[datatable-help] What is the point of SJ?

[Gabor Grothendieck]

Consider SJ which I assume was intended to be used like this
   X[ SJ(Y) ]
where X and Y are two data tables.  What is the point of SJ?  It seems
similar to J except it also adds a key to its argument; however, is it
not the case that that the key on Y will not be used since it has to
do a full scan of Y anyways?

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