tidb_cast to Int16 is not supported

[lilinghai]

Mini reproduce:

CREATE TABLE `t2` (
  `a` smallint NOT NULL,
  PRIMARY KEY (`a`) /*T![clustered_index] CLUSTERED */
);
insert into t2 values(1);
select a,max(a) from t2 group by a having max(a)>1; -- expect successfully; but got ERROR 1105 (HY000): tidb_cast to Int16 is not supported

Cluster version:
Release Version: v4.0.0-beta.2-2561-g9c75cfa4e
Edition: Community
Git Commit Hash: 9c75cfa4e2bbb854ab90562ecec0409f84a989b0
Git Branch: master
UTC Build Time: 2021-04-02 18:50:29
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[fzhedu]

similar to pingcap/tidb#23907

[windtalker]

Both TiFlash and MySQL does not support cast as Int16, the plan generated by TiDB is not as expected.

mysql> explain select a,max(a) from t2 group by a having max(a)>1;
+------------------------------+---------+--------------+---------------+-------------------------------------------------------------------------------------------+
| id                           | estRows | task         | access object | operator info                                                                             |
+------------------------------+---------+--------------+---------------+-------------------------------------------------------------------------------------------+
| Projection_7                 | 0.80    | root         |               | test.t2.a, Column#2                                                                       |
| └─Projection_8               | 0.80    | root         |               | test.t2.a, cast(test.t2.a, smallint(6))->Column#2, cast(test.t2.a, smallint(6))->Column#2 |
|   └─TableReader_14           | 0.80    | root         |               | data:Selection_13                                                                         |
|     └─Selection_13           | 0.80    | cop[tiflash] |               | gt(cast(test.t2.a, smallint(6)), 1)                                                       |
|       └─TableFullScan_12     | 1.00    | cop[tiflash] | table:t2      | keep order:false, stats:pseudo                                                            |
+------------------------------+---------+--------------+---------------+-------------------------------------------------------------------------------------------+

The implicit cast cast(test.t2.a, smallint(6)) should be something like cast(test.t2.a, bigint(20))

[windtalker]

close it as duplicated with pingcap/tidb#23907