Is it possible to dynamically loop through a table's columns? - postgresql

Question

I have a trigger function for a table test which has the following code snippet:
IF TG_OP='UPDATE' THEN
IF OLD.locked > 0 AND
( OLD.org_id <> NEW.org_id OR
OLD.document_code <> NEW.document_code OR
-- other columns ...
)
THEN
RAISE EXCEPTION 'Message';
-- more code
So I am statically checking all the column's new value with its previous value to ensure integrity. Now every time my business logic changes and I have to add new columns into that table, I will have to modify this trigger each time. I thought it would be better if somehow I could dynamically check all the columns of that table, without explicitly typing their name.
How can it be done?

Answer 1

From 9.0 beta2 documentation about WHEN clause in triggers, which might be able to be used in earlier versions within the trigger body:
OLD.* IS DISTINCT FROM NEW.*
or possibly (from 8.2 release notes)
IF row(new.*) IS DISTINCT FROM row(old.*)

Answer 2

Take a look at the information_schema, there is a view "columns". Execute a query to get all current columnnames from the table that fired the trigger:
SELECT
column_name
FROM
information_schema.columns
WHERE
table_schema = TG_TABLE_SCHEMA
AND
table_name = TG_TABLE_NAME;
Loop through the result and there you go!
More information can be found in the fine manual.

Answer 3

In Postgres 9.0 or later add a WHEN clause to your trigger definition (CREATE TRIGGER statement):
CREATE TRIGGER foo
BEFORE UPDATE
FOR EACH ROW
WHEN (OLD IS DISTINCT FROM NEW) -- parentheses required!
EXECUTE PROCEDURE ...;
Only possible for triggers BEFORE / AFTER UPDATE, where both OLD and NEW are defined. You'd get an exception trying to use this WHEN clause with INSERT or DELETE triggers.
And radically simplify the trigger function accordingly:
...
IF OLD.locked > 0 THEN
RAISE EXCEPTION 'Message';
END IF;
...
No need to test IF TG_OP='UPDATE' ... since this trigger only works for UPDATE anyway.
Or move that condition in the WHEN clause, too:
CREATE TRIGGER foo
BEFORE UPDATE
FOR EACH ROW
WHEN (OLD.locked > 0
AND OLD IS DISTINCT FROM NEW)
EXECUTE PROCEDURE ...;
Leaving only an unconditional RAISE EXCEPTION in your trigger function, which is only called when needed to begin with.
Read the fine print:
In a BEFORE trigger, the WHEN condition is evaluated just before the
function is or would be executed, so using WHEN is not materially
different from testing the same condition at the beginning of the
trigger function. Note in particular that the NEW row seen by the
condition is the current value, as possibly modified by earlier
triggers. Also, a BEFORE trigger's WHEN condition is not allowed to
examine the system columns of the NEW row (such as oid), because those
won't have been set yet.
In an AFTER trigger, the WHEN condition is evaluated just after the
row update occurs, and it determines whether an event is queued to
fire the trigger at the end of statement. So when an AFTER trigger's
WHEN condition does not return true, it is not necessary to queue an
event nor to re-fetch the row at end of statement. This can result in
significant speedups in statements that modify many rows, if the
trigger only needs to be fired for a few of the rows.
Related:
Fire trigger on update of columnA or ColumnB or ColumnC
To also address the question title
Is it possible to dynamically loop through a table's columns?
Yes. Examples:
Handle result when dynamic SQL is in a loop
Removing all columns with given name

Answer 4

Iteration over RECORD variable inside trigger

Answer 5

Use pl/perl or pl/python. They are much better suited for such tasks. much better.
You can also install hstore-new, and use it's row->hstore semantics, but that's definitely not a good idea when using normal datatypes.