> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-detect-table-modification.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# lightweight_* session settings

> ClickHouse session settings in the lightweight_* 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="lightweight_delete_mode">
  lightweight\_delete\_mode
</h2>

<SettingsInfoBlock type="LightweightDeleteMode" default_value="alter_update" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.5"},{"label": "alter_update"},{"label": "A new setting"}]}]} />

A mode of internal update query that is executed as a part of lightweight delete.

Possible values:

* `alter_update` - run `ALTER UPDATE` query that creates a heavyweight mutation.
* `lightweight_update` - run lightweight update if possible, run `ALTER UPDATE` otherwise.
* `lightweight_update_force` - run lightweight update if possible, throw otherwise.

<h2 id="lightweight_deletes_sync">
  lightweight\_deletes\_sync
</h2>

<SettingsInfoBlock type="UInt64" default_value="2" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.4"},{"label": "2"},{"label": "The same as 'mutation_sync', but controls only execution of lightweight deletes"}]}]} />

The same as [`mutations_sync`](/reference/settings/session-settings/mutations#mutations_sync), but controls only execution of lightweight deletes.

Possible values:

| Value | Description                                                                                                                                            |
| ----- | ------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `0`   | Mutations execute asynchronously.                                                                                                                      |
| `1`   | The query waits for the lightweight deletes to complete on the current server.                                                                         |
| `2`   | The query waits for the lightweight deletes to complete on all replicas (if they exist).                                                               |
| `3`   | The query waits only for active replicas. Supported only for `SharedMergeTree`. For `ReplicatedMergeTree` it behaves the same as `mutations_sync = 2`. |

**See Also**

* [Synchronicity of ALTER Queries](/reference/statements/alter/index#synchronicity-of-alter-queries)
* [Mutations](/reference/statements/alter/index#mutations)

Cloud default value: `1`.