no_merge/merge vs no_unnest/unnest

I sometimes get confused about the difference between (no_)merge and(no_)unnest.
I just do some test here to make the difference clearly.

The original sql and its plan are below. I’ll hint the sql with no_merge andno_unnest.
You will find the difference quickly.

I try the hint no_merge in
order to avoid merging the subquery. This obviously don’t work.

19

Then I try the hint no_unnest.
It works now. That means (no_)unest works
only in the where clause. So I guess (no_)merge is
only suitable after the from.

I do another test with the original sql below:

you will see the no_unest hint
doesn’t work.

Now I hint the original sql with the hint no_merge.
It works. That’s it!