CREATE OPERATOR

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:

+ - * / < > = ~ ! @ # % ^ & | ` ? $
   

• $ 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.

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 . 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 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 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.

   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