PostgreSQL- Exclusion Constraint

ByAdmin

An exclusion constraint is a type of constraint that allows you to specify a set of conditions that, if satisfied by any two rows in a table, will cause an error to be raised when trying to insert or update the table. Exclusion constraints are useful for preventing conflicts or overlaps between rows in a table.

Below is the syntax to create an exclusion constraint in PostgreSQL with the CREATE TABLE statement:

CREATE TABLE table_name (
    column1 data_type,
    column2 data_type,
    -- other columns
    EXCLUDE constraint_name WITH (
        exclusion_constraint_specification
    )
);

The exclusion_constraint_specification can be one of the following:

  • USING index_method: This specifies that an index should be used to enforce the exclusion constraint. The index_method can be btree, hash, or gist, depending on the type of index you want to use.
  • WHERE condition: This specifies a Boolean expression that must be true for any two rows that violate the exclusion constraint.
  • WITH operator: This specifies a custom exclusion operator that defines the conditions for two rows to be excluded.

Below is an example of creating an exclusion constraint with the USING method:

CREATE TABLE employee_shifts (
    employee_id integer,
    shift_start timestamp,
    shift_end timestamp,
    EXCLUDE overlapping_shifts
        WITH gist (employee_id, tsrange(shift_start, shift_end))
);

Above sql creates a table employee_shifts with columns employee_id, shift_start, and shift_end, and an exclusion constraint named overlapping_shifts. The constraint uses the GiST index method and a range type (tsrange) to ensure that no two rows have overlapping shifts for the same employee.

An exclusion constraint can be added to an existing table using the ALTER TABLE statement with the ADD CONSTRAINT option.

If an exclusion constraint is violated during an insert or update operation, PostgreSQL will raise an error with a message that includes the name of the constraint.

Exclusion constraints are transactional and atomic, meaning that they are only checked at the end of a transaction and will cause a rollback if violated.

Exclusion constraints can be used to enforce a variety of conditions, including uniqueness, non-overlapping ranges, mutual exclusion, and more.

In addition to the built-in exclusion operators provided by PostgreSQL (such as int4range, numrange, and tsrange), you can create custom exclusion operators using the CREATE OPERATOR statement.

Exclusion constraints can be dropped using the ALTER TABLE statement with the DROP CONSTRAINT option.

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