At 14:31 07/07/2008, Cyril SCETBON wrote:
>What the difference between adding and removing a column ? (in the 
>case, there's no constraint)

Adding a column without using EXECUTE SCRIPT works because Slony logs 
the column names and builds statements with those names. For 
instance, if you have a table with two columns a and b, it will use 
statements of the form INSERT INTO table (a,b) VALUES(1,2) on the 
slaves. So if you add the column on the destination(s) first, those 
statements will work, and the new column will just have its default 
value (that's one of the reasons you don't want to have a constraint 
on that new column: the constraint is likely to break).

If you delete a column, then you should do it on the master first, 
and the destinations afterwards (actually, only once any 
inserts/updates on the table generated prior to the column deletion 
on the master have been processed on all nodes), otherwise you'll get 
statements referencing the columns that don't exist anymore. There 
should be no constraint on the column to delete either (otherwise you 
may end up with Slony inserting default values on the slave that 
don't match the constraint). You may also have to modify the trigger 
argument (removing one "v" in the right place), though that's not 
necessary if your primary key columns are the first ones. Of course 
you shouldn't delete a column that is part of the primary key (or 
change the primary key or anything like that).

In reality, there are a lot of things you can actually do without 
EXECUTE SCRIPT if there is no activity on the table you're modifying 
(and possibly any other tables that reference them etc.). The only 
problem being that you actually have to *think* (carefully) about the 
consequences of any changes, so as pointed out by Andrew, EXECUTE 
SCRIPT has to take such extensive locks because it can't know if the 
DDL you're submitting is "safe" to execute with minimal locking or not.

In short, EXECUTE SCRIPT will attempt very strongly not to let you 
shoot yourself in the foot, while doing DDL changes directly on the 
different nodes is an ICBM targeted at your foot if you don't know 
what you are doing (and even if you think you do)! :-)

Jacques.