CREATE OPERATOR name ( PROCEDURE = func_name [, LEFTARG = left_type ] [, RIGHTARG = right_type ] [, COMMUTATOR = com_op ] [, NEGATOR = neg_op ] [, RESTRICT = res_proc ] [, JOIN = join_proc ] [, HASHES ] [, SORT1 = left_sort_op ] [, SORT2 = right_sort_op ] ) |
The operator to be defined. See below for allowable characters.
The function used to implement this operator.
The type of the left-hand argument of the operator, if any. This option would be omitted for a left-unary operator.
The type of the right-hand argument of the operator, if any. This option would be omitted for a right-unary operator.
The commutator of this operator.
The negator of this operator.
The restriction selectivity estimator function for this operator.
The join selectivity estimator function for this operator.
Indicates this operator can support a hash join.
The operator that sorts the left-hand data type of this operator, if this operator can support a merge join.
If this operator can support a merge join, the operator that sorts the right-hand data type of this operator.
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:
+ - * / < > = ~ ! @ # % ^ & | ` ? $
$ 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:
~ ! @ # % ^ & | ` ? $
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 |
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))' |
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. 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. 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 do not 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 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.
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))' |
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))') |
MYBOXES.description === box '((0,0), (1,1))' |
Refer to the chapter on operators in Chapter 3 for further information.
Use DROP OPERATOR to delete user-defined operators from a database.
To define a new user operator:
CREATE OPERATOR @#@ ( rightarg = int4, –– left unary procedure = int4fac ); |
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 = <<< ); |