Distinct 优化

    聚合函数 DISTINCT

    1. mysql> desc select count(distinct a) from test.t;
    2. +-------------------------+----------+-----------+---------------+------------------------------------------+
    3. | id                      | estRows  | task      | access object | operator info                            |
    4. +-------------------------+----------+-----------+---------------+------------------------------------------+
    5. | StreamAgg_6             | 1.00     | root      |               | funcs:count(distinct test.t.a)->Column#4 |
    6. |   └─TableFullScan_9     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo           |
    7. +-------------------------+----------+-----------+---------------+------------------------------------------+
    8. 3 rows in set (0.01 sec)
    10. mysql> set session tidb_opt_distinct_agg_push_down = 1;
    11. Query OK, 0 rows affected (0.00 sec)
    13. +---------------------------+----------+-----------+---------------+------------------------------------------+
    14. | id                        | estRows  | task      | access object | operator info                            |
    15. +---------------------------+----------+-----------+---------------+------------------------------------------+
    16. | HashAgg_8                 | 1.00     | root      |               | funcs:count(distinct test.t.a)->Column#3 |
    17. | └─TableReader_9           | 1.00     | root      |               | data:HashAgg_5                           |
    18. |   └─HashAgg_5             | 1.00     | cop[tikv] |               | group by:test.t.a,                       |
    19. |     └─TableFullScan_7     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo           |
    20. 4 rows in set (0.00 sec)