txn: incompatbile lock behaviour for cop and point get for update read

[cfzjywxk]

Bug Report

Please answer these questions before submitting your issue. Thanks!

When select for update is executed outside explicit transactions, there would be SelectLockExec for coprocessor read queries but no lock for point get queries, the behaviours are in-consistent.

1. Minimal reproduce step (Required)

create table t(a int key);
explain select * from t for update;
explain select * from t where a = 1 for update;

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

Both select would not have lock operations.

3. What did you see instead (Required)

There is SelectLockExec on select * from t for update;

explain select * from t for update;
+-------------------------+----------+-----------+---------------+--------------------------------+
| id                      | estRows  | task      | access object | operator info                  |
+-------------------------+----------+-----------+---------------+--------------------------------+
| SelectLock_5            | 10000.00 | root      |               | for update 0                   |
| └─TableReader_7         | 10000.00 | root      |               | data:TableFullScan_6           |
|   └─TableFullScan_6     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+----------+-----------+---------------+--------------------------------+
3 rows in set (0.001 sec)

While the point get query dose not have lock flag

MySQL [test]> explain select * from t where a = 1 for update;
+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| Point_Get_1 | 1.00    | root | table:t       | handle:1      |
+-------------+---------+------+---------------+---------------+

Note though the SelectLockExec is generated in the SQL plan, it would be skipped when building SelectLockExec executor at https://github.com/pingcap/tidb/blob/master/pkg/executor/builder.go#L770.

4. What is your TiDB version? (Required)

Nightly 26378cb