# 尝试检查执行计划

__*执行计划*__

```8
tidb> ~!@DESC@!~ SELECT name FROM universe.planets LIMIT 8;
+----------------------------+---------+-----------+---------------------------------+--------------------------------+
| id                         | estRows | task      | access object                   | operator info                  |
+----------------------------+---------+-----------+---------------------------------+--------------------------------+
| ~!@Limit_7@!~                    | 8.00    | root      |                                 | offset:0, count:8              |
| └─IndexReader_12           | 8.00    | root      |                                 | index:Limit_11                 |
|   └─Limit_11               | 8.00    | cop[tikv] |                                 | offset:0, count:8              |
|     └─IndexFullScan_10     | 8.00    | cop[tikv] | table:planets, index:name(name) | keep order:false, stats:pseudo |
+----------------------------+---------+-----------+---------------------------------+--------------------------------+
4 rows in set (0.01 sec)

tidb> ~!@EXPLAIN@!~ SELECT name FROM universe.planets ORDER BY distance_from_sun LIMIT 8;
+------------------------------+---------+-----------+---------------+-------------------------------------------------------+
| id                           | estRows | task      | access object | operator info                                         |
+------------------------------+---------+-----------+---------------+-------------------------------------------------------+
| Projection_7                 | 8.00    | root      |               | universe.planets.name                                 |
| └─~!@TopN_8@!~                     | 8.00    | root      |               | universe.planets.distance_from_sun, offset:0, count:8 |
|   └─TableReader_15           | 8.00    | root      |               | data:TopN_14                                          |
|     └─TopN_14                | 8.00    | cop[tikv] |               | universe.planets.distance_from_sun, offset:0, count:8 |
|       └─TableFullScan_13     | 9.00    | cop[tikv] | table:planets | keep order:false, stats:pseudo                        |
+------------------------------+---------+-----------+---------------+-------------------------------------------------------+
5 rows in set (0.00 sec)
```
-------------------------------------------------------------
SQL 是一种声明式语言。它描述了查询结果应该是什么样子,而不是实际检索这些结果的方法。TiDB 考虑了所有可能的查询执行方式,包括使用什么顺序连接表以及是否可以使用任何潜在的索引。考虑查询执行计划的过程称为 SQL 优化。

有关执行计划的更多信息, 请参阅: `https://docs.pingcap.com/tidb/stable/explain-overview`