> ## Documentation Index
> Fetch the complete documentation index at: https://clickhouse.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.
# use_variant_* 会话设置
> ClickHouse 中 use_variant_* 自动生成分组的会话设置。
export const SettingsInfoBlock = ({type, default_value, changeable_without_restart}) => {
return
类型
{type}
默认值
{default_value}
{changeable_without_restart &&
无需重启即可更改
}
{changeable_without_restart &&
{changeable_without_restart}
}
;
};
这些设置可在 [system.settings](/docs/zh/reference/system-tables/settings) 中查看，并根据 [source](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp) 自动生成。
## use\_variant\_as\_common\_type
当各参数类型之间不存在共同类型时，允许将 `Variant` 类型用作 [if](/docs/zh/reference/functions/regular-functions/conditional-functions#if)/[multiIf](/docs/zh/reference/functions/regular-functions/conditional-functions#multiIf)/[array](/docs/zh/reference/functions/regular-functions/array-functions)/[map](/docs/zh/reference/functions/regular-functions/tuple-map-functions) 函数的结果类型。
示例：
```sql theme={null}
SET use_variant_as_common_type = 1;
SELECT toTypeName(if(number % 2, number, range(number))) as variant_type FROM numbers(1);
SELECT if(number % 2, number, range(number)) as variant FROM numbers(5);
```
```text theme={null}
┌─variant_type───────────────────┐
│ Variant(Array(UInt64), UInt64) │
└────────────────────────────────┘
┌─variant───┐
│ [] │
│ 1 │
│ [0,1] │
│ 3 │
│ [0,1,2,3] │
└───────────┘
```
```sql theme={null}
SET use_variant_as_common_type = 1;
SELECT toTypeName(multiIf((number % 4) = 0, 42, (number % 4) = 1, [1, 2, 3], (number % 4) = 2, 'Hello, World!', NULL)) AS variant_type FROM numbers(1);
SELECT multiIf((number % 4) = 0, 42, (number % 4) = 1, [1, 2, 3], (number % 4) = 2, 'Hello, World!', NULL) AS variant FROM numbers(4);
```
```text theme={null}
─variant_type─────────────────────────┐
│ Variant(Array(UInt8), String, UInt8) │
└──────────────────────────────────────┘
┌─variant───────┐
│ 42 │
│ [1,2,3] │
│ Hello, World! │
│ ᴺᵁᴸᴸ │
└───────────────┘
```
```sql theme={null}
SET use_variant_as_common_type = 1;
SELECT toTypeName(array(range(number), number, 'str_' || toString(number))) as array_of_variants_type from numbers(1);
SELECT array(range(number), number, 'str_' || toString(number)) as array_of_variants FROM numbers(3);
```
```text theme={null}
┌─array_of_variants_type────────────────────────┐
│ Array(Variant(Array(UInt64), String, UInt64)) │
└───────────────────────────────────────────────┘
┌─array_of_variants─┐
│ [[],0,'str_0'] │
│ [[0],1,'str_1'] │
│ [[0,1],2,'str_2'] │
└───────────────────┘
```
```sql theme={null}
SET use_variant_as_common_type = 1;
SELECT toTypeName(map('a', range(number), 'b', number, 'c', 'str_' || toString(number))) as map_of_variants_type from numbers(1);
SELECT map('a', range(number), 'b', number, 'c', 'str_' || toString(number)) as map_of_variants FROM numbers(3);
```
```text theme={null}
┌─map_of_variants_type────────────────────────────────┐
│ Map(String, Variant(Array(UInt64), String, UInt64)) │
└─────────────────────────────────────────────────────┘
┌─map_of_variants───────────────┐
│ {'a':[],'b':0,'c':'str_0'} │
│ {'a':[0],'b':1,'c':'str_1'} │
│ {'a':[0,1],'b':2,'c':'str_2'} │
└───────────────────────────────┘
```
## use\_variant\_default\_implementation\_for\_comparisons
启用或禁用比较函数中 Variant 类型的默认实现。