CREATE OPERATOR() SQL Commands CREATE OPERATOR()NAME
CREATE OPERATOR - Defines a new user operator
SYNOPSIS
CREATE OPERATOR name ( PROCEDURE = func_name
[, LEFTARG = type1 ] [, RIGHTARG = type2 ]
[, 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.
type1 The type of the left-hand argument of the operator, if any.
This option would be omitted for a left-unary operator.
type2 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 opera‐
tor.
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:
· "$" cannot be defined as a single-character operator, although it can
be part of a multi-character operator name.
· "--" and "/*" cannot appear anywhere in an operator name, since they
will be taken as the start of a comment.
· A multi-character 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 Postgres 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 Postgres 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 opera‐
tors, 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
Postgres can reverse the order of the operands if it wishes. For exam‐
ple, 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 sim‐
plify
NOT MYBOXES.description === box '((0,0), (1,1))'
to
MYBOXES.description !== box '((0,0), (1,1))'
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 newly cre‐
ated operator as its commutator. This applies to the negator, as well.
This is to allow the definition of two operators that are the commuta‐
tors or the negators of each other. The first operator should be
defined without a commutator or negator (as appropriate). When the sec‐
ond operator is defined, name the first as the commutator or 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.)
The HASHES, SORT1, and SORT2 options 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 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 rep‐
resent 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, Postgres 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 Postgres may have to estimate the
fraction of the instances in MYBOXES that satisfy the clause. The func‐
tion 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 Postgres attempts to optimize operators and can decide to use
an index to restrict the search space when operators are involved. How‐
ever, 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 Postgres extension. There is no CREATE OPERATOR
statement in SQL92.
SQL - Language Statements 29 March 2001 CREATE OPERATOR()