Visibility

Sometimes your trigger functions may run into the Multiversion Concurrency Control (MVCC) visibility rules of how PostgreSQL's system interacts with changes to data.

A function declared STABLE or IMMUTABLE will never see changes applied to the underlying table by previous triggers.

A VOLATILE function follows more complex rules, which are in a nutshell as follows:

• The statement-level BEFORE triggers see no changes made by the current statement, and statement-level AFTER triggers see all of the changes made by the statement.

• Data changes by the operation to the row causing the trigger to fire are of course not visible to BEFORE triggers, as the operation has not happened yet. Changes made by other triggers to other rows in the same statement are visible and as the order of the rows processed is undefined this needs caution!

• The same is true of INSTEAD OF triggers. The changes by the triggers fired in the same command on previous rows are visible to current invocation of trigger function...