perf-tuning: add docs for DISTINCT optimizations

agg-distinct-optimization.md

@@ -3,4 +3,55 @@ title: Distinct 优化
 category: performance
 ---
 
-# Distinct 优化
+# Distinct 优化
+
+## 简单 DISTINCT
+
+通常简单的 `DISTINCT` 会被优化成 GROUP BY 来执行。例如：
+
+```sql
+mysql> explain select DISTINCT a from t;
++--------------------------+---------+-----------+---------------+-------------------------------------------------------+
+| id                       | estRows | task      | access object | operator info                                         |
++--------------------------+---------+-----------+---------------+-------------------------------------------------------+
+| HashAgg_6                | 2.40    | root      |               | group by:test.t.a, funcs:firstrow(test.t.a)->test.t.a |
+| └─TableReader_11         | 3.00    | root      |               | data:TableFullScan_10                                 |
+|   └─TableFullScan_10     | 3.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo                        |
++--------------------------+---------+-----------+---------------+-------------------------------------------------------+
+3 rows in set (0.00 sec)
+```
+
+TODO([#15284](https://github.com/pingcap/tidb/issues/15284)): 当 LIMIT __row_count__ 和 `DISTINCT` 组合使用时, TiDB 应立即返回 __row_count__ 个不同的行。
+
+## 聚合函数 DISTINCT
+
+通常来说，带有 `DISTINCT` 的聚合函数会单线程的在 TiDB 侧执行。
+使用系统变量 [`tidb_opt_distinct_agg_push_down`](/tidb-specific-system-variables.md#tidb_opt_distinct_agg_push_down) 或者 TiDB 的配置项 [distinct-agg-push-down](/tidb-configuration-file.md#distinct-agg-push-down) 控制优化器是否执行带有 `DISTINCT` 的聚合函数（比如 `select count(distinct a) from t`）下推到 Coprocessor 的优化操作。
+
+在以下示例中，`tidb_opt_distinct_agg_push_down` 开启前，TiDB 需要从 TiKV 读取所有数据，并在 TiDB 侧执行 `disctinct`。`tidb_opt_distinct_agg_push_down` 开启后， `distinct a` 被下推到了 Coprocessor，在 `HashAgg_5` 里新增了一个 `group by` 列 `test.t.a`。
+
+```sql
+mysql> desc select count(distinct a) from test.t;
++-------------------------+----------+-----------+---------------+------------------------------------------+
+| id                      | estRows  | task      | access object | operator info                            |
++-------------------------+----------+-----------+---------------+------------------------------------------+
+| StreamAgg_6             | 1.00     | root      |               | funcs:count(distinct test.t.a)->Column#4 |
+| └─TableReader_10        | 10000.00 | root      |               | data:TableFullScan_9                     |
+|   └─TableFullScan_9     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo           |
++-------------------------+----------+-----------+---------------+------------------------------------------+
+3 rows in set (0.01 sec)
+
+mysql> set session tidb_opt_distinct_agg_push_down = 1;
+Query OK, 0 rows affected (0.00 sec)
+
+mysql> desc select count(distinct a) from test.t;
++---------------------------+----------+-----------+---------------+------------------------------------------+
+| id                        | estRows  | task      | access object | operator info                            |
++---------------------------+----------+-----------+---------------+------------------------------------------+
+| HashAgg_8                 | 1.00     | root      |               | funcs:count(distinct test.t.a)->Column#3 |
+| └─TableReader_9           | 1.00     | root      |               | data:HashAgg_5                           |
+|   └─HashAgg_5             | 1.00     | cop[tikv] |               | group by:test.t.a,                       |
+|     └─TableFullScan_7     | 10000.00 | cop[tikv] | table:t       | keep order:false, stats:pseudo           |
++---------------------------+----------+-----------+---------------+------------------------------------------+
+4 rows in set (0.00 sec)
+```