planner: read-only variables with uppercase name cannot be converted to constant and lead to sub-optimal plans #66339

mysql> create table t (a int, key(a));
Query OK, 0 rows affected (0.020 sec)

mysql> set @a=1;
Query OK, 0 rows affected (0.000 sec)

mysql> explain select a from t where a=@a;
+------------------------+---------+-----------+---------------------+---------------------------------------------+
| id                     | estRows | task      | access object       | operator info                               |
+------------------------+---------+-----------+---------------------+---------------------------------------------+
| IndexReader_6          | 10.00   | root      |                     | index:IndexRangeScan_5                      |
| └─IndexRangeScan_5     | 10.00   | cop[tikv] | table:t, index:a(a) | range:[1,1], keep order:false, stats:pseudo |
+------------------------+---------+-----------+---------------------+---------------------------------------------+
2 rows in set (0.001 sec)

mysql> 
mysql> set @A=1;
Query OK, 0 rows affected (0.000 sec)

mysql> explain select a from t where a=@A;
+-------------------------+----------+-----------+---------------------+--------------------------------+
| id                      | estRows  | task      | access object       | operator info                  |
+-------------------------+----------+-----------+---------------------+--------------------------------+
| Selection_5             | 8000.00  | root      |                     | eq(test.t.a, getvar("a"))      |
| └─IndexReader_9         | 10000.00 | root      |                     | index:IndexFullScan_8          |
|   └─IndexFullScan_8     | 10000.00 | cop[tikv] | table:t, index:a(a) | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------------+--------------------------------+