ddl: primary column flag changed after alter table statement

[cfzjywxk]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

drop table if exists t1; 
create table t1(c1 int key, c2 int);

mysql> select c1 from t1;
Field   1:  `c1`
Catalog:    `def`
Database:   `test`
Table:      `t1`
Org_table:  `t1`
Type:       LONG
Collation:  binary (63)
Length:     11
Max_length: 0
Decimals:   0
Flags:      NOT_NULL PRI_KEY NUM 

Do the alter table

ALTER TABLE t1 CHANGE c1 c1 INT NOT NULL;

Check the flags again

mysql> select c1 from t1;
Field   1:  `c1`
Catalog:    `def`
Database:   `test`
Table:      `t1`
Org_table:  `t1`
Type:       LONG
Collation:  binary (63)
Length:     11
Max_length: 0
Decimals:   0
Flags:      NOT_NULL PRI_KEY NO_DEFAULT_VALUE NUM

There will be a new NO_DEFAULT_VALUE NUM on the primary key column but we changed nothing.

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

A new flag NO_DEFAULT_VALUE is added onto the new column.

3. What did you see instead (Required)

Flags are the same after alter table for the primary key column.

4. What is your TiDB version? (Required)

release-4.0 2862cd7

[cfzjywxk]

@bb7133 @wjhuang2016 PTAL

[ti-srebot]

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