> ## 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_rewrite_* session settings

> ClickHouse session settings in the optimize_rewrite_* 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_rewrite_aggregate_function_with_if">
  optimize\_rewrite\_aggregate\_function\_with\_if
</h2>

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

Rewrite aggregate functions with if expression as argument when logically equivalent.
For example, `avg(if(cond, col, null))` can be rewritten to `avgOrNullIf(cond, col)`. It may improve performance.

<Note>
  Supported only with the analyzer (`enable_analyzer = 1`).
</Note>

<h2 id="optimize_rewrite_array_exists_to_has">
  optimize\_rewrite\_array\_exists\_to\_has
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.4"},{"label": "1"},{"label": "Enable arrayExists to has rewrite optimization by default, now that type compatibility is checked before rewriting."}]}]} />

Rewrite arrayExists() functions to has() when logically equivalent. For example, arrayExists(x -> x = 1, arr) can be rewritten to has(arr, 1)

<h2 id="optimize_rewrite_has_to_in">
  optimize\_rewrite\_has\_to\_in
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.6"},{"label": "1"},{"label": "New setting"}]}]} />

Rewrite `has` functions to `IN` when the first argument is a constant array. For example, `has([1, 2, 3], x)` can be rewritten to `x IN [1, 2, 3]` for better performance with constant arrays

<h2 id="optimize_rewrite_like_perfect_affix">
  optimize\_rewrite\_like\_perfect\_affix
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.10"},{"label": "1"},{"label": "New setting"}]}]} />

Rewrite LIKE expressions with perfect prefix or suffix (e.g. `col LIKE 'ClickHouse%'`) to startsWith or endsWith functions (e.g. `startsWith(col, 'ClickHouse')`).

<h2 id="optimize_rewrite_regexp_functions">
  optimize\_rewrite\_regexp\_functions
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.8"},{"label": "1"},{"label": "A new setting"}]}]} />

Rewrite regular expression related functions into simpler and more efficient forms

<h2 id="optimize_rewrite_sum_if_to_count_if">
  optimize\_rewrite\_sum\_if\_to\_count\_if
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.4"},{"label": "1"},{"label": "Only available for the analyzer, where it works correctly"}]}]} />

Rewrite sumIf() and sum(if()) function countIf() function when logically equivalent