> ## 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.

# group_by_* session settings

> ClickHouse session settings in the group_by_* 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="group_by_overflow_mode">
  group\_by\_overflow\_mode
</h2>

<SettingsInfoBlock type="OverflowModeGroupBy" default_value="throw" />

Sets what happens when the number of unique keys for aggregation exceeds the limit:

* `throw`: throw an exception
* `break`: stop executing the query and return the partial result
* `any`: continue aggregation for the keys that got into the set, but do not add new keys to the set.

Using the 'any' value lets you run an approximation of GROUP BY. The quality of
this approximation depends on the statistical nature of the data.

<h2 id="group_by_two_level_threshold">
  group\_by\_two\_level\_threshold
</h2>

<SettingsInfoBlock type="UInt64" default_value="100000" />

From what number of keys, a two-level aggregation starts. 0 - the threshold is not set.

<h2 id="group_by_two_level_threshold_bytes">
  group\_by\_two\_level\_threshold\_bytes
</h2>

<SettingsInfoBlock type="UInt64" default_value="50000000" />

From what size of the aggregation state in bytes, a two-level aggregation begins to be used. 0 - the threshold is not set. Two-level aggregation is used when at least one of the thresholds is triggered.

<h2 id="group_by_use_nulls">
  group\_by\_use\_nulls
</h2>

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

Changes the way the [GROUP BY clause](/docs/reference/statements/select/group-by) treats the types of aggregation keys.
When the `ROLLUP`, `CUBE`, or `GROUPING SETS` specifiers are used, some aggregation keys may not be used to produce some result rows.
Columns for these keys are filled with either default value or `NULL` in corresponding rows depending on this setting.

Possible values:

* 0 — The default value for the aggregation key type is used to produce missing values.
* 1 — ClickHouse executes `GROUP BY` the same way as the SQL standard says. The types of aggregation keys are converted to [Nullable](/docs/reference/data-types/nullable). Columns for corresponding aggregation keys are filled with [NULL](/docs/reference/syntax#null) for rows that didn't use it.

See also:

* [GROUP BY clause](/docs/reference/statements/select/group-by)