Optimizations Database Operations: Join

[aannleax]

Suppose you want to evaluate the rule

   a(x, y) b(y, z) c(z) -> d(x, y)

Currently this would compute the temporary table tmp(x, y, z) and then project this down to d(x, y).

This is inefficient because the result does not care about every valid z-value. Hence it would suffice to know that one such value exists and move on to the next x, y pair.

To illustrate, consider a database with a(1, 2), b(2, 5), b(2, 6), c(5), c(6). We'd obtain tmp(1, 2, 5) and tmp(1, 2, 6), even though we only need to save d(1, 2).

[aannleax]

Implemented in #191.