> ## 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.

# read_in_* session settings

> ClickHouse session settings in the read_in_* generated group.

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="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="1" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.8"},{"label": "1"},{"label": "Enable the virtual row optimization by default. When reading in order of the primary key over many parts, it lets `MergingSortedTransform` reprioritize sources using primary key values from the sparse index, so parts that are not relevant for the query are not read, plus a bounded read-ahead window of at most `max_threads` parts that keeps reads parallel. This significantly reduces peak memory consumption (see https:\/\/github.com\/ClickHouse\/ClickHouse\/issues\/52624)."}]}, {"id": "row-2","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 the parts that can actually contribute to the result are read, plus a bounded read-ahead window of at most `max_threads` parts that keeps reads parallel.

<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.