The _tidb_rowid is not correct when using index scan only

[jackysp]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

mysql> drop table t;
Query OK, 0 rows affected (0.02 sec)

mysql> create table t (i varchar(20) primary key);
Query OK, 0 rows affected (0.01 sec)

mysql> insert into t values ("a");
Query OK, 1 row affected (0.00 sec)

mysql> select _tidb_rowid from t;
+-------------+
| _tidb_rowid |
+-------------+
|           1 |
+-------------+
1 row in set (0.00 sec)

mysql> select _tidb_rowid from t use index (`primary`);
+-------------+
| _tidb_rowid |
+-------------+
|          97 |
+-------------+
1 row in set (0.00 sec)

mysql> explain select _tidb_rowid from t use index (`primary`);
+-------------------+-------+------+---------------------------------------------------------------------+
| id                | count | task | operator info                                                       |
+-------------------+-------+------+---------------------------------------------------------------------+
| IndexReader_5     | 1.00  | root | index:IndexScan_4                                                   |
| └─IndexScan_4     | 1.00  | cop  | table:t, index:i, range:[NULL,+inf], keep order:false, stats:pseudo |
+-------------------+-------+------+---------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql> explain select _tidb_rowid from t;
+-------------------+-------+------+------------------------------------------------------------+
| id                | count | task | operator info                                              |
+-------------------+-------+------+------------------------------------------------------------+
| TableReader_5     | 1.00  | root | data:TableScan_4                                           |
| └─TableScan_4     | 1.00  | cop  | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+-------------------+-------+------+------------------------------------------------------------+
2 rows in set (0.00 sec)

2. What did you expect to see?

mysql> select _tidb_rowid from t use index (`primary`);
+-------------+
| _tidb_rowid |
+-------------+
|           1 |
+-------------+
1 row in set (0.00 sec)

3. What did you see instead?

mysql> select _tidb_rowid from t use index (`primary`);
+-------------+
| _tidb_rowid |
+-------------+
|          97 |
+-------------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   d07e5fb

[winoros]

This should be fixed in c23a30c by #8118

[jackysp]

😯
I've checked the latest master. It's fixed.