> I think that you must not change anything that can affect result

of select.

I try avoiding it, but there are useful patterns that require it. For
instance, you want to synchronize table1 and table2. You do a full
outer joint between them and check on the non-matching records on
either side (one side gets you new records, the other side gets you
records you should delete). How do you add/delete those records in
table1 to sync it to table2?