Jan Wieck wrote:
> Dale Johnson wrote:
> > I know we're not in the business of copying mySQL,
> > but the REPLACE INTO table (...) values (...) could be
> > a useful semantic. This is a combination INSERT or
> > UPDATE statement. For one thing, it is atomic, and
> > easier to work with at the application level. Also
> > if the application doesn't care about previous values,
> > then execution has fewer locking issues and race
> > conditions.
> >
> > comments?
>
> First it's not standard SQL, so chances aren't that good.
> Second, how do you think the system should behave in the
> following case:
>
> * Table A has one trigger BEFORE INSERT doing some checks
> plus inserting a row into table newA and updating a row in
> table balanceA. It also has triggers BEFORE UPDATE and
> BEFORE DELETE that update balanceA.
>
> * Now we do your REPLACE INTO
>
> The problem is that in a concurrent multiuser environment you
> cannot know if that row exists until you actually do the
> insert (except you lock the entire table and check for).
> Since there's a BEFORE trigger which potentially could
> suppress the INSERT, you can't do the insert before fireing
> it. Now it has been run, did it's inserts and updates and the
> statement must be converted into an UPDATE because the row
> exists - how do you undo the trigger work?
>
> I know, mySQL doesn't have triggers, referential integrity
> and all that damned complicated stuff. That's why it can have
> such a powerful non-standard command like REPLACE INTO.
>
> Jan
Perhaps it is as easy as saying that this feature is a non-standard
extension to SQL, thus a non-standard trigger mechanism is used.
The trigger will be on the statement replace. The trigger function will
carry with it the tuple, and the previous one if one exists.
create trigger my_trigger before update or insert or delete or replace
