Fix duplicate cast in insert (#78)

Author: lewiszlw

README.md

@@ -19,7 +19,7 @@ test command
 ```sql
 create table t1 (a int, b int);
 
-insert into t1 (a, b) values (1, 1), (5, 3), (5, 2);
+insert into t1 (a, b) values (1, 1), (5, 3), (6, 2);
 
 update t1 set a = 0 where b > 1;
 

src/binder/insert.rs

@@ -59,7 +59,9 @@ impl<S: Storage> Binder<S> {
                         }
                         ScalarExpression::Unary { expr, op, .. } => {
                             if let ScalarExpression::Constant(value) = expr.as_ref() {
-                                row.push(Arc::new(unary_op(value, op)?))
+                                row.push(Arc::new(
+                                    unary_op(value, op)?.cast(columns[i].datatype())?,
+                                ))
                             } else {
                                 unreachable!()
                             }

src/execution/executor/dml/insert.rs

@@ -63,10 +63,9 @@ impl Insert {
                 let mut tuple_map = HashMap::new();
                 for (i, value) in values.into_iter().enumerate() {
                     let col = &columns[i];
-                    let cast_val = DataValue::clone(&value).cast(&col.datatype())?;
 
                     if let Some(col_id) = col.id {
-                        tuple_map.insert(col_id, Arc::new(cast_val));
+                        tuple_map.insert(col_id, value);
                     }
                 }
                 let primary_col_id = primary_key_index.get_or_insert_with(|| {