operators, only
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.IR arg1
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should be defined, while for left unary operators only
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.IR arg2
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should be defined.
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.PP
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The name of the operator,
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.IR operator_name ,
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can be composed of symbols only. Also, the
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.IR func_name
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procedure must have been previously defined using
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.IR "create function" (l)
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and must have one or two arguments.
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.PP
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.\" that multiple instances of the
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.\" operator must be be evaluated
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.\" For example, consider the area-intersection operator,
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.\" .q A,
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.\" and the following expression:
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.\" .(l
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.\" MYBOXES2.description A \*(lq0,0,1,1\*(rq A MYBOXES.description
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.\" .)l
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.\" .in .5i
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.\" The associativity flag indicates that
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.\" .(l
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.\" (MYBOXES2.description A \*(lq0,0,1,1\*(rq) A MYBOXES.description
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.\" .)l
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.\" .in .5i
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.\" is the same as
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.\" .(l
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.\" MYBOXES2.description A (\*(lq0,0,1,1\*(rq A MYBOXES.description).
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.\" .)l
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The commutator operator is present so that Postgres can reverse the order
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of the operands if it wishes. For example, the operator
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area-less-than, >>>, would have a commutator operator,
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area-greater-than, <<<. Suppose that an operator, area-equal, ===,
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exists, as well as an area not equal, !==. Hence, the query optimizer
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could freely convert:
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.nf
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"0,0,1,1"::box >>> MYBOXES.description
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.fi
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to
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.nf
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MYBOXES.description <<< "0,0,1,1"::box
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.fi
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This allows the execution code to always use the latter representation
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and simplifies the query optimizer somewhat.
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.PP
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The negator operator allows the query optimizer to convert
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.nf
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not MYBOXES.description === "0,0,1,1"::box
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.fi
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to
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.nf
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MYBOXES.description !== "0,0,1,1"::box
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.fi
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If a commutator operator name is supplied, Postgres searches for it in
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the catalog. If it is found and it does not yet have a commutator
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itself, then the commutator's entry is updated to have the current
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(new) operator as its commutator. This applies to the negator, as
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well.
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.PP
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This is to allow the definition of two operators that are the
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commutators or the negators of each other. The first operator should
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be defined without a commutator or negator (as appropriate). When the
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second operator is defined, name the first as the commutator or
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negator. The first will be updated as a side effect.
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.PP
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The next two specifications are present to support the query optimizer
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in performing joins. Postgres can always evaluate a join (i.e.,
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processing a clause with two tuple variables separated by an operator
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that returns a boolean) by iterative substitution [WONG76]. In
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addition, Postgres is planning on implementing a hash-join algorithm
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along the lines of [SHAP86]; however, it must know whether this
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strategy is applicable. For example, a hash-join algorithm is usable
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for a clause of the form:
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.nf
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MYBOXES.description === MYBOXES2.description
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.fi
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but not for a clause of the form:
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.nf
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MYBOXES.description <<< MYBOXES2.description.
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.fi
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The
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.BR hashes
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flag gives the needed information to the query optimizer concerning
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whether a hash join strategy is usable for the operator in question.
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.PP
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Similarly, the two sort operators indicate to the query optimizer
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whether merge-sort is a usable join strategy and what operators should
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be used to sort the two operand classes. For the === clause above,
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the optimizer must sort both relations using the operator, <<<. On
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the other hand, merge-sort is not usable with the clause:
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.nf
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MYBOXES.description <<< MYBOXES2.description
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.fi
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If other join strategies are found to be practical, Postgres will change
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the optimizer and run-time system to use them and will require
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additional specification when an operator is defined. Fortunately,
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the research community invents new join strategies infrequently, and
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the added generality of user-defined join strategies was not felt to
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be worth the complexity involved.
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.PP
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The last two pieces of the specification are present so the query
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optimizer can estimate result sizes. If a clause of the form:
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.nf
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MYBOXES.description <<< "0,0,1,1"::box
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.fi
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is present in the qualification, then Postgres may have to estimate the
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fraction of the instances in MYBOXES that satisfy the clause. The
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function res_proc must be a registered function (meaning it is already
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defined using
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.IR "define function" (l))
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which accepts one argument of the correct data type and returns a
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floating point number. The query optimizer simply calls this
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function, passing the parameter
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.nf
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"0,0,1,1"
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.fi
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and multiplies the result by the relation size to get the desired
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expected number of instances.
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.PP
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Similarly, when the operands of the operator both contain instance
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variables, the query optimizer must estimate the size of the resulting
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join. The function join_proc will return another floating point
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number which will be multiplied by the cardinalities of the two
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classes involved to compute the desired expected result size.
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.PP
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The difference between the function
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.nf
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my_procedure_1 (MYBOXES.description, "0,0,1,1"::box)
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.fi
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and the operator
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.nf
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MYBOXES.description === "0,0,1,1"::box
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.fi
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is that Postgres attempts to optimize operators and can decide to use an
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index to restrict the search space when operators are involved.
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However, there is no attempt to optimize functions, and they are
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performed by brute force. Moreover, functions can have any number of
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arguments while operators are restricted to one or two.
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.SH EXAMPLE
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.nf
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--
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--The following command defines a new operator,
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--area-equality, for the BOX data type.
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--
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create operator === (
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leftarg = box,
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rightarg = box,
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procedure = area_equal_procedure,
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commutator = ===,
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negator = !==,
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restrict = area_restriction_procedure,
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hashes,
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join = area-join-procedure,
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sort = <<<, <<<)
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.\" arg is (box, box)
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.fi
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.SH "SEE ALSO"
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create function(l),
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drop operator(l).
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.SH BUGS
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Operator names cannot be composed of alphabetic characters in
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Postgres.
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.PP
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If an operator is defined before its commuting operator has been defined
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(a case specifically warned against above), a dummy operator with invalid
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fields will be placed in the system catalogs. This may interfere with
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the definition of later operators.
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