> ## 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.

# enable_parallel_* 会话设置

> ClickHouse 中由 enable_parallel_* 自动生成的会话设置组。

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>;
};

这些设置位于 [system.settings](/docs/zh/reference/system-tables/settings) 中，并从[源代码](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp)自动生成。

<div id="enable_parallel_blocks_marshalling">
  ## enable\_parallel\_blocks\_marshalling
</div>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.6"},{"label": "true"},{"label": "新增设置"}]}]} />

仅影响分布式查询。启用后，在向发起节点发送数据前后，将由管道线程对块进行 (反) 序列化和 (解) 压缩 (即采用高于默认值的并行度) 。

<div id="enable_parallel_single_level_merge">
  ## enable\_parallel\_single\_level\_merge
</div>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.8"},{"label": "1"},{"label": "新增设置：将键空间拆分为互不重叠的哈希分区，由各线程独立合并，从而并行执行单层聚合哈希表的最终合并。"}]}]} />

并行执行各线程单层聚合哈希表的最终合并。键空间会根据键的哈希值拆分为互不重叠的分区，每个分区独立合并：合并线程遍历所有线程本地表，并合并属于其分区的键，因此不会有两个线程同时处理同一个键。如果关闭此设置，单层表将在单个线程上串行合并。无论此设置是否启用，按 8 位和 16 位键进行的聚合均会保留其自身基于范围分区的并行合并。