文档目录

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 DESCDESCRIBEEXPLAIN 的别名。EXPLAIN <tableName> 的替代用法记录在 SHOW [FULL] COLUMNS FROM 下。

语法图

ExplainSym:

ExplainSym

ExplainStmt:

ExplainStmt

ExplainableStmt:

ExplainableStmt

示例

EXPLAIN SELECT 1;
+-------------------+-------+------+---------------+
| id                | count | task | operator info |
+-------------------+-------+------+---------------+
| Projection_3      | 1.00  | root | 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          | count | task | 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          | count | task | 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          | count | task | 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);
ERROR 1105 (HY000): Unsupported type *core.Insert
EXPLAIN UPDATE t1 SET c1=5 WHERE c1=3;
+---------------------+----------+------+-------------------------------------------------------------+
| id                  | count    | task | operator info                                               |
+---------------------+----------+------+-------------------------------------------------------------+
| TableReader_6       | 10.00    | root | data:Selection_5                                            |
| └─Selection_5       | 10.00    | cop  | eq(test.t1.c1, 3)                                           |
|   └─TableScan_4     | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+------+-------------------------------------------------------------+
3 rows in set (0.00 sec)
EXPLAIN DELETE FROM t1 WHERE c1=3;
+---------------------+----------+------+-------------------------------------------------------------+
| id                  | count    | task | operator info                                               |
+---------------------+----------+------+-------------------------------------------------------------+
| TableReader_6       | 10.00    | root | data:Selection_5                                            |
| └─Selection_5       | 10.00    | cop  | eq(test.t1.c1, 3)                                           |
|   └─TableScan_4     | 10000.00 | cop  | table:t1, range:[-inf,+inf], keep order:false, stats:pseudo |
+---------------------+----------+------+-------------------------------------------------------------+
3 rows in set (0.00 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 HashRightJoin_7 {
subgraph cluster7{
node [style=filled, color=lightgrey]
color=black
label = "root"
"HashRightJoin_7" -> "TableReader_10"
"HashRightJoin_7" -> "TableReader_12"
}
subgraph cluster9{
node [style=filled, color=lightgrey]
color=black
label = "cop"
"Selection_9" -> "TableScan_8"
}
subgraph cluster11{
node [style=filled, color=lightgrey]
color=black
label = "cop"
"TableScan_11"
}
"TableReader_10" -> "Selection_9"
"TableReader_12" -> "TableScan_11"
}
 |
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
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:

Explain Dot

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

另请参阅