core: Fix equivalence between variable expressions to be always false

Since until the bind to a value they are treated as NULL. https://sqlite.org/lang_expr.html#varparam

core/util.rs

@@ -399,7 +399,9 @@ pub fn exprs_are_equivalent(expr1: &Expr, expr2: &Expr) -> bool {
         (Expr::Unary(op1, expr1), Expr::Unary(op2, expr2)) => {
             op1 == op2 && exprs_are_equivalent(expr1, expr2)
         }
-        (Expr::Variable(var1), Expr::Variable(var2)) => var1 == var2,
+        // Variables that are not bound to a specific value, are treated as NULL
+        // https://sqlite.org/lang_expr.html#varparam
+        (Expr::Variable(..), Expr::Variable(..)) => false,
         (Expr::Parenthesized(exprs1), Expr::Parenthesized(exprs2)) => {
             exprs1.len() == exprs2.len()
                 && exprs1
@@ -945,6 +947,13 @@ pub mod tests {
         assert_eq!(normalize_ident("\"foo\""), "foo");
     }
 
+    #[test]
+    fn test_variable_comparison() {
+        let expr1 = Expr::Variable("?".to_string());
+        let expr2 = Expr::Variable("?".to_string());
+        assert!(!exprs_are_equivalent(&expr1, &expr2));
+    }
+
     #[test]
     fn test_basic_addition_exprs_are_equivalent() {
         let expr1 = Expr::Binary(