SET CONSTRAINTS — set constraint check timing for the current transaction
SET CONSTRAINTS { ALL | name [, ...] } { DEFERRED | IMMEDIATE }   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.
  
   SET CONSTRAINTS with a list of constraint names changes
   the mode of just those constraints (which must all be deferrable).  Each
   constraint name can be schema-qualified.  The
   current schema search path is used to find the first matching name if
   no schema name is specified.  SET CONSTRAINTS ALL
   changes the mode of all deferrable constraints.
  
   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.
  
The firing of triggers that are declared as “constraint triggers” is also controlled by this setting — they fire at the same time that the associated constraint should be checked.
   Because PostgreSQL does not require constraint
   names to be unique within a schema (but only per-table), it is possible
   that there is more than one match for a specified constraint name.
   In this case SET CONSTRAINTS will act on all matches.
   For a non-schema-qualified name, once a match or matches have been found in
   some schema in the search path, schemas appearing later in the path are not
   searched.
  
This command only alters the behavior of constraints within the current transaction. Issuing this outside of a transaction block emits a warning and otherwise has no effect.
   This command complies with the behavior defined in the SQL
   standard, except for the limitation that, in
   PostgreSQL, it does not apply to
   NOT NULL and CHECK constraints.
   Also, PostgreSQL checks non-deferrable
   uniqueness constraints immediately, not at end of statement as the
   standard would suggest.