deferrability

Deferrability is a concept in database management systems that relates to the timing of constraint checking. When a constraint is deferred, its enforcement is postponed until the end of the transaction, rather than being checked immediately after the statement that might violate it. This allows for temporary states that would otherwise be invalid to exist within a single transaction, as long as the constraint is satisfied by the time the transaction commits.

This mechanism is particularly useful in complex transactional scenarios where multiple operations need to occur in a specific order, and intermediate states might violate a constraint. For example, in a transfer of funds between two accounts, a simple balance check might fail mid-transaction if the debit is processed before the credit. By deferring the check, the system can ensure both operations complete, and then verify that the total balance remains consistent.

The ability to defer constraints can improve performance in certain situations by reducing the overhead of frequent constraint checks. However, it also introduces the possibility of transaction rollback if the deferred constraint is ultimately violated, meaning the entire transaction must be undone. The specific implementation and syntax for deferrability vary between different database systems.