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。

语法图

ExplainSym:

ExplainStmt:

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：

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

• Understanding the Query Execution Plan
• EXPLAIN ANALYZE
• ANALYZE TABLE
• TRACE