> ## 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.

# shrink_over_* 会话设置

> shrink_over_* 生成组中的 ClickHouse 会话设置。

export const VersionHistory = ({rows = []}) => {
  if (rows.length === 0) {
    return null;
  }
  const headers = ["版本", "默认值", "注释"];
  const border = "1px solid rgba(128, 128, 128, 0.3)";
  const cell = {
    border,
    padding: "0.25rem 0.5rem",
    textAlign: "start",
    verticalAlign: "top"
  };
  return <details className="not-prose" style={{
    border,
    borderRadius: "0.5rem",
    margin: "0.5rem 0",
    padding: "0.5rem 0.75rem",
    fontSize: "0.8125rem",
    lineHeight: "1.125rem"
  }}>
      <summary style={{
    cursor: "pointer",
    fontWeight: 600,
    opacity: 0.72
  }}>
        版本历史
      </summary>
      <table style={{
    borderCollapse: "collapse",
    width: "100%",
    margin: "0.5rem 0 0"
  }}>
        <thead>
          <tr>
            {headers.map(header => <th key={header} style={{
    ...cell,
    fontWeight: 600,
    opacity: 0.72
  }}>
                {header}
              </th>)}
          </tr>
        </thead>
        <tbody>
          {rows.map((row, row_index) => <tr key={row.id ?? row_index}>
              {(row.items ?? []).map((item, item_index) => <td key={item_index} style={{
    ...cell,
    overflowWrap: "anywhere"
  }}>
                  {item?.label}
                </td>)}
            </tr>)}
        </tbody>
      </table>
    </details>;
};

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，并由[源代码](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp)自动生成。

<div id="shrink_over_allocated_columns_min_waste_bytes">
  ## shrink\_over\_allocated\_columns\_min\_waste\_bytes
</div>

<SettingsInfoBlock type="UInt64" default_value="16777216" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.8"},{"label": "16777216"},{"label": "新增设置：在 INSERT 时将列收缩至实际大小所需的列内最小绝对浪费内存。"}]}]} />

在 INSERT 时将列收缩至实际大小所需的列内最小绝对浪费内存量 (已预留但未使用) ，以字节为单位。请参阅 [shrink\_over\_allocated\_columns\_min\_waste\_ratio](/docs/zh/reference/settings/session-settings/shrink-over#shrink_over_allocated_columns_min_waste_ratio)。

避免重新分配过度分配量较小的列，因为节省的内存不足以抵消其成本。

<div id="shrink_over_allocated_columns_min_waste_ratio">
  ## shrink\_over\_allocated\_columns\_min\_waste\_ratio
</div>

<SettingsInfoBlock type="Float" default_value="1" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.8"},{"label": "1"},{"label": "新增设置：在 INSERT 时将过度分配的列收缩至实际大小，从而降低峰值内存占用。默认禁用（1.0）。"}]}]} />

在 INSERT 时，会在开销较大的物化和分片写入阶段之前将过度分配的列收缩至实际大小 (即预留内存超过已用内存的列，例如可变长度列按 2 的幂扩容时) ，以降低峰值内存占用。

仅当 `allocatedBytes() > byteSize() * shrink_over_allocated_columns_min_waste_ratio` 且绝对浪费量至少达到 `shrink_over_allocated_columns_min_waste_bytes` 时，才会缩减列。

可能的值：

* 大于 1 的 Float。
* 小于或等于 1 的值会禁用缩减功能 (默认值 `1.0` 会保持禁用状态) 。