Incorrect datatype transformation when compare decimal and float about tidb HOT 2 OPEN

[16:34:51]TiDB root:test> explain SELECT v0.c2 FROM v0 WHERE (CASE v0.c2 WHEN v0.c1 THEN 1 ELSE 1 END );
+-----------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------+
| id                    | estRows  | task      | access object | operator info                                                                            |
+-----------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------+
| Projection_7          | 8000.00  | root      |               | cast(test.t1.c1, decimal(48,30) BINARY)->Column#4                                        |
| └─TableReader_10      | 8000.00  | root      |               | data:Selection_9                                                                         |
|   └─Selection_9       | 8000.00  | cop[tikv] |               | case(eq(cast(cast(test.t1.c1, decimal(48,30) BINARY), double BINARY), test.t1.c0), 1, 1) |
|     └─TableFullScan_8 | 10000.00 | cop[tikv] | table:t1      | keep order:false, stats:pseudo                                                           |
+-----------------------+----------+-----------+---------------+------------------------------------------------------------------------------------------+