eejbyfeldt commented on code in PR #13249:
URL: https://github.com/apache/datafusion/pull/13249#discussion_r1834439966


##########
datafusion/optimizer/src/simplify_expressions/expr_simplifier.rs:
##########
@@ -1675,12 +1675,34 @@ impl<'a, S: SimplifyInfo> TreeNodeRewriter for 
Simplifier<'a, S> {
                 }
             }
 
+            // null <op> A --> null,
+            Expr::BinaryExpr(BinaryExpr {
+                left,
+                op: Eq | NotEq | Gt | GtEq | Lt | LtEq,
+                right: _,
+            }) if always_null(&left, info) => 
Transformed::yes(lit_bool_null()),
+
+            // A <op> null --> null,
+            Expr::BinaryExpr(BinaryExpr {
+                left: _,
+                op: Eq | NotEq | Gt | GtEq | Lt | LtEq,
+                right,
+            }) if always_null(&right, info) => 
Transformed::yes(lit_bool_null()),
+
             // no additional rewrites possible
             expr => Transformed::no(expr),
         })
     }
 }
 
+fn always_null<S: SimplifyInfo>(expr: &Expr, info: &S) -> bool {
+    is_null(expr)

Review Comment:
   Do we actually need that distinction? Or could we make all current usage of 
`is_null` use `always_null`?
   
   If that is the case it seems like we should just improve `is_null` instead.



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