> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-postgresql-tls-support.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# optimize_limit_* 会话设置

> 按 optimize_limit_* 自动生成的分组中的 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](/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="optimize_limit_by_function_keys">
  ## optimize\_limit\_by\_function\_keys
</div>

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

消除 LIMIT BY 部分中可由其他键推导出的函数键。

示例：`LIMIT 5 BY x, f(x)` 会变为 `LIMIT 5 BY x`。

<div id="optimize_limit_by_in_order">
  ## optimize\_limit\_by\_in\_order
</div>

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

当 `<cols>` (顺序不限) 构成表排序键的前缀时，或者在 `WHERE col = const` 固定前导列后能够构成此前缀时，可优化 `SELECT ... LIMIT N BY <cols>` 查询。启用后，源端会按主键顺序读取数据，因此在每个流内，`BY` 列值相同的行会相邻到达。当数据以单个已排序流到达时，`LIMIT BY` 会以流式模式进行过滤，只需 O(1) 内存，而无需为已见到的每种不同 `BY` 列组合构建哈希表。当已排序数据通过多个流到达，且相同的 `BY` 值可能出现在多个流中时，系统会先以流式模式对每个流进行预过滤，将每个分组最多保留 `LIMIT + OFFSET` 行；然后再合并这些流，并通过最终基于哈希的 `LIMIT BY` 对跨多个流的分组进行去重。最后这一轮仍会为 `BY` 列的每种不同组合保留一个条目，但只处理预过滤后的行。