NULLs in indexed columns

Oracle btree indexes do not store null values, while Informix^® btree indexes do. This means that if you index a single column and select all the rows where that column is null, Informix will do an indexed read to fetch just those rows, but Oracle will do a sequential scan of all rows to find them. Having an index unusable for "is null" criteria can also completely change the behavior and performance of more complicated selects without causing a sequential scan.