> ## 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.

# max_size_* 会话设置

> ClickHouse 的 max_size_* 生成分组中的会话设置。

export const SettingsInfoBlock = ({type, default_value, changeable_without_restart}) => {
  return <div className="not-prose" style={{
    display: "flex",
    flexWrap: "wrap",
    alignItems: "baseline",
    columnGap: "0.5rem",
    rowGap: "0.125rem",
    margin: "0.375rem 0",
    fontSize: "0.8125rem",
    lineHeight: "1.125rem"
  }}>
      <div style={{
    fontWeight: 600,
    opacity: 0.72
  }}>类型</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{type}</div>
      <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>默认值</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{default_value}</div>
      {changeable_without_restart && <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>
          无需重启即可更改
        </div>}
      {changeable_without_restart && <div style={{
    overflowWrap: "anywhere"
  }}>
          {changeable_without_restart}
        </div>}
    </div>;
};

这些设置可在 [system.settings](/docs/zh/reference/system-tables/settings) 中查看，并根据 [源代码](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp) 自动生成。

<div id="max_size_to_preallocate_for_aggregation">
  ## max\_size\_to\_preallocate\_for\_aggregation
</div>

<SettingsInfoBlock type="UInt64" default_value="1000000000000" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.12"},{"label": "1000000000000"},{"label": "为更大的表启用优化。"}]}, {"id": "row-2","items": [{"label": "22.12"},{"label": "100000000"},{"label": "可优化性能"}]}]} />

在聚合前，所有哈希表总共允许为多少个元素预分配空间

<div id="max_size_to_preallocate_for_joins">
  ## max\_size\_to\_preallocate\_for\_joins
</div>

<SettingsInfoBlock type="UInt64" default_value="1000000000000" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.7"},{"label": "100000000"},{"label": "新增设置。"}]}, {"id": "row-2","items": [{"label": "24.12"},{"label": "1000000000000"},{"label": "为更大的表启用优化。"}]}]} />

在 joi 之前，总共允许在所有哈希表中预分配可容纳多少个元素的空间