Suppose we want to join two relations on a predicate. Is this in NC?I realize that a proof of it not being in NC would amount to a proof that $P\not=NC$, so I'd accept evidence of it being an open problem as an answer.I'm interested in the general case as well as specific cases (e.g. perhaps with some specific data structure it can be parallelized). EDIT: to bring some clarifications from the comments into this post:We could consider an equijoin $A.x = B.y$. On a single processor, a hash-based algorithm runs in $O(|A|+|B|)$ and this is the best we can do since we have to read each setIf the predicate is a black box where we have to check each pair, there are $|A|\cdot|B|$ pairs, and each one could be in or not, so $2^{ab}$ possibilities. Checking each pair divides the possibilities in half, so the best we can do is $O(ab)$.Could either of these (or some third type of join) be improved to $\log^k n$ on multiple processors?
Can joins be parallelized?
complexity theory;time complexity;parallel computing;database theory;descriptive complexity
$n^2$ processors can compare all ${n \choose 2}$ possibilities in constant depth, so yes it's in NC.