For the last I'd rather say, select a row by the clustered index if you know it, it's unique and you want just that row. Not sure other databases use the same terminology as MSSQL, but in MSSQL the clustered index is the index that controls the order in which the data are actually stored so it's the fastest one to use. And it doesn't have to be the primary key.
I agree completely with the rest!
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