PostgreSQL Server Programming - Second Edition

Visibility

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

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

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

• The statement-level BEFORE triggers detects whether no changes are made by the current statement, and the statement-level AFTER triggers detects 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 the BEFORE triggers, as the operation has not occurred yet. Changes made by other ...