Function rebase_expr 

pub(crate) fn rebase_expr(
    expr: &Expr,
    base_exprs: &[Expr],
    plan: &LogicalPlan,
) -> Result<Expr>

Rebuilds an Expr as a projection on top of a collection of Expr’s.

For example, the expression a + b < 1 would require, as input, the 2 individual columns, a and b. But, if the base expressions already contain the a + b result, then that may be used in lieu of the a and b columns.

This is useful in the context of a query like:

SELECT a + b < 1 … GROUP BY a + b

where post-aggregation, a + b need not be a projection against the individual columns a and b, but rather it is a projection against the a + b found in the GROUP BY.