Need support window function push down to tiflash

[LittleFall]

Feature Request

Is your feature request related to a problem? Please describe:

create table employee(empid int, deptid int, salary decimal(10,2));
insert into employee values(1,10,5500.00);
insert into employee values(2,10,4500.00);
insert into employee values(3,20,1900.00);
insert into employee values(4,20,4800.00);
insert into employee values(5,40,6500.00);
insert into employee values(6,40,14500.00);
insert into employee values(7,40,44500.00);
insert into employee values(8,50,6500.00);
insert into employee values(9,50,7500.00);
insert into employee values(10,10,3500.00);
insert into employee values(11,10,2500.00);
insert into employee values(12,20,3500.00);
insert into employee values(13,20,2500.00);
insert into employee values(14,40,3500.00);
insert into employee values(15,40,2500.00);
insert into employee values(16,50,3500.00);
insert into employee values(17,50,2500.00);


select *, row_number() over (partition by deptid ORDER BY salary desc) FROM employee;
+-------+--------+--------+--------------------------------------------------------------+
| empid | deptid | salary | row_number() OVER (PARTITION BY deptid ORDER BY salary DESC) |
+-------+--------+--------+--------------------------------------------------------------+
|     1 |     10 | 5500   |                                                            1 |
|     2 |     10 | 4500   |                                                            2 |
|    10 |     10 | 3500   |                                                            3 |
|    11 |     10 | 2500   |                                                            4 |
|     4 |     20 | 4800   |                                                            1 |
|    12 |     20 | 3500   |                                                            2 |
|    13 |     20 | 2500   |                                                            3 |
|     3 |     20 | 1900   |                                                            4 |
|     7 |     40 | 44500  |                                                            1 |
|     6 |     40 | 14500  |                                                            2 |
|     5 |     40 | 6500   |                                                            3 |
|    14 |     40 | 3500   |                                                            4 |
|    15 |     40 | 2500   |                                                            5 |
|     9 |     50 | 7500   |                                                            1 |
|     8 |     50 | 6500   |                                                            2 |
|    16 |     50 | 3500   |                                                            3 |
|    17 |     50 | 2500   |                                                            4 |
+-------+--------+--------+--------------------------------------------------------------+

alter table employee set tiflash replica 1;
set tidb_enforce_mpp=1;
desc select  *, row_number() over (partition by deptid ORDER BY salary desc) FROM employee;
+------------------------------+---------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------+
| id                           | estRows | task         | access object  | operator info                                                                                                                              |
+------------------------------+---------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------+
| Shuffle_14                   | 17.00   | root         |                | execution info: concurrency:5, data sources:[TableReader_12]                                                                               |
| └─Window_8                   | 17.00   | root         |                | row_number()->Column#6 over(partition by test.employee.deptid order by test.employee.salary desc rows between current row and current row) |
|   └─Sort_13                  | 17.00   | root         |                | test.employee.deptid, test.employee.salary:desc                                                                                            |
|     └─TableReader_12         | 17.00   | root         |                | data:TableFullScan_11                                                                                                                      |
|       └─TableFullScan_11     | 17.00   | cop[tiflash] | table:employee | keep order:false, stats:pseudo                                                                                                             |
+------------------------------+---------+--------------+----------------+--------------------------------------------------------------------------------------------------------------------------------------------+
5 rows in set, 2 warnings (0.00 sec)

mysql> show warnings;
+---------+------+-------------------------------------------------------------------------+
| Level   | Code | Message                                                                 |
+---------+------+-------------------------------------------------------------------------+
| Warning | 1105 | MPP mode may be blocked because operator `Window` is not supported now. |
| Warning | 1105 | MPP mode may be blocked because operator `Window` is not supported now. |
+---------+------+-------------------------------------------------------------------------+
2 rows in set (0.00 sec)

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[LittleFall]

another syntax:

desc select /*+read_from_storage(tiflash[employee]) */  *, row_number() over w, max(deptid) over w from employee window w as  (partition by deptid);

[LittleFall]

euqal to #4200