Question

Is is possible with PostgreSql without a trigger to not allow the update of a column, just the insertion is allowed.

Answer 1

Completely untested but as Postgres SQL supports column level permissions it looks like it might be. http://www.postgresql.org/docs/current/static/sql-grant.html

Does this work?

GRANT SELECT (col1, col2), INSERT(col1, col2), UPDATE (col1) ON mytable TO userX;