EXPLAIN

显示 SQL 语句的执行计划。执行计划以由不同操作符组成的树形结构展示，你可以借此查看 TiDB Cloud Lake 将如何执行该 SQL 语句。一个操作符通常包含一个或多个字段，用于描述 TiDB Cloud Lake 将执行的操作或与查询相关的对象。

例如，以下由 EXPLAIN 命令返回的执行计划中包含一个名为 TableScan 的操作符，并带有多个字段。

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 TiDB Cloud Lake，可以利用 Query Profile 功能将 SQL 语句的执行计划可视化。

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]