prefixed column inside clustered index's TableScan only read prefix data

[lysu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

start a cluster with tikv(not unistore)

set @@tidb_enable_clustered_index=1;
CREATE TABLE `t1` (
  `cb` varchar(12) NOT NULL,
  `ci` int(11) DEFAULT NULL,
  `v` int(11) DEFAULT NULL,
  PRIMARY KEY (`cb`(1)) /*T![clustered_index] CLUSTERED */,
  KEY `idx_1` (`cb`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin;
insert into t1 values('PvtYW2', 1, 1);

run queries:

select * from t1;
select cb from t1;

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

MySQL [test]> select * from t1;
+--------+------+------+
| cb     | ci   | v    |
+--------+------+------+
| PvtYW2 |    1 |    1 |
+--------+------+------+
1 row in set (0.004 sec)

MySQL [test]> select cb from t1;
+----+
| cb |
+----+
| PvtYW2  |
+----+
1 row in set (0.003 sec)

3. What did you see instead (Required)

MySQL [test]> select * from t1;
+--------+------+------+
| cb     | ci   | v    |
+--------+------+------+
| PvtYW2 |    1 |    1 |
+--------+------+------+
1 row in set (0.004 sec)

MySQL [test]> select cb from t1;
+----+
| cb |
+----+
| P  |
+----+
1 row in set (0.003 sec)

4. What is your TiDB version? (Required)

Release Version: v4.0.0-beta.2-2136-gc9af43099-dirty
Edition: Community
Git Commit Hash: c9af430996323e81a7919ba0e3f30db934cf4aee
Git Branch: master
UTC Build Time: 2021-02-17 13:08:36
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[sticnarf]

To my understanding, the IndexScan here means TableScan using the key only mode, and actually we cannot use the key only mode here?

[ti-srebot]

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1. Root Cause Analysis (RCA) (optional)

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5. Affected versions

master

6. Fixed versions

master