EXPLAIN
显示 SQL 语句的执行计划。执行计划以树状结构展示,由不同的算子(Operator)组成,从中可以看到 Databend 将如何执行该 SQL 语句。一个算子(Operator)通常包含一个或多个字段,用于描述 Databend 将执行的操作或与查询相关的对象。
例如,以下 EXPLAIN 命令返回的执行计划包含一个名为 TableScan 的算子(Operator),该算子(Operator)包含多个字段。
EXPLAIN SELECT * FROM allemployees;
---
TableScan
├── table: default.default.allemployees
├── read rows: 5
├── read bytes: 592
├── partitions total: 5
├── partitions scanned: 5
└── push downs: [filters: [], limit: NONE]
如果你正在使用 Databend Cloud,可以利用查询分析(Query Profile)功能来可视化 SQL 语句的执行计划。
语法
EXPLAIN <statement>
示例
EXPLAIN select t.number from numbers(1) as t, numbers(1) as t1 where t.number = t1.number;
----
Project
├── columns: [number (#0)]
└── HashJoin
├── join type: INNER
├── build keys: [numbers.number (#1)]
├── probe keys: [numbers.number (#0)]
├── filters: []
├── TableScan(Build)
│ ├── table: default.system.numbers
│ ├── read rows: 1
│ ├── read bytes: 8
│ ├── partitions total: 1
│ ├── partitions scanned: 1
│ └── push downs: [filters: [], limit: NONE]
└── TableScan(Probe)
├── table: default.system.numbers
├── read rows: 1
├── read bytes: 8
├── partitions total: 1
├── partitions scanned: 1
└── push downs: [filters: [], limit: NONE]