用 EXPLAIN 查看子查询的执行计划
TiDB 会执行多种子查询相关的优化，以提升子查询的执行性能。本文档介绍一些常见子查询的优化方式，以及如何解读 EXPLAIN 语句返回的执行计划信息。
本文档所使用的示例表数据如下:
CREATE TABLE t1 (id BIGINT NOT NULL PRIMARY KEY auto_increment, pad1 BLOB, pad2 BLOB, pad3 BLOB, int_col INT NOT NULL DEFAULT 0);
CREATE TABLE t2 (id BIGINT NOT NULL PRIMARY KEY auto_increment, t1_id BIGINT NOT NULL, pad1 BLOB, pad2 BLOB, pad3 BLOB, INDEX(t1_id));
CREATE TABLE t3 (
id INT NOT NULL PRIMARY KEY auto_increment,
t1_id INT NOT NULL,
UNIQUE (t1_id)
);
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM dual;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t1 SELECT NULL, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024), 0 FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
INSERT INTO t2 SELECT NULL, a.id, RANDOM_BYTES(1024), RANDOM_BYTES(1024), RANDOM_BYTES(1024) FROM t1 a JOIN t1 b JOIN t1 c LIMIT 10000;
UPDATE t1 SET int_col = 1 WHERE pad1 = (SELECT pad1 FROM t1 ORDER BY RAND() LIMIT 1);
INSERT INTO t3 SELECT NULL, id FROM t1 WHERE id < 1000;
SELECT SLEEP(1);
ANALYZE TABLE t1, t2, t3;
Inner join（无 UNIQUE 约束的子查询）
以下示例中，IN 子查询会从表 t2 中搜索一列 ID。为保证语义正确性，TiDB 需要保证 t1_id 列的值具有唯一性。使用 EXPLAIN 可查看到该查询的执行计划去掉重复项并执行 Inner Join 内连接操作：
EXPLAIN SELECT * FROM t1 WHERE id IN (SELECT t1_id FROM t2);
+--------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+--------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------+
| IndexMergeJoin_19 | 45.00 | root | | inner join, inner:TableReader_14, outer key:test.t2.t1_id, inner key:test.t1.id |
| ├─HashAgg_38(Build) | 45.00 | root | | group by:test.t2.t1_id, funcs:firstrow(test.t2.t1_id)->test.t2.t1_id |
| │ └─IndexReader_39 | 45.00 | root | | index:HashAgg_31 |
| │ └─HashAgg_31 | 45.00 | cop[tikv] | | group by:test.t2.t1_id, |
| │ └─IndexFullScan_37 | 90000.00 | cop[tikv] | table:t2, index:t1_id(t1_id) | keep order:false |
| └─TableReader_14(Probe) | 1.00 | root | | data:TableRangeScan_13 |
| └─TableRangeScan_13 | 1.00 | cop[tikv] | table:t1 | range: decided by [test.t2.t1_id], keep order:true |
+--------------------------------+----------+-----------+------------------------------+---------------------------------------------------------------------------------+
7 rows in set (0.00 sec)
由上述查询结果可知，TiDB 首先执行 Index Join 索引连接（即 Merge Join 合并连接的变体）操作，开始读取 t2.t1_id 列的索引。先是 └─HashAgg_31 算子的部分任务在 TiKV 中对 t1_id 值进行去重，然后├─HashAgg_38(Build) 算子的部分任务在 TiDB 中对 t1_id 值再次进行去重。去重操作由聚合函数 firstrow(test.t2.t1_id) 执行，之后会将操作结果与 t1 表的主键相连接。
Inner join（有 UNIQUE 约束的子查询）
在上述示例中，为了确保 t1_id 值在与表 t1 连接前具有唯一性，需要执行聚合运算。在以下示例中，由于 UNIQUE 约束已能确保 t3.t1_id 列值的唯一：
EXPLAIN SELECT * FROM t1 WHERE id IN (SELECT t1_id FROM t3);
+-----------------------------+---------+-----------+------------------------------+---------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+---------+-----------+------------------------------+---------------------------------------------------------------------------------+
| IndexMergeJoin_20 | 999.00 | root | | inner join, inner:TableReader_15, outer key:test.t3.t1_id, inner key:test.t1.id |
| ├─IndexReader_39(Build) | 999.00 | root | | index:IndexFullScan_38 |
