> ## 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_streams_* session settings

> ClickHouse session settings in the max_streams_* generated group.

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
  }}>Type</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{type}</div>
      <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>Default</div>
      <div style={{
    overflowWrap: "anywhere"
  }}>{default_value}</div>
      {changeable_without_restart && <div style={{
    fontWeight: 600,
    opacity: 0.72,
    marginInlineStart: "0.5rem"
  }}>
          Changeable without restart
        </div>}
      {changeable_without_restart && <div style={{
    overflowWrap: "anywhere"
  }}>
          {changeable_without_restart}
        </div>}
    </div>;
};

These settings are available in [system.settings](/docs/reference/system-tables/settings) and are autogenerated from [source](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp).

<h2 id="max_streams_for_files_processing_in_cluster_functions">
  max\_streams\_for\_files\_processing\_in\_cluster\_functions
</h2>

<SettingsInfoBlock type="UInt64" default_value="0" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.12"},{"label": "0"},{"label": "Add a new setting that allows to limit number of streams for files processing in *Cluster table functions"}]}]} />

If is not zero, limit the number of threads reading data from files in \*Cluster table functions.

<h2 id="max_streams_for_merge_tree_reading">
  max\_streams\_for\_merge\_tree\_reading
</h2>

<SettingsInfoBlock type="UInt64" default_value="0" />

If is not zero, limit the number of reading streams for MergeTree table.

<h2 id="max_streams_for_union_step">
  max\_streams\_for\_union\_step
</h2>

<SettingsInfoBlock type="UInt64" default_value="0" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.5"},{"label": "0"},{"label": "New setting to limit the number of simultaneously active data streams in a UNION step to reduce peak memory usage."}]}]} />

Limits the number of simultaneously active data streams in a `UNION` step (applies to both `UNION ALL` and `UNION DISTINCT`, because `UNION DISTINCT` is implemented via a `UNION ALL` step followed by a `DISTINCT` step). When a `UNION` query has many subqueries, all of them open their read buffers at the same time, leading to memory usage proportional to the number of subqueries. This setting inserts `Concat` processors to narrow the pipeline so that at most this many streams are active at once, drastically reducing peak memory. The actual limit is the minimum of this value and `max_threads * max_streams_for_union_step_to_max_threads_ratio` (either one being 0 means it is ignored). When both are 0, no narrowing is applied. The limit is also not applied when the query plan requires each output stream of the `UNION` to stay individually sorted (for example, when the read-in-order optimization is applied across the `UNION`); in that case correctness of the ordering takes precedence and narrowing is skipped.

<h2 id="max_streams_for_union_step_to_max_threads_ratio">
  max\_streams\_for\_union\_step\_to\_max\_threads\_ratio
</h2>

<SettingsInfoBlock type="Float" default_value="8" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.5"},{"label": "8"},{"label": "New setting: the limit on simultaneously active streams in a UNION step is computed as min(max_streams_for_union_step, max_threads * max_streams_for_union_step_to_max_threads_ratio), either being 0 disables that input."}]}]} />

This ratio multiplied by `max_threads` determines a limit on simultaneously active streams in a `UNION` step (applies to both `UNION ALL` and `UNION DISTINCT`). The actual limit is the minimum of this computed value and `max_streams_for_union_step` (either one being 0 means it is ignored). For example, with `max_threads = 8` and this ratio set to 1, at most 8 streams will be active. Set to 0 to disable this ratio-based limit. Like `max_streams_for_union_step`, the limit is not applied when the query plan requires each output stream of the `UNION` to stay individually sorted.

<h2 id="max_streams_multiplier_for_merge_tables">
  max\_streams\_multiplier\_for\_merge\_tables
</h2>

<SettingsInfoBlock type="Float" default_value="5" />

Ask more streams when reading from Merge table. Streams will be spread across tables that Merge table will use. This allows more even distribution of work across threads and is especially helpful when merged tables differ in size.

<h2 id="max_streams_to_max_threads_ratio">
  max\_streams\_to\_max\_threads\_ratio
</h2>

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

Allows you to use more sources than the number of threads - to more evenly distribute work across threads. It is assumed that this is a temporary solution since it will be possible in the future to make the number of sources equal to the number of threads, but for each source to dynamically select available work for itself.