# SQL explain / describe

> 可以用來分析 sql statement

# 比較重要的 columns

https://mariadb.com/kb/en/explain/#columns-in-explain-select

| Column 名稱 | 用途 |
| -------- | -------- |
| type | 有多少資料被撈出來 (join type) |
| possible_keys | 可能被用到的 key |
| key | 實際用到的 key |
| Extra | 額外資訊 |

# type
https://mariadb.com/kb/en/explain/#type-column

- ALL => full table scan 非常不好
- index => full scan 比 ALL 好但還是差

[有列出 type 最優到最差](https://mgleon08.github.io/blog/2017/09/01/mysql-sql-optimization-with-explain/)

最優到最差

- System
- const
- eq_ref
- ref
- fulltext
- ref_or_null
- index_merge
- unique_subquery
- index_subquery
- range
- index
- ALL

# extra

https://mariadb.com/kb/en/explain/#extra-column

# examples
https://mariadb.com/kb/en/explain/#extra-column

# 參考資料
- https://mariadb.com/kb/en/explain/#syntax
- https://mgleon08.github.io/blog/2017/09/01/mysql-sql-optimization-with-explain/

###### tags: `2022` `草稿` `sql` `perfromance` `mariadb` `索引` `index`
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