  We prove tight upper and lower bounds on approximation ratios of all Boolean
Max-2CSP problems in the streaming model. Specifically, for every type of
Max-2CSP problem, we give an explicit constant $\alpha$, s.t. for any
$\epsilon>0$ (i) there is an $(\alpha-\epsilon)$-streaming approximation using
space $O(\log{n})$; and (ii) any $(\alpha+\epsilon)$-streaming approximation
requires space $\Omega(\sqrt{n})$. This generalizes the celebrated work of
[Kapralov, Khanna, Sudan SODA 2015; Kapralov, Krachun STOC 2019], who showed
that the optimal approximation ratio for Max-CUT was $1/2$.
  Prior to this work, the problem of determining this ratio was open for all
other Max-2CSPs. Our results are quite surprising for some specific Max-2CSPs.
For the Max-DCUT problem, there was a gap between an upper bound of $1/2$ and a
lower bound of $2/5$ [Guruswami, Velingker, Velusamy APPROX 2017]. We show that
neither of these bounds is tight, and the optimal ratio for Max-DCUT is $4/9$.
We also establish that the tight approximation for Max-2SAT is $\sqrt{2}/2$,
and for Exact Max-2SAT it is $3/4$. As a byproduct, our result gives a
separation between space-efficient approximations for Max-2SAT and Exact
Max-2SAT. This is in sharp contrast to the setting of polynomial-time
algorithms with polynomial space, where the two problems are known to be
equally hard to approximate. Finally, we prove that the tight streaming
approximation for \mksat{} is $\sqrt{2}/2$ for every $k\geq2$.
