> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-trino-dialect.mintlify.site/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 ExperimentalBadge = () => {
  return <a href="https://clickhouse.com/docs/reference/settings/beta-and-experimental-features#experimental-features" className="experimentalBadge">
            <div className="experimentalIcon">
            <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
                <path strokeWidth="1.25" d="M5.5 2H10.5" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
                <path strokeWidth="1.25" d="M9.50015 2V6.19625L13.4283 12.7425C13.4738 12.8183 13.4985 12.9049 13.4996 12.9934C13.5008 13.0818 13.4785 13.169 13.435 13.246C13.3914 13.323 13.3283 13.3871 13.2519 13.4317C13.1755 13.4764 13.0886 13.4999 13.0002 13.5H3.00015C2.91164 13.5 2.8247 13.4766 2.74822 13.432C2.67174 13.3874 2.60847 13.3233 2.56487 13.2463C2.52126 13.1693 2.49889 13.082 2.50004 12.9935C2.50119 12.905 2.52582 12.8184 2.5714 12.7425L6.50015 6.19625V2" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
                <path strokeWidth="1.25" d="M4.47656 9.56754C5.30344 9.41254 6.47656 9.47942 7.99969 10.25C10.0153 11.2707 11.4216 11.0569 12.2184 10.7282" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
            </svg>
        </div>
            Experimental feature
        </a>;
};

export const VersionHistory = ({rows = []}) => {
  if (rows.length === 0) {
    return null;
  }
  const headers = ["Version", "Default value", "Comment"];
  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
  }}>
        Version history
      </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
  }}>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](/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_top_k_optimization_observation_rows">
  group\_by\_top\_k\_optimization\_observation\_rows
</h2>

<ExperimentalBadge />

<SettingsInfoBlock type="UInt64" default_value="65536" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.8"},{"label": "65536"},{"label": "New experimental setting: rows each aggregation stream observes before declaring a full top-K heap that never rejected anything pure overhead and freezing it."}]}]} />

For `enable_group_by_top_k_optimization`: the number of rows each aggregation stream observes before freezing a full top-K heap that skipped less than 10% of input rows and evicted fewer keys than its capacity. A frozen heap means aggregation continues as if the optimization were disabled.

The effective window is at least twice the heap's reserved size, so a heap always gets a chance to fill before being judged. `0` disables this freeze (the heap still freezes when a boundary tie-set overgrows it).

This has no effect on `GROUP BY keys LIMIT K` queries without `ORDER BY`, where the freeze is always disabled: that shape's plan contains a synthesized sort that only pays off while the heap bounds the hash table, so freezing the heap would leave a plan slower than the un-optimized one.

<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](/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](/reference/data-types/nullable). Columns for corresponding aggregation keys are filled with [NULL](/reference/syntax#null) for rows that didn't use it.

See also:

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