executor, planner: eliminate projection after aggregation pruning

[XuHuaiyu]

What problem does this PR solve?

Eliminate redundant projection after aggregation pruning.

What is changed and how it works?

e.g.

create table t(a int primary key, b varchar(10));
insert into t value(1,11), (2, null);
select a, min(b), max(b) from t group by a;

Before this commit, the above query will panic.
Rationale:
explain result:

+-----------------------+----------+------+------------------------------------------------------------+
| id                    | count    | task | operator info                                              |
+-----------------------+----------+------+------------------------------------------------------------+
| Projection_5          | 10000.00 | root | test.t.a, 2_col_0, 2_col_1                                 |
| └─Projection_6        | 10000.00 | root | test.t.b, test.t.b, test.t.a                               |
|   └─TableReader_8     | 10000.00 | root | data:TableScan_7                                           |
|     └─TableScan_7     | 10000.00 | cop  | table:t, range:[-inf,+inf], keep order:false, stats:pseudo |
+-----------------------+----------+------+------------------------------------------------------------+

After aggregation pruning, all the expressions in Projection_6 are type Column.
And Projection5.2_col_0.Index is 0, Projection_5.2_col_1.Index is 1.

As the image in #7841 shows,
when evaluating 2_col_0, an empty chunk.column will be swapped to the first test.t.b of Prj_6, and the second test.t.b points to it.

When evaluating 2_col_1, the same empty chunk.column will be swapped to 2_col_1, which will raise panic later on.

Note:
I call projectionEliminator.optimize here because that I cannot find a proper way to reset the Index info of the Schema in Prj_5.

Check List

Tests

• Unit test
• Integration test

Code changes

• Has exported function/method change

Side effects

N/A

Related changes

[XuHuaiyu]

PTAL @zz-jason @eurekaka @lamxTyler

[zz-jason]

LGTM

[zz-jason]

/run-all-tests

[alivxxx]

In https://github.com/pingcap/tidb/pull/7531/files#diff-6a7fc6244769b48bc271de3d766c529eR37, we introduce the flagEliminateProjection2, could this be done by setting that flag? Or we may eliminate the projection multiply times.

[XuHuaiyu]

Refer https://github.com/pingcap/tidb/pull/7531/files#r224341848,
I changed the implementation to solve the problem.
PTAL @zz-jason @lamxTyler

[XuHuaiyu]

/run-all-tests

[alivxxx]

LGTM

[zz-jason]

LGTM