mysql> create table t (a int, key(a));
Query OK, 0 rows affected (0.05 sec)

mysql> prepare st from 'select * from t use index(a) where a < ?';
Query OK, 0 rows affected (0.01 sec)

mysql> set @a1=1.1;
Query OK, 0 rows affected (0.00 sec)

mysql> execute st using @a1; -- sub-optimal plan with fullscan
Empty set (0.01 sec)

+---------------------------+----------+---------+-----------+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------+--------+------+
| id                        | estRows  | actRows | task      | access object       | execution info                                                                                                                                             | operator info                                 | memory | disk |
+---------------------------+----------+---------+-----------+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------+--------+------+
| Selection_8               | 8000.00  | 0       | root      |                     | time:1.05ms, loops:1                                                                                                                                       | lt(cast(test.t.a, decimal(10,0) BINARY), 1.1) | N/A    | N/A  |
| └─IndexReader_7           | 8000.00  | 0       | root      |                     | time:1.04ms, loops:1, cop_task: {num: 1, max: 898.4µs, proc_keys: 0, rpc_num: 1, rpc_time: 814.5µs, copr_cache_hit_ratio: 0.00, distsql_concurrency: 15}   | index:Selection_6                             | N/A    | N/A  |
|   └─Selection_6           | 8000.00  | 0       | cop[tikv] |                     | tikv_task:{time:706µs, loops:0}                                                                                                                            | lt(cast(test.t.a, decimal(10,0) BINARY), 1.1) | N/A    | N/A  |
|     └─IndexFullScan_5     | 10000.00 | 0       | cop[tikv] | table:t, index:a(a) | tikv_task:{time:706µs, loops:0}                                                                                                                            | keep order:false, stats:pseudo                | N/A    | N/A  |
+---------------------------+----------+---------+-----------+---------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------+-----------------------------------------------+--------+------+

mysql> explain select * from t use index(a) where a < 1.1;  -- range scan
+------------------------+---------+-----------+---------------------+------------------------------------------------+
| id                     | estRows | task      | access object       | operator info                                  |
+------------------------+---------+-----------+---------------------+------------------------------------------------+
| IndexReader_6          | 3323.33 | root      |                     | index:IndexRangeScan_5                         |
| └─IndexRangeScan_5     | 3323.33 | cop[tikv] | table:t, index:a(a) | range:[-inf,2), keep order:false, stats:pseudo |
+------------------------+---------+-----------+---------------------+------------------------------------------------+
2 rows in set (0.01 sec)