What it does

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. Each

constraint has its own IMMEDIATE or DEFERRED mode.

Upon creation, a constraint is given one of three characteristics: DEFERRABLE

INITIALLY DEFERRED, DEFERRABLE INITIALLY IMMEDIATE, or NOT DEFERRABLE. The

third class is always IMMEDIATE and is not affected by the SET CONSTRAINTS

command. The first two classes start every transaction in the indicated mode,

but their behavior can be changed within a transaction by SET CONSTRAINTS.

Note: NOT DEFERRABLE means we can't change the deferrability of the constraint inside a particular transaction.

Note: Constraints are NOT DEFERRABLE INITIALLY IMMEDIATE by default in PostgreSQL

Surprise!

When SET CONSTRAINTS changes the mode of a constraint from DEFERRED to

IMMEDIATE, the new mode takes effect retroactively: any outstanding data

modifications that would have been checked at the end of the transaction are

instead checked during the execution of the SET CONSTRAINTS command. If any

such constraint is violated, the SET CONSTRAINTS fails (and does not change

the constraint mode). Thus, SET CONSTRAINTS can be used to force checking of

constraints to occur at a specific point in a transaction.

Currently, only UNIQUE, PRIMARY KEY, REFERENCES (foreign key), and EXCLUDE

constraints are affected by this setting. NOT NULL and CHECK constraints are

always checked immediately when a row is inserted or modified (not at the end

of the statement). Uniqueness and exclusion constraints that have not been

declared DEFERRABLE are also checked immediately.