> ## 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.

# read_in_* session settings

> ClickHouse session settings in the read_in_* 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="read_in_order_two_level_merge_threshold">
  read\_in\_order\_two\_level\_merge\_threshold
</h2>

<SettingsInfoBlock type="UInt64" default_value="100" />

Minimal number of parts to read to run preliminary merge step during multithread reading in order of primary key.

<h2 id="read_in_order_use_buffering">
  read\_in\_order\_use\_buffering
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.7"},{"label": "1"},{"label": "Use buffering before merging while reading in order of primary key"}]}]} />

Use buffering before merging while reading in order of primary key. It increases the parallelism of query executio

<h2 id="read_in_order_use_virtual_row">
  read\_in\_order\_use\_virtual\_row
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.11"},{"label": "0"},{"label": "Use virtual row while reading in order of primary key or its monotonic function fashion. It is useful when searching over multiple parts as only relevant ones are touched."}]}]} />

Use virtual row while reading in order of primary key or its monotonic function fashion. It is useful when searching over multiple parts as only relevant ones are touched.

<h2 id="read_in_order_use_virtual_row_per_block">
  read\_in\_order\_use\_virtual\_row\_per\_block
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.4"},{"label": "0"},{"label": "Emit virtual row after each block during read-in-order to allow more frequent source reprioritization in MergingSortedTransform."}]}]} />

When enabled together with `read_in_order_use_virtual_row`, emit a virtual row after each block read (not only at the beginning of each part).
This allows `MergingSortedTransform` to reprioritize sources more frequently, which is useful when downstream filters discard many rows and data is distributed unevenly across parts.
Note that it disables `read_in_order_use_buffering` optimization and preliminary merge (`read_in_order_two_level_merge_threshold`) for reading.