actual plan of a running stmt is not the same as the result of explain stmt

[XuHuaiyu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. What did you do?

create table kankan1(id int, name text);

insert into kankan1 values(1, 'a');

insert into kankan1 values(2, 'a');

create table kankan2(id int, h1 text);

insert into kankan2 values(2, 'z');

In session1:

tidb> explain select t1.id,sleep(20) from kankan1 t1 left join kankan2 t2 on t1.id = t2.id where (case  when t1.name='b' then 'case2' when t1.name='a' then 'case1' else NULL
+---------------------------+-------+------+-----------------------------------------------------------------------------------------+
| id                        | count | task | operator info                                                                           |
+---------------------------+-------+------+-----------------------------------------------------------------------------------------+
| Projection_6              | 1.60  | root | test.t1.id, sleep(20)                                                                   |
| └─HashLeftJoin_7          | 1.60  | root | left outer join, inner:TableReader_13, equal:[eq(test.t1.id, test.t2.id)]    -- left outer join           |
|   ├─TableReader_10        | 1.60  | root | data:Selection_9                                                                        |
|   │ └─Selection_9         | 1.60  | cop  | eq(case(eq(test.t1.name, "b"), "case2", eq(test.t1.name, "a"), "case1", NULL), "case1") |
|   │   └─TableScan_8       | 2.00  | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo                             |
|   └─TableReader_13        | 1.00  | root | data:Selection_12                                                                       |
|     └─Selection_12        | 1.00  | cop  | not(isnull(test.t2.id))                                                                 |
|       └─TableScan_11      | 1.00  | cop  | table:t2, range:[-inf,+inf], keep order:false, stats:pseudo                             |
+---------------------------+-------+------+-----------------------------------------------------------------------------------------+
8 rows in set (0.00 sec)

tidb> select t1.id,sleep(20) from kankan1 t1 left join kankan2 t2 on t1.id = t2.id where (case  when t1.name='b' then 'case2' when t1.name='a' then 'case1' else NULL end) = 'case1';

In session2:

tidb> show processlist; 
+------+------+-----------+------+---------+------+-------+------------------------------------------------------------------------------------------------------+
| Id   | User | Host      | db   | Command | Time | State | Info                                                                                                 |
+------+------+-----------+------+---------+------+-------+------------------------------------------------------------------------------------------------------+
|    1 | root | 127.0.0.1 | test | Query   |   10 | 2     | select t1.id,sleep(20) from kankan1 t1 left join kankan2 t2 on t1.id = t2.id where (case  when t1.na |
|    2 | root | 127.0.0.1 | test | Query   |    0 | 2     | show processlist                                                                                     |
+------+------+-----------+------+---------+------+-------+------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

tidb> explain for connection 1;
+---------------------------+-------+------+------------------------------------------------------------------------------------------------------------------+
| id                        | count | task | operator info                                                                                                    |
+---------------------------+-------+------+------------------------------------------------------------------------------------------------------------------+
| Projection_6              | 1.25  | root | test.t1.id, sleep(20)                                                                                            |
| └─HashLeftJoin_7          | 1.25  | root | inner join, inner:TableReader_14, equal:[eq(test.t1.id, test.t2.id)]      -- inner join                                       |
|   ├─TableReader_11        | 1.60  | root | data:Selection_10                                                                                                |
|   │ └─Selection_10        | 1.60  | cop  | eq(case(eq(test.t1.name, "b"), "case2", eq(test.t1.name, "a"), "case1", NULL), "case1"), not(isnull(test.t1.id)) |
|   │   └─TableScan_9       | 2.00  | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo                                                      |
|   └─TableReader_14        | 1.00  | root | data:Selection_13                                                                                                |
|     └─Selection_13        | 1.00  | cop  | not(isnull(test.t2.id))                                                                                          |
|       └─TableScan_12      | 1.00  | cop  | table:t2, range:[-inf,+inf], keep order:false, stats:pseudo                                                      |
+---------------------------+-------+------+------------------------------------------------------------------------------------------------------------------+
8 rows in set (0.00 sec)

2. What did you expect to see?
   The same explain result.

3. What did you see instead?
   N/A

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   21d2590ac37bf53e5e687b8a341d50105ddbffe8 in master branch

[XuHuaiyu]

The root cause is that StmtCtx of explain is not set correctly.

[XuHuaiyu]

related issue:
#11102