What is a deferrable foreign key?

What is a deferrable foreign key?

I read this about the SQL keyword DEFERRABLE in Database Systems – The Complete Book. The latter [NOT DEFERRABLE] is default, and means that every time a database modification statement is executed, the constraint is checked immediately afterwards, if the modification could violate the foreign-key constraint.

What is a deferrable constraint?

If a constraint is deferrable, this clause specifies the default time to check the constraint. If the constraint is INITIALLY IMMEDIATE, it is checked after each statement, which is the default. If the constraint is INITIALLY DEFERRED, it is checked only at the end of the transaction.

What does deferrable initially deferred mean?

If a constraint is created with the DEFERRABLE keyword it can act in one of two ways ( INITIALLY IMMEDIATE, INITIALLY DEFERRED ). The INITIALLY DEFERRED keyword causes constraint validation to defer until commit, unless immediate is secifically requested.

What is deferrable in SQL?

SET CONSTRAINTS sets the behavior of constraint checking within the current transaction. IMMEDIATE constraints are checked at the end of each statement. DEFERRED constraints are not checked until transaction commit.

Is deferrable a word?

or de·fer·a·ble capable of being deferred or postponed: a deferrable project. qualified or eligible to receive a military deferment.

What are examples of non deferrable expenses?

Wages and other employment expenses to independent (arm’s length) third parties.

Rent or lease payments for real estate used for business purposes;

Rent or lease payments for capital equipment used for business purposes;

Payments incurred for insurance-related costs;

Payments incurred for property taxes;

Why are foreign keys used as deferrable constraints?

The reasoning for having the foreign keys as being deferrable,initially deferred was to allow for easier programming in the application. There was no need to alter the constraints to be deferred or to worry about the order of inserts into the tables. Is this a sensible guideline to follow for using deferred integrity constraints.

What happens when a DEFERRABLE Constraint is declared?

There’s one other important caveat. As the table above shows, declaring UNIQUE or PRIMARY KEY constraint DEFERRABLE INITIALLY IMMEDIATE actually changes it from per-row to per-statement checking. Even if a transaction does not choose to defer those constraints, their granularity is still subtly altered.

Why do you need to defer foreign keys in SQL?

Correct modeling in this case will elegantly remove the need for deferring constraints. Deferring constraints while loading data can make the process more convenient. For instance, deferred foreign keys allow tables to be loaded in any order, children before parents.

Can You defer constraints outside of a transaction?

Attempting to defer constraints outside of a transaction will do nothing and warn, WARNING: 25P01: SET CONSTRAINTS can only be used in transaction blocks. There’s one other important caveat. As the table above shows, declaring UNIQUE or PRIMARY KEY constraint DEFERRABLE INITIALLY IMMEDIATE actually changes it from per-row to per-statement checking.