More 'conservative' variant of ICols property: For projection, numbering operators and aggregations require all input columns, regardless of wether the operator's output is actually required. This means that an operator might only be pruned once the operators that used its output have been pruned themselves.
More 'conservative' variant of ICols property: For projection, numbering
operators and aggregations require all input columns, regardless of
wether the operator's output is actually required. This means that an
operator might only be pruned once the operators that used its output
have been pruned themselves.

This makes the dependency between icols rewrites explicit and allows us
to use an arbitrary traversal strategy and not rely on a preorder
traversal.

ulricha/dsh