If you execute SQL commands in your trigger function, and these commands access the table that the trigger is for, then you need to be aware of the data visibility rules, because they determine whether these SQL commands will see the data change that the trigger is fired for. Briefly:
Statement-level triggers follow simple visibility rules: none of
the changes made by a statement are visible to statement-level
BEFORE triggers, whereas all
modifications are visible to statement-level AFTER
triggers.
The data change (insertion, update, or deletion) causing the
trigger to fire is naturally not visible
to SQL commands executed in a row-level BEFORE trigger,
because it hasn't happened yet.
However, SQL commands executed in a row-level BEFORE
trigger will see the effects of data
changes for rows previously processed in the same outer
command. This requires caution, since the ordering of these
change events is not in general predictable; a SQL command that
affects multiple rows can visit the rows in any order.
Similarly, a row-level INSTEAD OF trigger will see the
effects of data changes made by previous firings of INSTEAD
OF triggers in the same outer command.
When a row-level AFTER trigger is fired, all data
changes made
by the outer command are already complete, and are visible to
the invoked trigger function.
If your trigger function is written in any of the standard procedural
languages, then the above statements apply only if the function is
declared VOLATILE. Functions that are declared
STABLE or IMMUTABLE will not see changes made by
the calling command in any case.
Further information about data visibility rules can be found in Section 46.5. The example in Section 37.4 contains a demonstration of these rules.