- 关于 TiDB
- 主要概念
- 操作指南
- 快速上手
- 部署
- 配置
- 安全
- 安全传输层协议 (TLS)
- 生成自签名证书
- 监控
- 迁移
- 运维
- 扩容缩容
- 升级
- 故障诊断
- 参考手册
- SQL
- 与 MySQL 兼容性对比
- SQL 语言结构
- 数据类型
- 函数与操作符
- SQL 语句
ADD COLUMN
ADD INDEX
ADMIN
ADMIN CANCEL DDL
ADMIN CHECKSUM TABLE
ADMIN CHECK [TABLE|INDEX]
ADMIN SHOW DDL [JOBS|QUERIES]
ALTER DATABASE
ALTER TABLE
ALTER USER
ANALYZE TABLE
BEGIN
CHANGE COLUMN
COMMIT
CREATE DATABASE
CREATE INDEX
CREATE TABLE LIKE
CREATE TABLE
CREATE USER
DEALLOCATE
DELETE
DESC
DESCRIBE
DO
DROP COLUMN
DROP DATABASE
DROP INDEX
DROP TABLE
DROP USER
EXECUTE
EXPLAIN ANALYZE
EXPLAIN
FLUSH PRIVILEGES
FLUSH STATUS
FLUSH TABLES
GRANT <privileges>
INSERT
KILL [TIDB]
LOAD DATA
LOAD STATS
MODIFY COLUMN
PREPARE
RENAME INDEX
RENAME TABLE
REPLACE
REVOKE <privileges>
ROLLBACK
SELECT
SET [NAMES|CHARACTER SET]
SET PASSWORD
SET TRANSACTION
SET [GLOBAL|SESSION] <variable>
SHOW CHARACTER SET
SHOW COLLATION
SHOW [FULL] COLUMNS FROM
SHOW CREATE TABLE
SHOW DATABASES
SHOW ENGINES
SHOW ERRORS
SHOW [FULL] FIELDS FROM
SHOW GRANTS
SHOW INDEXES [FROM|IN]
SHOW INDEX [FROM|IN]
SHOW KEYS [FROM|IN]
SHOW PRIVILEGES
SHOW [FULL] PROCESSSLIST
SHOW SCHEMAS
SHOW STATUS
SHOW [FULL] TABLES
SHOW TABLE STATUS
SHOW [GLOBAL|SESSION] VARIABLES
SHOW WARNINGS
START TRANSACTION
TRACE
TRUNCATE
UPDATE
USE
- 约束
- 生成列
- 字符集
- 配置
- tidb-server
- pd-server
- tikv-server
- 安全
- 事务
- 系统数据库
- 错误码
- 支持的连接器和 API
- 垃圾回收 (GC)
- 性能调优
- 监控指标
- 报警规则
- 最佳实践
- TiSpark 使用指南
- TiDB Binlog
- 周边工具
- 常见问题 (FAQ)
- 技术支持
- 贡献
- 版本发布历史
- 术语表
重要
你正在查看 TiDB 数据库的较旧版本 (TiDB v2.1) 的文档。
如无特殊需求,建议使用 TiDB 数据库的最新 LTS 版本。
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
mysql> 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)
mysql> CREATE TABLE t1 (id INT NOT NULL PRIMARY KEY AUTO_INCREMENT, c1 INT NOT NULL);
Query OK, 0 rows affected (0.10 sec)
mysql> INSERT INTO t1 (c1) VALUES (1), (2), (3);
Query OK, 3 rows affected (0.02 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> 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)
mysql> 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)
mysql> 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)
mysql> EXPLAIN INSERT INTO t1 (c1) VALUES (4);
ERROR 1105 (HY000): Unsupported type *core.Insert
mysql> 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)
mysql> 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;
TiDB > desc format = "dot" select A.a, B.b from t A join t B on A.a > B.b where A.a < 10;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
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