Then it is checking for every detail table whether the key exists in the master table.
Just a discussion out of curiosity: Only informix chose to implement PKY->FKY relationship
via an index. In Oracle, Db2, and SQL Server, FKY does not need an index.
There are both plus and minus of Informix approach. The plus is that the join of master
and child will always use index. In other dbs, if the DBA forgets to create it, it will
suffer in performance.
The minus is that, sometimes for small code tables, where there is no need of index,
Informix will still create an index. On a large table, this is a waste of space.