Explicit semi- and antijoin operators would be a nice addition to the language, but do note that they present some additional challenges for the optimizer. In particular, when the join condition becomes explicit, it can take on more forms than before, which can limit the reorderings allowed or allow new ones. E.g. you now need to deal with this structure:
(a SEMIJOIN b ON a.x=b.y) JOIN c ON b.z=c.z
This was an impossible structure before, since IN and EXISTS both hide b's columns (all semi- and antijoins effectively come last), and your optimizer will now need to know whether e.g. this associative rewrite is allowed or not:
a SEMIJOIN (b JOIN c ON b.z=c.z) ON a.x=b.y
Also, you'll need to deal with LATERAL semijoins, which you didn't before…
None of this is impossible, but there's more to it than just a small syntactic change.
