It's possible that this is due to the underlying implementation. In a unique column normally you'll have an index, so NULL becomes a special value in an index,

It's possible that this is due to the underlying implementation. In a unique column normally you'll have an index, so NULL becomes a special value in an index, but in SELECT DISTINCT you probably won't have an index, which means a full scan is performed, then every row has to be compared with every other row.