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
triggers, whereas all modifications are visible to
The data change (insertion, update, or deletion) causing
the trigger to fire is naturally not visible to SQL commands
executed in a row-level
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
Similarly, a row-level
OF trigger will see the effects of data changes made
by previous firings of
OF triggers in the same outer command.
When a row-level
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
Functions that are declared
IMMUTABLE will not see changes
made by the calling command in any case.
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