can't find column while use index merge

[aytrack]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t1(a int primary key, b int, c int, key(b), key(c));
explain select /*+ use_index_merge(t1) */ * from t1 where c=10 or (b=10 and a=10);

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

execute successfully

3. What did you see instead (Required)

MySQL root@127.0.0.1:test> explain select /*+ use_index_merge(t1) */ * from t1 where c=10 or (b=10 and a=10);
(1105, "Can't find column test.t1.b in schema Column: [test.t1.a] Unique key: []")

4. What is your TiDB version? (Required)

MySQL root@127.0.0.1:test> select tidb_version()\G
***************************[ 1. row ]***************************
tidb_version() | Release Version: v5.1.0-alpha-87-g0158f71cd
Edition: Community
Git Commit Hash: 0158f71cda6ccbbd382c6e8759904234baca259c
Git Branch: HEAD
UTC Build Time: 2021-06-01 11:51:39
GoVersion: go1.16.4
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[zhuo-zhi]

/assign

[ti-srebot]

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5. Affected versions

6. Fixed versions

[vivid392845427]

@zhuo-zhi ，please check pick 5.0