Is it possibe to support Projection push down on IndexLookUp?

[Yriuns]

Feature Request

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

Describe the feature you'd like:

set tidb_opt_projection_push_down = on;

CREATE TABLE `t` (
    `i` BIGINT(11) NOT NULL,
    `long_string` VARCHAR(1024) NOT NULL,
    KEY `idx_str`(`long_string`(10))
);

mysql> explain SELECT i FROM t FORCE INDEX(`idx_str`) WHERE long_string LIKE 'a%';
+---------------------------------+---------+-----------+-------------------------------------+-------------------------------------------------+
| id                              | estRows | task      | access object                       | operator info                                   |
+---------------------------------+---------+-----------+-------------------------------------+-------------------------------------------------+
| Projection_4                    | 250.00  | root      |                                     | test.t.i                                        |
| └─IndexLookUp_9                 | 250.00  | root      |                                     |                                                 |
|   ├─IndexRangeScan_6(Build)     | 250.00  | cop[tikv] | table:t, index:idx_str(long_string) | range:["a","b"), keep order:false, stats:pseudo |
|   └─Selection_8(Probe)          | 250.00  | cop[tikv] |                                     | like(test.t.long_string, "a%", 92)              |
|     └─TableRowIDScan_7          | 250.00  | cop[tikv] | table:t                             | keep order:false, stats:pseudo                  |
+---------------------------------+---------+-----------+-------------------------------------+-------------------------------------------------+
5 rows in set (0.00 sec)

As we can see, the IndexRangeScan will return i and long_string to tidb. However, it is unnecessary to return long_string, which will result in a large overhead when the string is very long. The optimal execution plan may looks like:

+-------------------------------+---------+-----------+-------------------------------------+-------------------------------------------------+
| id                            | estRows | task      | access object                       | operator info                                   |
+-------------------------------+---------+-----------+-------------------------------------+-------------------------------------------------+
| IndexLookUp_9                 | 250.00  | root      |                                     |                                                 |
| ├─Projection_4                | 250.00  | cop[tikv] |                                     | test.t.i                                        |
| | └─IndexRangeScan_6(Build)   | 250.00  | cop[tikv] | table:t, index:idx_str(long_string) | range:["a","b"), keep order:false, stats:pseudo |
| └─Selection_8(Probe)          | 250.00  | cop[tikv] |                                     | like(test.t.long_string, "a%", 92)              |
|   └─TableRowIDScan_7          | 250.00  | cop[tikv] | table:t                             | keep order:false, stats:pseudo                  |
+-------------------------------+---------+-----------+-------------------------------------+-------------------------------------------------+

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[yibin87]

/cc @yibin87