> ## 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.

# map_serialization_version_* MergeTree table settings

> ClickHouse MergeTree table settings in the map_serialization_version_* 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.merge\_tree\_settings](/docs/reference/system-tables/merge_tree_settings) and are autogenerated from ClickHouse source.

<h2 id="map_serialization_version">
  map\_serialization\_version
</h2>

<SettingsInfoBlock type="MergeTreeMapSerializationVersion" default_value="basic" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "basic"},{"label": "Add a setting to control Map serialization version"}]}]} />

Controls the serialization method used for `Map` columns.

Possible values:

* basic — Use the standard serialization for `Map`.
* with\_buckets — Split keys into buckets during serialization. Using buckets improves reading individual keys from the Map.

The number of buckets in `with_buckets` serialization is determined by [max\_buckets\_in\_map](/docs/reference/settings/merge-tree-settings/max#max_buckets_in_map) and [map\_buckets\_strategy](/docs/reference/settings/merge-tree-settings/map-buckets#map_buckets_strategy).

<h2 id="map_serialization_version_for_zero_level_parts">
  map\_serialization\_version\_for\_zero\_level\_parts
</h2>

<SettingsInfoBlock type="MergeTreeMapSerializationVersion" default_value="basic" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "basic"},{"label": "Add a setting to control Map serialization version for zero-level parts"}]}]} />

This setting allows to specify a different serialization version of
`Map` columns for zero level parts that are created during inserts.
It can be useful to keep `basic` serialization for zero level parts to avoid
performance degradation during inserts, while using `with_buckets` for merged parts.