Should not fold a non-constant expression when it is not clear to do so

[NGA-TRAN]

Describe the bug
This expression col < 10 and boolean(NULL) can either be NULL or false depending on the result of col < 10. Hence it should not be rewritten to a constant but it is current can.

To Reproduce
Add the following test in optimize_expr_bool_and, you will see it was rewritten to c1 BETWEEN Int32(0) AND Int32(10) which is wrong.

      // c1 BETWEEN Int32(0) AND Int32(10) AND Boolean(NULL)
      // it can be either NULL or false depending on the value of `c1 BETWEEN Int32(0) AND Int32(10`
       let expr = Expr::Between{ expr: Box::new(col("c1")), negated: false, low: Box::new(lit(0)), high: Box::new(lit(10))};
       let expr = expr.and(Expr::Literal(ScalarValue::Boolean(None)));
       let result = expr.clone().rewrite(&mut rewriter)?;
       println!("expr: {:#?} \n result: {:#?}", expr, result);
       assert_eq!(expr, result);

Expected behavior
Do not rewritten expression that does not have deterministic result yet

[NGA-TRAN]

I am working on a fix for this @alamb

[alamb]

thank you @NGA-TRAN -- FYI @matthewmturner I lead you astray in #1208 🤦 about nulls. NULLs!!!!

[matthewmturner]

@alamb got it - thanks for keeping me posted!

[alamb]

👍 FWIW I think handling NULLs correctly is the single hardest thing in databases (at least the thing that is the most fraught with strange corner cases). The fact we are dealing with them in DataFusion now I think is a sign of its maturity