COERCIBILITY is wrong for json type

[Defined2014]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a json, b blob);
insert into t values ('{"a": "b"}', 0x3f);
SELECT COERCIBILITY(a), COERCIBILITY(b) from t;

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

mysql> SELECT COERCIBILITY(a), COERCIBILITY(b) from t;
+-----------------+-----------------+
| COERCIBILITY(a) | COERCIBILITY(b) |
+-----------------+-----------------+
|               2 |               2 |
+-----------------+-----------------+

3. What did you see instead (Required)

mysql> SELECT COERCIBILITY(a), COERCIBILITY(b) from t;
+-----------------+-----------------+
| COERCIBILITY(a) | COERCIBILITY(b) |
+-----------------+-----------------+
|               5 |               2 |
+-----------------+-----------------+

4. What is your TiDB version? (Required)

mysql> SELECT tidb_version();
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tidb_version()                                                                                                                                                                                                                                |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Release Version: None
Edition: Community
Git Commit Hash: None
Git Branch: None
UTC Build Time: None
GoVersion: go1.17.2
Race Enabled: false
TiKV Min Version: v3.0.0-60965b006877ca7234adaced7890d7b029ed1306
Check Table Before Drop: false |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

[Defined2014]

/severity moderate

[Defined2014]

/sig sql-infra

[Defined2014]

/label sig/sql-infra

[ti-chi-bot]

@Defined2014: The label(s) sig/sql-infra cannot be applied. These labels are supported: challenge-program, compatibility-breaker, first-time-contributor, contribution, require-LGT3, good first issue, correctness, duplicate, proposal, security, needs-more-info, needs-cherry-pick-3.0, needs-cherry-pick-3.1, needs-cherry-pick-4.0, needs-cherry-pick-5.0, needs-cherry-pick-5.1, needs-cherry-pick-5.2, needs-cherry-pick-5.3, needs-cherry-pick-5.4, affects-4.0, affects-5.0, affects-5.1, affects-5.2, affects-5.3, backport-4.0.14, backport-4.0.15, backport-5.0.3, backport-5.0.4, backport-5.1.0, backport-5.1.1, backport-5.1.2.

In response to this:

/label sig/sql-infra

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