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; an 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 47.5. The example in Section 39.4 contains a demonstration of these rules.