> ## 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.

# count_distinct_* session settings

> ClickHouse session settings in the count_distinct_* 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="count_distinct_implementation">
  count\_distinct\_implementation
</h2>

<SettingsInfoBlock type="String" default_value="uniqExact" />

Specifies which of the `uniq*` functions should be used to perform the [COUNT(DISTINCT ...)](/docs/reference/functions/aggregate-functions/count) construction.

Possible values:

* [uniq](/docs/reference/functions/aggregate-functions/uniq)
* [uniqCombined](/docs/reference/functions/aggregate-functions/uniqCombined)
* [uniqCombined64](/docs/reference/functions/aggregate-functions/uniqCombined64)
* [uniqHLL12](/docs/reference/functions/aggregate-functions/uniqHLL12)
* [uniqExact](/docs/reference/functions/aggregate-functions/uniqExact)

<h2 id="count_distinct_optimization">
  count\_distinct\_optimization
</h2>

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

Rewrite count distinct to subquery of group by