CREATE OPERATOSQL)- Language Statements (2002-1CREATE OPERATOR(l)
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 ] [, MERGES ]
[, SORT1 = left_sort_op ] [, SORT2 = right_sort_op ]
[, LTCMP = less_than_op ] [, GTCMP = greater_than_op ] )
INPUTS
name The operator to be defined. See below for allowable
characters. The name may be schema-qualified, for
example CREATE OPERATOR myschema.+ (...).
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.
MERGES
Indicates this operator can support a merge join.
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left_sort_op
If this operator can support a merge join, the less-
than operator that sorts the left-hand data type of
this operator.
right_sort_op
If this operator can support a merge join, the less-
than operator that sorts the right-hand data type of
this operator.
less_than_op
If this operator can support a merge join, the less-
than operator that compares the input data types of
this operator.
greater_than_op
If this operator can support a merge join, the
greater-than operator that compares the input data
types of this operator.
OUTPUTS
CREATE OPERATOR
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.
If a schema name is given then the operator is created in
the specified schema. Otherwise it is created in the current
schema (the one at the front of the search path; see
CURRENT_SCHEMA()).
Two operators in the same schema can have the same name if
they operate on different data types. This is called
overloading. The system will attempt to pick the intended
operator based on the actual input data types when there is
ambiguity.
The operator name is a sequence of up to NAMEDATALEN-1 (63
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,
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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
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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, MERGES, SORT1, SORT2, LTCMP, and GTCMP 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 .
In addition, PostgreSQL can use a hash-join algorithm ;
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.
Similarly, the MERGES flag indicates whether merge-sort is a
usable join strategy for this operator. A merge join
requires that the two input data types have consistent
orderings, and that the merge-join 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 merge-join
equality operator: less-than comparison for the left input
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data type, less-than comparison for the right input data
type, less-than comparison between the two data types, and
greater-than comparison between the two data types. 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 <, <, <, > 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.
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
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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.
To give a schema-qualified operator name in com_op or the
other optional arguments, use the OPERATOR() syntax, for
example
COMMUTATOR = OPERATOR(myschema.===) ,
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 = <<<
-- Since sort operators were given, MERGES is implied.
-- LTCMP and GTCMP are assumed to be < and > respectively
);
COMPATIBILITY
SQL92
CREATE OPERATOR is a PostgreSQL extension. There is no
CREATE OPERATOR statement in SQL92.
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