Planner: Need to ignore the ordering index path even if the index selectivity is 1.

[King-Dylan]

Bug Report

Please answer these questions before submitting your issue. Thanks!
In some versions, we still need to set tidb_opt_ordering_index_selectivity_threshold to control whether we want to use the index order property to scan rows.
However, when the index selectivity is equal to 1 and we set tidb_opt_ordering_index_selectivity_threshold=1, we still can't ignore the ordering index plan. This is because we always require index_selectivity < selectivity_threshold to eliminate the ordering path.
Actually, we just need to replace < with <=.

1. Minimal reproduce step (Required)

+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table                                                                                                                                                                                                                                                                                                     |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tttt1 | CREATE TABLE `tttt1` (
  `id1` int NOT NULL,
  `id2` int NOT NULL,
  `id3` varchar(20) NOT NULL,
  `id4` int NOT NULL,
  `id5` int NOT NULL,
  `id6` int NOT NULL,
  KEY `idx_id1` (`id3`,`id2`),
  KEY `idx_id2` (`id4`,`id2`),
  KEY `idx` (`id1`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin |
+-------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.03 sec)
mysql> set session tidb_opt_ordering_index_selectivity_threshold=1;
Query OK, 0 rows affected (0.00 sec)

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

mysql> explain select * from tttt1 where id6=1 order by id1 limit 10;
+------------------------------+------------+-----------+---------------+----------------------------------------------------+
| id                           | estRows    | task      | access object | operator info                                      |
+------------------------------+------------+-----------+---------------+----------------------------------------------------+
| TopN_8                       | 10.00      | root      |               | test.tttt1.id1, offset:0, count:10                 |
| └─TableReader_16             | 10.00      | root      |               | data:TopN_15                                       |
|   └─TopN_15                  | 10.00      | cop[tikv] |               | test.tttt1.id1, offset:0, count:10                 |
|     └─Selection_14           | 2097.15    | cop[tikv] |               | eq(test.tttt1.id6, 1)                              |
|       └─TableFullScan_13     | 2097152.00 | cop[tikv] | table:tttt1   | keep order:false, stats:partial[id6:unInitialized] |
+------------------------------+------------+-----------+---------------+----------------------------------------------------+
5 rows in set (0.00 sec)

3. What did you see instead (Required)

mysql> explain select * from tttt1 where id6=1 order by id1 limit 10;
+-----------------------------------+----------+-----------+-----------------------------+------------------------------------------------------------------------------------------------+
| id                                | estRows  | task      | access object               | operator info                                                                                  |
+-----------------------------------+----------+-----------+-----------------------------+------------------------------------------------------------------------------------------------+
| Limit_12                          | 10.00    | root      |                             | offset:0, count:10                                                                             |
| └─Projection_21                   | 10.00    | root      |                             | test.tttt1.id1, test.tttt1.id2, test.tttt1.id3, test.tttt1.id4, test.tttt1.id5, test.tttt1.id6 |
|   └─IndexLookUp_20                | 10.00    | root      |                             |                                                                                                |
|     ├─IndexFullScan_17(Build)     | 10000.00 | cop[tikv] | table:tttt1, index:idx(id1) | keep order:true, stats:partial[id6:unInitialized]                                              |
|     └─Selection_19(Probe)         | 10.00    | cop[tikv] |                             | eq(test.tttt1.id6, 1)                                                                          |
|       └─TableRowIDScan_18         | 10000.00 | cop[tikv] | table:tttt1                 | keep order:false, stats:partial[id6:unInitialized]                                             |
+-----------------------------------+----------+-----------+-----------------------------+------------------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

v7.1.6