Data may be lost when changing column type with tidb_enable_amend_pessimistic_txn=on.

[wjhuang2016]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set global tidb_enable_amend_pessimistic_txn=ON;
CREATE TABLE `t` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL
);

insert into t many rows (about 1 000 000)

                                    /* txn */ begin pessimistic;
                                    /* txn */ insert into t values (52000002, 502);
/* DDL */ alter table t change column b b smallint;
                                    /* txn  during DDL */  commit;
/* DDL */ finish DDL
mysql> select * from t where a=52000002;
+----------+------+
| a        | b    |
+----------+------+
| 52000002 | NULL |
+----------+------+
1 row in set (0.00 sec)

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

mysql> select * from t where a=52000002;
+----------+------+
| a        | b    |
+----------+------+
| 52000002 | 502  |
+----------+------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

NULL

4. What is your TiDB version? (Required)

master

[cfzjywxk]

@wjhuang2016
The current implementation of transaction amend is not compatible with column type change, in the earlier versions the column type is only allowed to be changed to certain types which does not need data reorgnization, but now the assumption is not true. It's needed to add rejection logic at https://github.com/pingcap/tidb/blob/master/session/schema_amender.go#L163.Also some compatibility test case are needed to verify the rejection.

[cfzjywxk]

Seems the column id is also needed to be checked to ensure only field length is increased and other changes will be rejected.

[zimulala]

@cfzjywxk
We have rejection logic at https://github.com/pingcap/tidb/blob/master/session/schema_amender.go#L163. But the state of the column we check should not be public https://github.com/pingcap/tidb/blob/master/session/schema_amender.go#L197.

[ti-srebot]

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