Normalize filter predicates

[aalexandrov]

Complex boolean predicates should be normalized to CNF and treated as a chain of filters in the comprehend method time.

For example, the following comprehension

val res = for (
  x <- X; 
  y <- Y; 
  z <- Z; 
  if x.a = "a" && x.b = "b" && x.a = y.a && x.b = z.b) yield (x,y,z)

should be executed effectively rewritten as

val res = for (
  x <- X; 
  y <- Y; 
  z <- Z; 
  if x.a = "a"; if x.b = "b"; if x.a = y.a; if x.b = z.b) yield (x,y,z)

The CombinatorRewriteEngine state machine will then handle predicate push-down and generate a plan with early filters and joins instead of a crosses followed by a filter.

[aalexandrov]

A good overview of the role of normalization in the classic relational setting can be found in this article.

[akunft]

@fschueler For the normalization to CNF I have some code already that i can provide.

[fschueler]

@akunft can you point me to the code?

[akunft]

https://github.com/akunft/ozone/tree/3f093e6b2cf5709327fb77f12a95772f3652e118/pact/pact-common/src/main/java/eu/stratosphere/pact/common/io/pax/selection

This package contains all the predicate conversion stuff. The CNFConverter does the normalization.