>I've tried recreating the PK and various other stuff, the only thing
>that seems to help is to use a constant rather than a parameter.

Actually, using

UPDATE MyTable M
SET M.MyField = 'Y'
WHERE M.MyPK+0 = :MyPK

and thus eliminate the use of the primary key index, also seems to work. Though I'm still puzzled and not happy with that 'solution'.

Set