EXPLAIN

EXPLAIN 语句仅用于显示查询的执行计划，而不执行查询。EXPLAIN ANALYZE 可执行查询，补充 EXPLAIN 语句。如果 EXPLAIN 的输出与预期结果不匹配，可考虑在查询的每个表上执行 ANALYZE TABLE。

语句 DESC 和 DESCRIBE 是 EXPLAIN 的别名。EXPLAIN <tableName> 的替代用法记录在 SHOW [FULL] COLUMNS FROM 下。

TiDB 支持 EXPLAIN [options] FOR CONNECTION connection_id，但与 MySQL 的 EXPLAIN FOR 有一些区别，请参见 EXPLAIN FOR CONNECTION。

语法图

ExplainSym:

ExplainStmt:

ExplainableStmt:

EXPLAIN 输出格式

目前 TiDB 的 EXPLAIN 会输出 5 列，分别是：id，estRows，task，access object， operator info。执行计划中每个算子都由这 5 列属性来描述，EXPLAIN 结果中每一行描述一个算子。每个属性的具体含义如下：

示例

EXPLAIN SELECT 1;

+-------------------+---------+------+---------------+---------------+
| id                | estRows | task | access object | operator info |
+-------------------+---------+------+---------------+---------------+
| Projection_3      | 1.00    | root |               | 1->Column#1   |
| └─TableDual_4     | 1.00    | root |               | rows:1        |
+-------------------+---------+------+---------------+---------------+
2 rows in set (0.00 sec)

CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);

Query OK, 0 rows affected (0.10 sec)

INSERT INTO t1 (c1) VALUES (1), (2), (3);

Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

EXPLAIN SELECT * FROM t1 WHERE id = 1;

+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| Point_Get_1 | 1.00    | root | table:t1      | handle:1      |
+-------------+---------+------+---------------+---------------+
1 row in set (0.00 sec)

DESC SELECT * FROM t1 WHERE id = 1;

+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| Point_Get_1 | 1.00    | root | table:t1      | handle:1      |
+-------------+---------+------+---------------+---------------+
1 row in set (0.00 sec)

DESCRIBE SELECT * FROM t1 WHERE id = 1;

+-------------+---------+------+---------------+---------------+
| id          | estRows | task | access object | operator info |
+-------------+---------+------+---------------+---------------+
| Point_Get_1 | 1.00    | root | table:t1      | handle:1      |
+-------------+---------+------+---------------+---------------+
1 row in set (0.00 sec)

EXPLAIN INSERT INTO t1 (c1) VALUES (4);

+----------+---------+------+---------------+---------------+
| id       | estRows | task | access object | operator info |
+----------+---------+------+---------------+---------------+
| Insert_1 | N/A     | root |               | N/A           |
+----------+---------+------+---------------+---------------+
1 row in set (0.00 sec)

EXPLAIN UPDATE t1 SET c1=5 WHERE c1=3;

+---------------------------+---------+-----------+---------------+--------------------------------+
| id                        | estRows | task      | access object | operator info                  |
+---------------------------+---------+-----------+---------------+--------------------------------+
| Update_4                  | N/A     | root      |               | N/A                            |
| └─TableReader_8           | 0.00    | root      |               | data:Selection_7               |
|   └─Selection_7           | 0.00    | cop[tikv] |               | eq(test.t1.c1, 3)              |
|     └─TableFullScan_6     | 3.00    | cop[tikv] | table:t1      | keep order:false, stats:pseudo |
+---------------------------+---------+-----------+---------------+--------------------------------+
4 rows in set (0.00 sec)

EXPLAIN DELETE FROM t1 WHERE c1=3;

+---------------------------+---------+-----------+---------------+--------------------------------+
| id                        | estRows | task      | access object | operator info                  |
+---------------------------+---------+-----------+---------------+--------------------------------+
| Delete_4                  | N/A     | root      |               | N/A                            |
| └─TableReader_8           | 0.00    | root      |               | data:Selection_7               |
|   └─Selection_7           | 0.00    | cop[tikv] |               | eq(test.t1.c1, 3)              |
|     └─TableFullScan_6     | 3.00    | cop[tikv] | table:t1      | keep order:false, stats:pseudo |
+---------------------------+---------+-----------+---------------+--------------------------------+
4 rows in set (0.01 sec)

如果未指定 FORMAT，或未指定 FORMAT ="row"，那么 EXPLAIN 语句将以表格格式输出结果。更多信息，可参阅 Understand the Query Execution Plan。

除 MySQL 标准结果格式外，TiDB 还支持 DotGraph。需按照下列所示指定 FORMAT ="dot"：

create table t(a bigint, b bigint);
desc format = "dot" select A.a, B.b from t A join t B on A.a > B.b where A.a < 10;

+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| dot contents                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| 
digraph Projection_8 {
subgraph cluster8{
node [style=filled, color=lightgrey]
color=black
label = "root"
"Projection_8" -> "HashJoin_9"
"HashJoin_9" -> "TableReader_13"
"HashJoin_9" -> "Selection_14"
"Selection_14" -> "TableReader_17"
}
subgraph cluster12{
node [style=filled, color=lightgrey]
color=black
label = "cop"
"Selection_12" -> "TableFullScan_11"
}
subgraph cluster16{
node [style=filled, color=lightgrey]
color=black
label = "cop"
"Selection_16" -> "TableFullScan_15"
}
"TableReader_13" -> "Selection_12"
"TableReader_17" -> "Selection_16"
}
 |
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

如果你的计算机上安装了 dot 程序（在 graphviz 包中），可使用以下方法生成 PNG 文件：

dot xx.dot -T png -O

The xx.dot is the result returned by the above statement.

如果你的计算机上未安装 dot 程序，可将结果复制到 本网站 以获取树形图：

MySQL 兼容性

• EXPLAIN 的格式和 TiDB 中潜在的执行计划都与 MySQL 有很大不同。
• TiDB 不像 MySQL 那样支持 EXPLAIN FORMAT = JSON。
• TiDB 目前不支持插入语句的 EXPLAIN。

EXPLAIN FOR CONNECTION

EXPLAIN FOR CONNECTION 用于获得一个连接中最后执行的查询的执行计划，其输出格式与 EXPLAIN 完全一致。但 TiDB 中的实现与 MySQL 不同，除了输出格式之外，还有以下区别：

• MySQL 返回的是正在执行的查询计划，而 TiDB 返回的是最后执行的查询计划。
• MySQL 的文档中指出，MySQL 要求登录用户与被查询的连接相同，或者拥有 PROCESS 权限，而 TiDB 则要求登录用户与被查询的连接相同，或者拥有 SUPER 权限。

另请参阅

• 理解 TiDB 执行计划
• EXPLAIN ANALYZE
• ANALYZE TABLE
• TRACE