Question

Is it possible to prevent deletion of the first row in table on PostgreSQL side?

I have a category table and I want to prevent deletion of default category as it could break the application. Of course I could easily do it in application code, but it would be a lot better to do it in database.

I think it has something to do with rules on delete statement, but I couldn’t find anything remotely close to my problem in documentation.

Answer 1

The best way I see to accomplish this is by creating a delete trigger on this table. Basically, you’ll have to write a stored procedure to make sure that this ‘default’ category will always exist, and then enforce it using a trigger ON DELETE event on this table. A good way to do this is create a per-row trigger that will guarantee that on DELETE events the ‘default’ category row will never be deleted.

Please check out PostgreSQL’s documentation about triggers and stored procedures:

http://www.postgresql.org/docs/8.3/interactive/trigger-definition.html

http://www.postgresql.org/docs/8.3/interactive/plpgsql.html

There’s also valuable examples in this wiki:

http://wiki.postgresql.org/wiki/A_Brief_Real-world_Trigger_Example