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CREATE
NAME
CREATE OPERATOR - define a new operator
SYNOPSIS
CREATE OPERATOR name ( PROCEDURE = func_name
[, LEFTARG = lefttype
] [, RIGHTARG = righttype ]
[, COMMUTATOR = com_op ] [, NEGATOR = neg_op ]
[, RESTRICT = res_proc ] [, JOIN = join_proc ]
[, HASHES ] [, SORT1 = left_sort_op ] [, SORT2 = right_sort_op ] )
INPUTS
name The operator to be defined. See below for allowable characters.
func_name
The function used to implement this operator.
lefttype
The type of the left-hand argument of the operator, if any. This
option would be omitted for a left-unary operator.
righttype
The type of the right-hand argument of the operator, if any. This
option would be omitted for a right-unary operator.
com_op
The commutator of this operator.
neg_op
The negator of this operator.
res_proc
The restriction selectivity estimator function for this operator.
join_proc
The join selectivity estimator function for this operator.
HASHES
Indicates this operator can support a hash join.
left_sort_op
If this operator can support a merge join, the operator that sorts the
left-hand data type of this operator.
right_sort_op
If this operator can support a merge join, the operator that sorts the
right-hand data type of this operator.
OUTPUTS
CREATE
Message returned if the operator is successfully created.
DESCRIPTION
CREATE OPERATOR defines a new operator, name. The user who defines an
operator becomes its owner.
The operator name is a sequence of up to NAMEDATALEN-1 (31 by default)
characters from the following list:
+ - * / < > = ~ ! @ # % ^ & | ` ? $
There are a few restrictions on your choice of name:
o+ $ cannot be defined as a single-character operator, although it can be
part of a multicharacter operator name.
o+ -- and /* cannot appear anywhere in an operator name, since they will be
taken as the start of a comment.
o+ A multicharacter operator name cannot end in + or -, unless the name also
contains at least one of these characters:
~ ! @ # % ^ & | ` ? $
For example, @- is an allowed operator name, but *- is not. This
restriction allows PostgreSQL to parse SQL-compliant queries without
requiring spaces between tokens.
Note: When working with non-SQL-standard operator names, you will
usually need to separate adjacent operators with spaces to avoid
ambiguity. For example, if you have defined a left-unary operator
named @, you cannot write X*@Y; you must write X* @Y to ensure that
PostgreSQL reads it as two operator names not one.
The operator != is mapped to <> on input, so these two names are always
equivalent.
At least one of LEFTARG and RIGHTARG must be defined. For binary operators,
both should be defined. For right unary operators, only LEFTARG should be
defined, while for left unary operators only RIGHTARG should be defined.
The func_name procedure must have been previously defined using CREATE
FUNCTION and must be defined to accept the correct number of arguments
(either one or two) of the indicated types.
The commutator operator should be identified if one exists, so that
PostgreSQL can reverse the order of the operands if it wishes. For
example, the operator area-less-than, <<<, would probably have a commutator
operator, area-greater-than, >>>. Hence, the query optimizer could freely
convert:
box '((0,0), (1,1))' >>> MYBOXES.description
to
MYBOXES.description <<< box '((0,0), (1,1))'
This allows the execution code to always use the latter representation and
simplifies the query optimizer somewhat.
Similarly, if there is a negator operator then it should be identified.
Suppose that an operator, area-equal, ===, exists, as well as an area not
equal, !==. The negator link allows the query optimizer to simplify
NOT MYBOXES.description === box '((0,0), (1,1))'
to
MYBOXES.description !== box '((0,0), (1,1))'
If a commutator operator name is supplied, PostgreSQL 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 newly created operator
as its commutator. This applies to the negator, as well. 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. (As of PostgreSQL 6.5, it also works to just have both operators
refer to each other.)
The HASHES, SORT1, and SORT2 options are present to support the query
optimizer in performing joins. PostgreSQL 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,
PostgreSQL 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 hashjoined.)
The HASHES flag indicates to the query optimizer that a hash join may
safely be used with this operator.
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.
If other join strategies are found to be practical, PostgreSQL will change
the optimizer and run-time system to use them and will require additional
specification when an operator is defined. Fortunately, the research
community invents new join strategies infrequently, and the added
generality of user-defined join strategies was not felt to be worth the
complexity involved.
The RESTRICT and JOIN options assist the query optimizer in estimating
result sizes. If a clause of the form:
MYBOXES.description <<< box '((0,0), (1,1))'
is present in the qualification, then PostgreSQL may have to estimate the
fraction of the instances in MYBOXES that satisfy the clause. The function
res_proc must be a registered function (meaning it is already defined using
CREATE FUNCTION) which accepts arguments of the correct data types and
returns a floating-point number. The query optimizer simply calls this
function, passing the parameter ((0,0), (1,1)) and multiplies the result by
the relation size to get the expected number of instances.
Similarly, when the operands of the operator both contain instance
variables, the query optimizer must estimate the size of the resulting
join. The function join_proc will return another floating-point number
which will be multiplied by the cardinalities of the two tables involved to
compute the expected result size.
The difference between the function
my_procedure_1 (MYBOXES.description, box '((0,0), (1,1))')
and the operator
MYBOXES.description === box '((0,0), (1,1))'
is that PostgreSQL attempts to optimize operators and can decide to use an
index to restrict the search space when operators are involved. However,
there is no attempt to optimize functions, and they are performed by brute
force. Moreover, functions can have any number of arguments while
operators are restricted to one or two.
NOTES
Refer to the chapter on operators in the PostgreSQL User's Guide for
further information. Refer to DROP OPERATOR to delete user-defined
operators from a database.
USAGE
The following command defines a new operator, area-equality, for the BOX
data type:
CREATE OPERATOR === (
LEFTARG = box,
RIGHTARG = box,
PROCEDURE = area_equal_procedure,
COMMUTATOR = ===,
NEGATOR = !==,
RESTRICT = area_restriction_procedure,
JOIN = area_join_procedure,
HASHES,
SORT1 = <<<,
SORT2 = <<<
);
COMPATIBILITY
SQL92
CREATE OPERATOR is a PostgreSQL extension. There is no CREATE OPERATOR
statement in SQL92.
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Index for
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Alphabetical
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