date_sub(date, interval Expression week) shouldn't return datetime type result

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a date);
alter table t set tiflash replica 1;
insert into t values("2020-10-10");
set tidb_enforce_mpp=1;
select date_sub(a, interval 1 week) from t; -- expect "2020-10-03" but got "2020-10-03 00:00:00.000000"
/*
+---------------------------+---------+--------------+---------------+--------------------------------------------+
| id                        | estRows | task         | access object | operator info                              |
+---------------------------+---------+--------------+---------------+--------------------------------------------+
| TableReader_9             | 1.00    | root         |               | data:ExchangeSender_8                      |
| └─ExchangeSender_8        | 1.00    | cop[tiflash] |               | ExchangeType: PassThrough                  |
|   └─Projection_4          | 1.00    | cop[tiflash] |               | date_sub(pushdown2.t.a, 1, WEEK)->Column#3 |
|     └─TableFullScan_7     | 1.00    | cop[tiflash] | table:t       | keep order:false, stats:pseudo             |
+---------------------------+---------+--------------+---------------+--------------------------------------------+
*/

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiFlash version? (Required)

master

Start with the provided SQL reproduction in TiFlash, focusing on the date_sub expression with a DATE column and a WEEK interval under TiFlash MPP execution. Trace where the expression's result type is determined and verify the query result against the expected DATE value without a time component.