ddl:creating a secondary index may isn't compatible with mysql when the table contains cluster index

[TonsnakeLin]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t9(
id1 varchar(3000),
id2 varchar(3000),
primary key (id1) CLUSTERED,
key id2_key(id2)
) charset = latin1;

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

success

3. What did you see instead (Required)

ERROR 1071 (42000): Specified key was too long; max key length is 3072 bytes

4. What is your TiDB version? (Required)

Release Version: v5.0.0
Edition: Community
Git Commit Hash: bdac088
Git Branch: heads/refs/tags/v5.0.0
UTC Build Time: 2021-04-06 16:36:29
GoVersion: go1.13
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false

[github-actions]

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