point get for update reading from tiflash

[ChenPeng2013]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

use test;
drop table if exists t;
create table t(a int, b int, c int, primary key(a, b));
insert into t values(1,2,3), (4,5,6);
alter table t set tiflash replica 1;
select sleep(10);
set tidb_enable_tiflash_read_for_write_stmt = on;
set tidb_isolation_read_engines='tidb, tiflash';

begin;
explain select a, b from t where a = 1 and b = 2 for update;

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

MySQL [test]> explain select a, b from t where a = 1 and b = 2 for update;
(1815, "Internal : No access path for table 't' is found with 'tidb_isolation_read_engines' = 'tidb,tiflash', valid values can be 'tikv, tiflash'.")

3. What did you see instead (Required)

MySQL [test]> begin;
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> explain select a, b from t where a = 1 and b = 2 for update;
+--------------------------+---------+--------------+---------------+-------------------------------------------------+
| id                       | estRows | task         | access object | operator info                                   |
+--------------------------+---------+--------------+---------------+-------------------------------------------------+
| SelectLock_6             | 0.01    | root         |               | for update 0                                    |
| └─TableReader_8          | 1.00    | root         |               | data:TableRangeScan_7                           |
|   └─TableRangeScan_7     | 1.00    | cop[tiflash] | table:t       | range:[1 2,1 2], keep order:false, stats:pseudo |
+--------------------------+---------+--------------+---------------+-------------------------------------------------+
3 rows in set (0.00 sec)

4. What is your TiDB version? (Required)

Release Version: v6.5.0-alpha
Edition: Community
Git Commit Hash: 8fc4535575b7cb8b3ed0ef9fe042b510815f423d
Git Branch: heads/refs/tags/v6.5.0-alpha
UTC Build Time: 2022-11-22 14:33:54
GoVersion: go1.19.2
Race Enabled: false
TiKV Min Version: 6.2.0-alpha
Check Table Before Drop: false
Store: tikv

[gengliqi]

Point get + for update is a particular case for TiDB transaction model.
TiKV will add a lock for the predicate during processing point get even if there is no data.
However, only the existing data can add lock when using coprocessor in the current implementation.

So the plan of using point get is not equal to the plan of using coprocessor in the presence of for update.
/cc @chrysan @Yisaer