- 关于 TiDB
- 快速上手
- 部署集群
- 数据迁移
- 运维操作
- 升级 TiDB 版本
- 扩缩容
- 备份与恢复
- 使用 BR 工具(推荐)
- 读取历史数据
- 修改时区
- 日常巡检
- TiFlash 常用运维操作
- TiUP 常用运维操作
- 在线修改集群配置
- 监控与告警
- 故障诊断
- 性能调优
- 系统调优
- 软件调优
- SQL 性能调优
- SQL 性能调优概览
- 理解 TiDB 执行计划
- SQL 优化流程
- 控制执行计划
- 教程
- TiDB 生态工具
- 参考指南
- 架构
- 监控指标
- 安全加固
- 权限
- SQL
- SQL 语言结构和语法
- 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 INDEX
ALTER INSTANCE
ALTER TABLE
ALTER USER
ANALYZE TABLE
BACKUP
BEGIN
CHANGE COLUMN
CHANGE DRAINER
CHANGE PUMP
COMMIT
CREATE [GLOBAL|SESSION] BINDING
CREATE DATABASE
CREATE INDEX
CREATE ROLE
CREATE SEQUENCE
CREATE TABLE LIKE
CREATE TABLE
CREATE USER
CREATE VIEW
DEALLOCATE
DELETE
DESC
DESCRIBE
DO
DROP [GLOBAL|SESSION] BINDING
DROP COLUMN
DROP DATABASE
DROP INDEX
DROP ROLE
DROP SEQUENCE
DROP STATS
DROP TABLE
DROP USER
DROP VIEW
EXECUTE
EXPLAIN ANALYZE
EXPLAIN
FLASHBACK TABLE
FLUSH PRIVILEGES
FLUSH STATUS
FLUSH TABLES
GRANT <privileges>
GRANT <role>
INSERT
KILL [TIDB]
LOAD DATA
LOAD STATS
MODIFY COLUMN
PREPARE
RECOVER TABLE
RENAME INDEX
RENAME TABLE
REPLACE
RESTORE
REVOKE <privileges>
REVOKE <role>
ROLLBACK
SELECT
SET DEFAULT ROLE
SET [NAMES|CHARACTER SET]
SET PASSWORD
SET ROLE
SET TRANSACTION
SET [GLOBAL|SESSION] <variable>
SHOW [BACKUPS|RESTORES]
SHOW ANALYZE STATUS
SHOW [GLOBAL|SESSION] BINDINGS
SHOW BUILTINS
SHOW CHARACTER SET
SHOW COLLATION
SHOW [FULL] COLUMNS FROM
SHOW CONFIG
SHOW CREATE SEQUENCE
SHOW CREATE TABLE
SHOW CREATE USER
SHOW DATABASES
SHOW DRAINER STATUS
SHOW ENGINES
SHOW ERRORS
SHOW [FULL] FIELDS FROM
SHOW GRANTS
SHOW INDEX [FROM|IN]
SHOW INDEXES [FROM|IN]
SHOW KEYS [FROM|IN]
SHOW MASTER STATUS
SHOW PLUGINS
SHOW PRIVILEGES
SHOW [FULL] PROCESSSLIST
SHOW PROFILES
SHOW PUMP STATUS
SHOW SCHEMAS
SHOW STATS_HEALTHY
SHOW STATS_HISTOGRAMS
SHOW STATS_META
SHOW STATUS
SHOW TABLE NEXT_ROW_ID
SHOW TABLE REGIONS
SHOW TABLE STATUS
SHOW [FULL] TABLES
SHOW [GLOBAL|SESSION] VARIABLES
SHOW WARNINGS
SHUTDOWN
SPLIT REGION
START TRANSACTION
TRACE
TRUNCATE
UPDATE
USE
- 数据类型
- 函数与操作符
- 聚簇索引
- 约束
- 生成列
- SQL 模式
- 事务
- 垃圾回收 (GC)
- 视图
- 分区表
- 字符集和排序规则
- 系统表
mysql
- INFORMATION_SCHEMA
- Overview
ANALYZE_STATUS
CHARACTER_SETS
CLUSTER_CONFIG
CLUSTER_HARDWARE
CLUSTER_INFO
CLUSTER_LOAD
CLUSTER_LOG
CLUSTER_SYSTEMINFO
COLLATIONS
COLLATION_CHARACTER_SET_APPLICABILITY
COLUMNS
DDL_JOBS
ENGINES
INSPECTION_RESULT
INSPECTION_RULES
INSPECTION_SUMMARY
KEY_COLUMN_USAGE
METRICS_SUMMARY
METRICS_TABLES
PARTITIONS
PROCESSLIST
SCHEMATA
SEQUENCES
SESSION_VARIABLES
SLOW_QUERY
STATISTICS
TABLES
TABLE_CONSTRAINTS
TABLE_STORAGE_STATS
TIDB_HOT_REGIONS
TIDB_INDEXES
TIDB_SERVERS_INFO
TIFLASH_REPLICA
TIKV_REGION_PEERS
TIKV_REGION_STATUS
TIKV_STORE_STATUS
USER_PRIVILEGES
VIEWS
METRICS_SCHEMA
- UI
- CLI
- 命令行参数
- 配置文件参数
- 系统变量
- 存储引擎
- TiKV
- TiFlash
- TiUP
- 遥测
- 错误码
- 通过拓扑 label 进行副本调度
- 常见问题解答 (FAQ)
- 术语表
- 版本发布历史
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:
属性名 | 含义 |
---|---|
id | 算子的 ID,是算子在整个执行计划中唯一的标识。在 TiDB 2.1 中,ID 会格式化地显示算子的树状结构。数据从孩子结点流向父亲结点,每个算子的父亲结点有且仅有一个。 |
estRows | 算子预计将会输出的数据条数,基于统计信息以及算子的执行逻辑估算而来。在 4.0 之前叫 count。 |
task | 算子属于的 task 种类。目前的执行计划分成为两种 task,一种叫 root task,在 tidb-server 上执行,一种叫 cop task,在 TiKV 或者 TiFlash 上并行执行。当前的执行计划在 task 级别的拓扑关系是一个 root task 后面可以跟许多 cop task,root task 使用 cop task 的输出结果作为输入。cop task 中执行的也即是 TiDB 下推到 TiKV 或者 TiFlash 上的任务,每个 cop task 分散在 TiKV 或者 TiFlash 集群中,由多个进程共同执行。 |
access object | 算子所访问的数据项信息。包括表 table ,表分区 partition 以及使用的索引 index (如果有)。只有直接访问数据的算子才拥有这些信息。 |
operator info | 算子的其它信息。各个算子的 operator info 各有不同,可参考下面的示例解读。 |
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。