Indexing Edge is not working in some scenario.

[sumitsharansatsangi]

I want to achieve this functionality.

I tried following.

CREATE INDEX Watched_out_in ON Watched (@out, @in) UNIQUE; is not working, creating error.
CREATE INDEX IF NOT EXISTS ON Watched (out, in) UNIQUE; This is working instead.

CREATE INDEX IF NOT EXISTS ON Watched (out, in) UNIQUE; is also not working, I mean it is generating error that Index already exist, while the expected behaviour is to execute silently, without any errors.