unexpected explain result _0 in the root operator.

[zz-jason]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

as shown in #14852, the result of explain for connection is:

TiDB(root@127.0.0.1:test) > explain for connection 1;
+-------------------------+-------+------+-----------------------------------------+
| id                      | count | task | operator info                           |
+-------------------------+-------+------+-----------------------------------------+
| _0                      | N/A   | root | N/A                                     |
| └─Batch_Point_Get_1     | 2.00  | root | table:bmsql_item, handles: %#v[123 456] |
+-------------------------+-------+------+-----------------------------------------+
2 rows in set (0.00 sec)

it outputs a _0 operator as the root operator, which is unexpected.

2. What did you expect to see?

disappear from the result

3. What did you see instead?

as described in the above.

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

the same as #14852

[sre-bot]

Dear contributor, the first Performance challenge program has been closed. But you can still contribute to this issue and we appreciate your contribution.

[sre-bot]

Dear contributor, the first Performance challenge program has been closed. But you can still contribute to this issue and we appreciate your contribution.

[zz-jason]

@LittleFall PTAL