> ## 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_threads_* session settings

> ClickHouse session settings in the max_threads_* 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_threads">
  max\_threads
</h2>

<SettingsInfoBlock type="MaxThreads" default_value="auto(N)" />

The maximum number of query processing threads, excluding threads for retrieving data from remote servers (see the ['max\_distributed\_connections'](/docs/reference/settings/session-settings/max-distributed#max_distributed_connections) parameter).

This parameter applies to threads that perform the same stages of the query processing pipeline in parallel.
For example, when reading from a table, if it is possible to evaluate expressions with functions, filter with `WHERE` and pre-aggregate for `GROUP BY` in parallel using at least 'max\_threads' number of threads, then 'max\_threads' are used.

For queries that are completed quickly because of a LIMIT, you can set a lower 'max\_threads'.
For example, if the necessary number of entries are located in every block and max\_threads = 8, then 8 blocks are retrieved, although it would have been enough to read just one.
The smaller the `max_threads` value, the less memory is consumed.

The `max_threads` setting by default matches the number of hardware threads (number of CPU cores) available to ClickHouse.
As a special case, for x86 processors with less than 32 CPU cores and SMT (e.g. Intel HyperThreading), ClickHouse uses the number of logical cores (= 2 x physical core count) by default.

Without SMT (e.g. Intel HyperThreading), this corresponds to the number of CPU cores.

For ClickHouse Cloud users, the default value will display as `auto(N)` where N matches the vCPU size of your service e.g. 2vCPU/8GiB, 4vCPU/16GiB etc.
See the settings tab in the Cloud console for a list of all service sizes.

<h2 id="max_threads_for_indexes">
  max\_threads\_for\_indexes
</h2>

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

The maximum number of threads process indices.

<h2 id="max_threads_min_free_memory_per_thread">
  max\_threads\_min\_free\_memory\_per\_thread
</h2>

<SettingsInfoBlock type="UInt64" default_value="1073741824" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.5"},{"label": "1073741824"},{"label": "New setting to limit the number of threads based on available free memory"}]}]} />

Lowers `max_threads` when the server is under memory pressure, to avoid starting highly-parallel queries that are likely to hit the memory limit.

Free memory is computed as the server's `max_server_memory_usage` minus the memory currently tracked by the global memory tracker. If that free memory is less than `max_threads` multiplied by this value, `max_threads` is reduced to the largest N such that `N * value <= free_memory`, with a minimum of `1`.

Set to `0` to disable this limit.

For example, with the default of 1 GiB and 32 GiB of free memory, `max_threads` is capped at 32; with 1 GiB of free memory it falls back to 1.

This setting applies to read-side parallelism (`SELECT`, `UNION`, `INTERSECT`/`EXCEPT`, and the `SELECT` side of `INSERT ... SELECT`). For the write side, see `max_insert_threads_min_free_memory_per_thread`.