| │ └─IndexFullScan_38 | 999.00 | cop[tikv] | table:t3, index:t1_id(t1_id) | keep order:false |
| └─TableReader_15(Probe) | 1.00 | root | | data:TableRangeScan_14 |
| └─TableRangeScan_14 | 1.00 | cop[tikv] | table:t1 | range: decided by [test.t3.t1_id], keep order:true |
+-----------------------------+---------+-----------+------------------------------+---------------------------------------------------------------------------------+
5 rows in set (0.00 sec)
从语义上看，因为约束保证了 t3.t1_id 列值的唯一性，TiDB 可以直接执行 INNER JOIN 查询。
Semi Join（关联查询）
在前两个示例中，通过 HashAgg 聚合操作或通过 UNIQUE 约束保证子查询数据的唯一性之后，TiDB 才能够执行 Inner Join 操作。这两种连接均使用了 Index Join（Merge Join 的变体）。
下面的例子中，TiDB 优化器则选择了一种不同的执行计划：
EXPLAIN SELECT * FROM t1 WHERE id IN (SELECT t1_id FROM t2 WHERE t1_id != t1.int_col);
+-----------------------------+-----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+-----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+
| MergeJoin_9 | 45446.40 | root | | semi join, left key:test.t1.id, right key:test.t2.t1_id, other cond:ne(test.t2.t1_id, test.t1.int_col) |
| ├─IndexReader_24(Build) | 180000.00 | root | | index:IndexFullScan_23 |
| │ └─IndexFullScan_23 | 180000.00 | cop[tikv] | table:t2, index:t1_id(t1_id) | keep order:true |
| └─TableReader_22(Probe) | 56808.00 | root | | data:Selection_21 |
| └─Selection_21 | 56808.00 | cop[tikv] | | ne(test.t1.id, test.t1.int_col) |
| └─TableFullScan_20 | 71010.00 | cop[tikv] | table:t1 | keep order:true |
+-----------------------------+-----------+-----------+------------------------------+--------------------------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)
由上述查询结果可知，TiDB 执行了 Semi Join。不同于 Inner Join，Semi Join 仅允许右键 (t2.t1_id) 上的第一个值，也就是该操作将去除 Join 算子任务中的重复数据。Join 算法也包含 Merge Join，会按照排序顺序同时从左侧和右侧读取数据，这是一种高效的 Zipper Merge。
可以将原语句视为关联子查询，因为它引入了子查询外的 t1.int_col 列。然而，EXPLAIN 语句的返回结果显示的是关联子查询去关联后的执行计划。条件 t1_id != t1.int_col 会被重写为 t1.id != t1.int_col。TiDB 可以从表 t1 中读取数据并且在 └─Selection_21 中执行此操作，因此这种去关联和重写操作会极大提高执行效率。
Anti Semi Join （NOT IN 子查询）
在以下示例中，除非子查询中存在 t3.t1_id，否则该查询将（从语义上）返回表 t3 中的所有行：
EXPLAIN SELECT * FROM t3 WHERE t1_id NOT IN (SELECT id FROM t1 WHERE int_col < 100);
+-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------+
| id | estRows | task | access object | operator info |
+-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------+
| IndexMergeJoin_20 | 1598.40 | root | | anti semi join, inner:TableReader_15, outer key:test.t3.t1_id, inner key:test.t1.id |
| ├─TableReader_28(Build) | 1998.00 | root | | data:TableFullScan_27 |
| │ └─TableFullScan_27 | 1998.00 | cop[tikv] | table:t3 | keep order:false |
| └─TableReader_15(Probe) | 1.00 | root | | data:Selection_14 |
| └─Selection_14 | 1.00 | cop[tikv] | | lt(test.t1.int_col, 100) |
| └─TableRangeScan_13 | 1.00 | cop[tikv] | table:t1 | range: decided by [test.t3.t1_id], keep order:true |
+-----------------------------+---------+-----------+---------------+-------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)
上述查询首先读取了表 t3，然后根据主键开始探测 (probe) 表 t1。连接类型是 anti semi join，即反半连接：之所以使用 anti，是因为上述示例有不存在匹配值（即 NOT IN）的情况；使用 Semi Join 则是因为仅需要匹配第一行后就可以停止查询。