> ## 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_limit_* session settings

> ClickHouse session settings in the optimize_limit_* 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_limit_by_function_keys">
  optimize\_limit\_by\_function\_keys
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.6"},{"label": "1"},{"label": "New setting that eliminates LIMIT BY keys that are functions of other LIMIT BY keys."}]}]} />

Eliminates functions of other keys in LIMIT BY section.

Example: `LIMIT 5 BY x, f(x)` becomes `LIMIT 5 BY x`.

<h2 id="optimize_limit_by_in_order">
  optimize\_limit\_by\_in\_order
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.6"},{"label": "1"},{"label": "New setting to optimize `LIMIT BY` queries when `BY` columns are a prefix of the table's sorting key."}]}]} />

Optimize `SELECT ... LIMIT N BY <cols>` queries when `<cols>` (in any order) form a prefix of the table's sorting key, or become one after `WHERE col = const` fixes leading columns. With this enabled the source reads data in primary-key order, so rows with equal values of the `BY` columns arrive adjacent to each other within each stream. When the data arrives in a single sorted stream, `LIMIT BY` filters it in streaming mode with O(1) memory, instead of building a hash table of every distinct combination of `BY` columns seen. When the sorted data arrives in multiple streams and the same `BY` values can appear in more than one of them, each stream is first prefiltered in streaming mode down to at most `LIMIT + OFFSET` rows per group, then the streams are combined and a final hash-based `LIMIT BY` deduplicates groups that span several streams. That final pass still keeps an entry for every distinct combination of `BY` columns, but it only processes the prefiltered rows.