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I'm trying to write a general-purpose trigger that will disallow updates
on certain fields (I could probably do this in other ways, but I have a
stubborn streak ...).

Given a table, I want to define a trigger on that table that will
"write-protect" one column by name:

CREATE TRIGGER tbl_nomod_create
BEFORE INSERT OR UPDATE ON tbl
FOR EACH ROW EXECUTE PROCEDURE
no_modification_allowed('create_date');

I.e.,

UPDATE tbl SET fld_1 = 'foo;

would be OK but

UPDATE tbl SET create_date = now();

would result in an exception.

My trigger function below attempts to create a dynamic SQL statement
that tests "old.<column-name>" against "new.<column-name>".

CREATE OR REPLACE FUNCTION no_modification_allowed()
RETURNS TRIGGER
LANGUAGE 'plpgsql'
AS '
DECLARE
tmp_stmt TEXT;
result RECORD;
BEGIN
IF TG_ARGV[0] IS NULL
THEN
RETURN new;
ELSE
tmp_stmt := ''SELECT 1 AS is_null FROM (SELECT 1) AS dual WHERE '';
FOR result IN EXECUTE (tmp_stmt
|| ''old.'' || quote_ident(TG_ARGV[0])
|| '' IS NULL'') LOOP
RETURN new;
END LOOP;

FOR result IN EXECUTE (tmp_stmt
|| ''old.'' || quote_ident(TG_ARGV[0])
|| '' = new.''
|| quote_ident(TG_ARGV[0])) LOOP
RETURN new;
END LOOP;

RAISE EXCEPTION ''Cannot modify % in %'', TG_ARGV[0], TG_RELNAME;
END IF;
END
';

I tried one or two other approaches in the dynamic statement, but
generally I get errors indicating that "new" and "old" can't be
referenced in this fashion:

ERROR: OLD used in query that is not in a rule

Is there a way to do what I want?

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Jeff Boes wrote:
> I'm trying to write a general-purpose trigger that will disallow updates
> on certain fields (I could probably do this in other ways, but I have a
> stubborn streak ...).
>
> Given a table, I want to define a trigger on that table that will
> "write-protect" one column by name:
>
> CREATE TRIGGER tbl_nomod_create
> BEFORE INSERT OR UPDATE ON tbl
> FOR EACH ROW EXECUTE PROCEDURE
> no_modification_allowed('create_date');

> CREATE OR REPLACE FUNCTION no_modification_allowed()
> RETURNS TRIGGER
> LANGUAGE 'plpgsql'
> AS '

It's a lot easier if you use TCL/Perl/one of the other interpreted
languages. Bound to be an example in the mailing list archives, I might
even have posted one.

--
Richard Huxton
Archonet Ltd

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Jeff Boes <jeff@endpoint.com> writes:
> I'm trying to write a general-purpose trigger that will disallow updates
> on certain fields (I could probably do this in other ways, but I have a
> stubborn streak ...).

I think it's pretty much impossible to do this in plpgsql. You could do
it in the other PLs that support triggers, or in C. From a performance
standpoint I'd think you'd want to do it in C anyway. There are some
closely related example trigger functions in the contrib tree.

regards, tom lane

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