user variable in GROUP BY might be wrongly pruned in logical optimization

[time-and-fate]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int, b int);
insert into t value(1,1), (2,2), (3,3);
set @n = 1;

explain select @n:=@n+1 as e from t group by e;
select @n:=@n+1 as e from t group by e;

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

+------+
| e    |
+------+
|    2 |
|    3 |
|    4 |
+------+

3. What did you see instead (Required)

+-----------------------------+---------+-----------+---------------+-------------------------------------------------------+
| id                          | estRows | task      | access object | operator info                                         |
+-----------------------------+---------+-----------+---------------+-------------------------------------------------------+
| Projection_4                | 1.00    | root      |               | setvar(n, plus(getvar(n), 1))->Column#4               |
| └─HashAgg_9                 | 1.00    | root      |               | group by:Column#8, funcs:firstrow(Column#8)->Column#7 |
|   └─TableReader_10          | 1.00    | root      |               | data:HashAgg_5                                        |
|     └─HashAgg_5             | 1.00    | cop[tikv] |               | group by:1,                                           |
|       └─TableFullScan_8     | 3.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo                        |
+-----------------------------+---------+-----------+---------------+-------------------------------------------------------+

+---+
| e |
+---+
| 2 |
+---+

4. What is your TiDB version? (Required)

current master

This should be introduced by enabling push down agg below projection by default in #22090.
Before #22090, this can also be triggered by turning on tidb_opt_agg_push_down.

[time-and-fate]

/sig planner

[time-and-fate]

/assign time-and-fate

[time-and-fate]

The same reason as #27106.

[time-and-fate]

Close this one since duplicated with #27106.

[ti-srebot]

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