3.5. Triggers

Triggers are objects associated with a table that are used to execute operations stored in special functions when some operation (or event) happens on a single row or over the entire table. The trigger can be activated before or after the following events: INSERT, DELETE, UPDATE, or TRUNCATE. Besides ordinary triggers, there is a special kind known as a constraint trigger, whose firing time can be adjusted.

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Attributes
#	Field	Group box	Description
1	Execution		The moment when the trigger fires in relation to the event: BEFORE, AFTER, or INSTEAD OF (the last one is meant for triggers on views and requires For each row). It is disabled for constraint triggers, which always fire AFTER.
2	For each row		Executes the trigger function once for each row affected by the event (FOR EACH ROW). When unchecked, it is executed once per statement (FOR EACH STATEMENT). It is always checked, and disabled, for constraint triggers.
3	Constraint trigger		Turns the trigger into a constraint trigger. Constraint triggers always fire AFTER the event and for each row, so the fields Execution and For each row are disabled; the fields of the tab Constraint are enabled instead.
4	Function		The function executed when the trigger fires. It must take no arguments and return trigger; the values defined in the tab Arguments are passed to it through its special variables.
5	INSERT	Events	Fires the trigger on INSERT commands.
6	DELETE	Events	Fires the trigger on DELETE commands.
7	OLD	Transition tables names	Name of the transition table that exposes the old versions of the affected rows (REFERENCING OLD TABLE AS). Enabled only for AFTER triggers with a single event, either UPDATE or DELETE.
8	UPDATE	Events	Fires the trigger on UPDATE commands.
9	TRUNCATE	Events	Fires the trigger on TRUNCATE commands.
10	NEW	Transition tables names	Name of the transition table that exposes the new versions of the affected rows (REFERENCING NEW TABLE AS). Enabled only for AFTER triggers with a single event, either UPDATE or INSERT.

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Arguments
#	Field	Description
1	Argument	A literal argument passed to the trigger function at runtime. These arguments must be handled inside the function.
2	Arguments grid	Lists the arguments to be passed to the trigger function. Fill the field above and use the add button to include an item.

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Columns
#	Field	Description
1	Column	The columns of the table that cause the trigger to fire in case of an UPDATE on them. This tab is enabled only when UPDATE is checked and the trigger belongs to a table (not to a view). A trigger configured with INSTEAD OF can't be used with a list of columns.
2	Columns grid	Lists the columns whose update fires the trigger.

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Constraint
#	Field	Description
1	Deferrable	Indicates whether the constraint trigger can be deferred. Constraint triggers work like regular triggers, but their firing time can be adjusted with the command SET CONSTRAINTS. The deferral mode is configurable only when this option is checked.
2	Deferral	Specifies when a deferrable constraint trigger is checked: INITIALLY IMMEDIATE (at the end of each statement) or INITIALLY DEFERRED (at the end of the transaction).
3	Reference table	Table referenced by the constraint trigger (FROM clause). It is enabled only when the option Constraint trigger is checked.
4	Condition	A boolean expression that determines whether the trigger function is executed (WHEN clause).

References

Trigger DDL
https://www.postgresql.org/docs/current/static/sql-createtrigger.html