CREATE OPERATOR
SQL - Language Statements
CREATE OPERATOR
define a new operator
2000-03-25
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 ] )
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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.
MERGES
Indicates this operator can support a merge join.
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.
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Outputs
CREATE
Message returned if the operator is successfully created.
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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:
$ cannot be defined as a single-character operator,
although it can be part of a multicharacter operator name.
-- and /* cannot appear anywhere in an operator name,
since they will be taken as the start of a comment.
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.
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, 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 [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 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
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 <>,
<>, <>, >> 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
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.
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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 = <<<
-- Since sort operators were given, MERGES is implied.
-- LTCMP and GTCMP are assumed to be < and > respectively
);
Compatibility
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SQL92
CREATE OPERATOR
is a PostgreSQL extension.
There is no CREATE OPERATOR
statement in SQL92.