Allow filter condition push down to IndexScan for prefix index.

Feature Request

Allow filter condition push down to IndexScan for prefix index.

mysql> create table t (a text, b text, index a_b (a(255), b(255)));
Query OK, 0 rows affected (0.01 sec)

mysql> explain select * from t where a between 'a' and 'b' and b = 'b';
+-------------------------------+---------+-----------+--------------------------+---------------------------------------------------------+
| id                            | estRows | task      | access object            | operator info                                           |
+-------------------------------+---------+-----------+--------------------------+---------------------------------------------------------+
| IndexLookUp_11                | 0.25    | root      |                          |                                                         |
| ├─IndexRangeScan_8(Build)     | 250.00  | cop[tikv] | table:t, index:a_b(a, b) | range:["a","b"], keep order:false, stats:pseudo         |
| └─Selection_10(Probe)         | 0.25    | cop[tikv] |                          | eq(test.t.b, "b"), ge(test.t.a, "a"), le(test.t.a, "b") |
|   └─TableRowIDScan_9          | 250.00  | cop[tikv] | table:t                  | keep order:false, stats:pseudo                          |
+-------------------------------+---------+-----------+--------------------------+---------------------------------------------------------+

The condition b = 'b' should be able to pushed down to IndexScan phase to reduce the lookup table cost.

We only need to make sure the equal condition value of b is less than the index column length 255.

The ideal plan should be

mysql> explain select * from t where a between 'a' and 'b' and b = 'b';
+--------------------------+---------+-----------+--------------------------+-------------------------------------------------+
| id                       | estRows | task      | access object            | operator info                                   |
+--------------------------+---------+-----------+--------------------------+-------------------------------------------------+
| IndexReader_7            | 0.25    | root      |                          | index:Selection_6                               |
| └─Selection_6            | 0.25    | cop[tikv] |                          | eq(test.t.b, "b")                               |
|   └─IndexRangeScan_5     | 250.00  | cop[tikv] | table:t, index:a_b(a, b) | range:["a","b"], keep order:false, stats:pseudo |
+--------------------------+---------+-----------+--------------------------+-------------------------------------------------+

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: