Ah, you were looking for a DEFERRABLE CONSTRAINT, which postpones the constraint-validation to the end of a transaction.

So, you'd do (again, postgres):

create table         tree (
   f_node_id   int primary key
 , f_parent_id int
 , f_name      text
 , f_sort      float
 , constraint tree_parent_sort_uniq_idx unique (f_parent_id, f_sort) d
+eferrable
);
[download]

This lets you mess about with non-unique states for the duration of a transaction. The constraint (here: uniqueness) is only then enforced.

I can't imagine MariaDB does not have this functionality but I can't find it in the documentation. (I looked for DEFERRED or DEFERRABLE. The MariaDB docs list 'DEFERRED' as a reserved word but I see no functionality associated with it.)