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Jun 16, 2020
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system_tables: add column description for several information_schema tables #3617

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add column description for several information_schema tables
bb7133 Jun 12, 2020
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address comment
bb7133 Jun 16, 2020
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57 changes: 26 additions & 31 deletions system-tables/system-table-information-schema.md
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Expand Up @@ -55,6 +55,13 @@ SELECT * FROM character_sets;
5 rows in set (0.00 sec)
```

`CHARACTER_SETS` 表中列的含义如下：

* `CHARACTER_SET_NAME`：字符集名称
* `DEFAULT_COLLATE_NAME`：字符集的默认排序规则名称
* `DESCRIPTION`：字符集的描述信息
* `MAXLEN`：该字符集存储一个字符所需要的最大字节数

## COLLATIONS 表

`COLLATIONS` 表提供了 `CHARACTER_SETS` 表中字符集对应的排序规则列表。TiDB 当前仅支持二进制排序规则，包含该表仅为兼容 MySQL。
Expand Down Expand Up @@ -99,6 +106,15 @@ SELECT * FROM collations WHERE character_set_name='utf8mb4';
26 rows in set (0.00 sec)
```

`COLLATION` 表中列的含义如下：

* `COLLATION_NAME`：排序规则名称
* `CHARACTER_SET_NAME`：排序规则所属的字符集名称
* `ID`：排序规则的 ID
* `IS_DEFAULT`：该排序规则是否是所属字符集的默认排序规则
* `IS_COMPILED`：字符集是否编译到服务器中
* `SORTLEN`：排序规则在对字符进行排序时，所分配内存的最小长度

## COLLATION_CHARACTER_SET_APPLICABILITY 表

`COLLATION_CHARACTER_SET_APPLICABILITY` 表将排序规则映射至适用的字符集名称。和 `COLLATIONS` 表一样，包含此表也是为了兼容 MySQL。
Expand All @@ -110,39 +126,18 @@ SELECT * FROM collation_character_set_applicability WHERE character_set_name='ut
```

```sql
+------------------------+--------------------+
| COLLATION_NAME | CHARACTER_SET_NAME |
+------------------------+--------------------+
| utf8mb4_general_ci | utf8mb4 |
| utf8mb4_bin | utf8mb4 |
| utf8mb4_unicode_ci | utf8mb4 |
| utf8mb4_icelandic_ci | utf8mb4 |
| utf8mb4_latvian_ci | utf8mb4 |
| utf8mb4_romanian_ci | utf8mb4 |
| utf8mb4_slovenian_ci | utf8mb4 |
| utf8mb4_polish_ci | utf8mb4 |
| utf8mb4_estonian_ci | utf8mb4 |
| utf8mb4_spanish_ci | utf8mb4 |
| utf8mb4_swedish_ci | utf8mb4 |
| utf8mb4_turkish_ci | utf8mb4 |
| utf8mb4_czech_ci | utf8mb4 |
| utf8mb4_danish_ci | utf8mb4 |
| utf8mb4_lithuanian_ci | utf8mb4 |
| utf8mb4_slovak_ci | utf8mb4 |
| utf8mb4_spanish2_ci | utf8mb4 |
| utf8mb4_roman_ci | utf8mb4 |
| utf8mb4_persian_ci | utf8mb4 |
| utf8mb4_esperanto_ci | utf8mb4 |
| utf8mb4_hungarian_ci | utf8mb4 |
| utf8mb4_sinhala_ci | utf8mb4 |
| utf8mb4_german2_ci | utf8mb4 |
| utf8mb4_croatian_ci | utf8mb4 |
| utf8mb4_unicode_520_ci | utf8mb4 |
| utf8mb4_vietnamese_ci | utf8mb4 |
+------------------------+--------------------+
26 rows in set (0.00 sec)
+----------------+--------------------+
| COLLATION_NAME | CHARACTER_SET_NAME |
+----------------+--------------------+
| utf8mb4_bin | utf8mb4 |
+----------------+--------------------+
```

`COLLATION_CHARACTER_SET_APPLICABILITY` 表中列的含义如下：

* `COLLATION_NAME`：排序规则名称
* `CHARACTER_SET_NAME`：排序规则所属的字符集名称

## COLUMNS 表

`COLUMNS` 表提供了表的所有列的信息。
Expand Down