> ## 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.

# optimize_aggregation_in_order_* session settings

> ClickHouse session settings in the optimize_aggregation_in_order_* 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="optimize_aggregation_in_order">
  optimize\_aggregation\_in\_order
</h2>

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

Enables [GROUP BY](/docs/reference/statements/select/group-by) optimization in [SELECT](/docs/reference/statements/select/index) queries for aggregating data in corresponding order in [MergeTree](/docs/reference/engines/table-engines/mergetree-family/mergetree) tables.

Possible values:

* 0 — `GROUP BY` optimization is disabled.
* 1 — `GROUP BY` optimization is enabled.

**See Also**

* [GROUP BY optimization](/docs/reference/statements/select/group-by#group-by-optimization-depending-on-table-sorting-key)

<h2 id="optimize_aggregation_in_order_limit">
  optimize\_aggregation\_in\_order\_limit
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.7"},{"label": "1"},{"label": "New setting to push the `LIMIT` into aggregation-in-order for early termination when the `ORDER BY` is a prefix of the `GROUP BY` sort description."}]}]} />

When enabled and aggregation in order is active, pushes LIMIT into the aggregation step to enable early termination after producing enough groups. This reduces the amount of data read when ORDER BY matches the GROUP BY key prefix. May reduce the value reported by `rows_before_limit_at_least`; use `exact_rows_before_limit` if exact counts are needed.