mysql> set @@session.tidb_opt_distinct_agg_push_down = 1;
Query OK, 0 rows affected (0.00 sec)

mysql> desc select /*+ agg_to_cop() */ avg(b), c, avg(b), count(distinct A, B),  count(distinct A), count(distinct c), sum(b) from t group by c;
+-----------------------------+---------+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| id                          | estRows | task      | operator info                                                                                                                                                                                                                                                            |
+-----------------------------+---------+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Projection_4                | 4.00    | root      | Column#5, test.t.c, Column#5, Column#6, Column#7, Column#8, Column#9                                                                                                                                                                                                     |
| └─HashAgg_8                 | 4.00    | root      | group by:test.t.c, funcs:avg(Column#10, Column#11)->Column#5, funcs:count(distinct test.t.a, test.t.b)->Column#6, funcs:count(distinct test.t.a)->Column#7, funcs:count(distinct test.t.c)->Column#8, funcs:sum(Column#12)->Column#9, funcs:firstrow(test.t.c)->test.t.c |
|   └─TableReader_9           | 4.00    | root      | data:HashAgg_5                                                                                                                                                                                                                                                           |
|     └─HashAgg_5             | 4.00    | cop[tikv] | group by:test.t.a, test.t.b, test.t.c, funcs:count(test.t.b)->Column#10, funcs:sum(test.t.b)->Column#11, funcs:sum(test.t.b)->Column#12                                                                                                                                  |
|       └─TableFullScan_7     | 5.00    | cop[tikv] | table:t, keep order:false, stats:pseudo                                                                                                                                                                                                                                  |
+-----------------------------+---------+-----------+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
5 rows in set (0.00 sec)

MySQL [test]> select count(distinct g), count(g) from test;
+-------------------+----------+
| count(distinct g) | count(g) |
+-------------------+----------+
|                10 | 50000000 |
+-------------------+----------+
1 row in set (0.43 sec)

MySQL [test]> set @@session.tidb_opt_distinct_agg_push_down = 0;
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> select count(distinct g) from test;
+-------------------+
| count(distinct g) |
+-------------------+
|                10 |
+-------------------+
1 row in set (4.37 sec)

MySQL [test]> set @@session.tidb_opt_distinct_agg_push_down = 1;
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> select count(distinct g) from test;
+-------------------+
| count(distinct g) |
+-------------------+
|                10 |
+-------------------+
1 row in set (0.43 sec)

MySQL [test]> select count(distinct id), count(id) from test;
+--------------------+-----------+
| count(distinct id) | count(id) |
+--------------------+-----------+
|           10000000 |  50000000 |
+--------------------+-----------+
1 row in set (16.57 sec)

MySQL [test]> set @@session.tidb_opt_distinct_agg_push_down = 0;
Query OK, 0 rows affected (0.00 sec)

MySQL [test]> select count(distinct id) from test;
+--------------------+
| count(distinct id) |
+--------------------+
|           10000000 |
+--------------------+
1 row in set (17.09 sec)

MySQL [test]> set @@session.tidb_opt_distinct_agg_push_down = 1;
Query OK, 0 rows affected (0.01 sec)

MySQL [test]> select count(distinct id) from test;
+--------------------+
| count(distinct id) |
+--------------------+
|           10000000 |
+--------------------+
1 row in set (24.80 sec)