> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-parallel-read-in-order-multi-part.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# allow_experimental_* session settings

> ClickHouse session settings in the allow_experimental_* generated group.

export const ExperimentalBadge = () => {
  return <a href="https://clickhouse.com/docs/reference/settings/beta-and-experimental-features#experimental-features" className="experimentalBadge">
            <div className="experimentalIcon">
            <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
                <path strokeWidth="1.25" d="M5.5 2H10.5" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
                <path strokeWidth="1.25" d="M9.50015 2V6.19625L13.4283 12.7425C13.4738 12.8183 13.4985 12.9049 13.4996 12.9934C13.5008 13.0818 13.4785 13.169 13.435 13.246C13.3914 13.323 13.3283 13.3871 13.2519 13.4317C13.1755 13.4764 13.0886 13.4999 13.0002 13.5H3.00015C2.91164 13.5 2.8247 13.4766 2.74822 13.432C2.67174 13.3874 2.60847 13.3233 2.56487 13.2463C2.52126 13.1693 2.49889 13.082 2.50004 12.9935C2.50119 12.905 2.52582 12.8184 2.5714 12.7425L6.50015 6.19625V2" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
                <path strokeWidth="1.25" d="M4.47656 9.56754C5.30344 9.41254 6.47656 9.47942 7.99969 10.25C10.0153 11.2707 11.4216 11.0569 12.2184 10.7282" stroke="currentColor" strokeLinecap="round" strokeLinejoin="round" />
            </svg>
        </div>
            Experimental feature
        </a>;
};

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="allow_experimental_analyzer">
  allow\_experimental\_analyzer
</h2>

**Aliases**: `enable_analyzer`

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.9"},{"label": "1"},{"label": "The setting is obsolete: the analyzer is mandatory and the old query analysis is no longer supported. A change that would disable it is accepted and replaced with `1`, and `compatibility` with a version below 24.3 no longer reverts it."}]}, {"id": "row-2","items": [{"label": "24.8"},{"label": "1"},{"label": "Added the alias `enable_analyzer`."}]}, {"id": "row-3","items": [{"label": "24.3"},{"label": "1"},{"label": "Enable analyzer and planner by default."}]}]} />

Obsolete since v26.9: the analyzer cannot be disabled anymore.

The analyzer is the query analysis and planning infrastructure that has been the default since v24.3. In v26.9 this setting was frozen at its only supported value, `1`, and the `compatibility` setting no longer reverts it. In v26.10 the query analysis it used to switch to was removed. For backward compatibility, setting it to `0` is accepted and replaced with `1`, so queries, session settings, settings profiles and client configurations that still set `enable_analyzer = 0` keep working, with the analyzer. To compare the behaviour or the performance of a query with the old query analysis, use a ClickHouse version older than v26.9.

<h2 id="allow_experimental_cleanup_old_data_files_compaction">
  allow\_experimental\_cleanup\_old\_data\_files\_compaction
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.5"},{"label": "0"},{"label": "New setting"}]}]} />

Allow to clean up old data files during Iceberg compaction.

<h2 id="allow_experimental_database_hms_catalog">
  allow\_experimental\_database\_hms\_catalog
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.5"},{"label": "0"},{"label": "Allow experimental database engine DataLakeCatalog with catalog_type = 'hive'"}]}]} />

Allow experimental database engine DataLakeCatalog with catalog\_type = 'hms'

<h2 id="allow_experimental_database_materialized_postgresql">
  allow\_experimental\_database\_materialized\_postgresql
</h2>

<ExperimentalBadge />

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

Allow to create database with Engine=MaterializedPostgreSQL(...).

<h2 id="allow_experimental_database_paimon_rest_catalog">
  allow\_experimental\_database\_paimon\_rest\_catalog
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.1"},{"label": "0"},{"label": "New setting"}]}]} />

Allow experimental database engine DataLakeCatalog with catalog\_type = 'paimon\_rest'

<h2 id="allow_experimental_eval_table_function">
  allow\_experimental\_eval\_table\_function
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.7"},{"label": "0"},{"label": "New setting to enable the experimental table function `eval`."}]}]} />

Enable experimental table function `eval`.

<h2 id="allow_experimental_expire_snapshots">
  allow\_experimental\_expire\_snapshots
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "0"},{"label": "New setting."}]}]} />

Allow to execute experimental Iceberg command `ALTER TABLE ... EXECUTE expire_snapshots`.

<h2 id="allow_experimental_hash_functions">
  allow\_experimental\_hash\_functions
</h2>

<ExperimentalBadge />

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

Enable experimental hash functions

<h2 id="allow_experimental_iceberg_compaction">
  allow\_experimental\_iceberg\_compaction
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.8"},{"label": "0"},{"label": "New setting"}]}]} />

Allow to explicitly use 'OPTIMIZE' for iceberg tables.

<h2 id="allow_experimental_kusto_dialect">
  allow\_experimental\_kusto\_dialect
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.1"},{"label": "0"},{"label": "A new setting"}]}]} />

Enable the Kusto Query Language (KQL) dialect - an alternative to SQL.

<h2 id="allow_experimental_lateral_join">
  allow\_experimental\_lateral\_join
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.10"},{"label": "0"},{"label": "New setting to allow `LATERAL JOIN` syntax."}]}]} />

Allow LATERAL JOIN syntax. When enabled, subqueries in the right side of a JOIN can reference columns from the left side, enabling correlated subqueries in the FROM clause (SQL standard LATERAL JOIN).

<h2 id="allow_experimental_nlp_functions">
  allow\_experimental\_nlp\_functions
</h2>

<ExperimentalBadge />

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

Enable experimental functions for natural language processing.

<h2 id="allow_experimental_object_storage_queue_hive_partitioning">
  allow\_experimental\_object\_storage\_queue\_hive\_partitioning
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.1"},{"label": "0"},{"label": "New setting."}]}]} />

Allow to use hive partitioning with S3Queue/AzureQueue engines

<h2 id="allow_experimental_paimon_storage_engine">
  allow\_experimental\_paimon\_storage\_engine
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.5"},{"label": "0"},{"label": "New setting."}]}]} />

Allow to create tables with Paimon\* table engines.

<h2 id="allow_experimental_parallel_reading_from_replicas">
  allow\_experimental\_parallel\_reading\_from\_replicas
</h2>

**Aliases**: `enable_parallel_replicas`

<SettingsInfoBlock type="UInt64" default_value="0" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.10"},{"label": "0"},{"label": "Parallel replicas with read tasks became the Beta tier feature. The setting is also known by its alias `enable_parallel_replicas`."}]}]} />

Use up to `max_parallel_replicas` the number of replicas from each shard for SELECT query execution. Reading is parallelized and coordinated dynamically. 0 - disabled, 1 - enabled, silently disable them in case of failure, 2 - enabled, throw an exception in case of failure

<h2 id="allow_experimental_polyglot_dialect">
  allow\_experimental\_polyglot\_dialect
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "0"},{"label": "New setting to enable the polyglot SQL transpiler dialect."}]}]} />

Enable polyglot SQL transpiler - transpiles SQL from 30+ dialects (MySQL, PostgreSQL, SQLite, Snowflake, DuckDB, etc.) into ClickHouse SQL.

<h2 id="allow_experimental_prql_dialect">
  allow\_experimental\_prql\_dialect
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.1"},{"label": "0"},{"label": "A new setting"}]}]} />

Enable PRQL - an alternative to SQL.

<h2 id="allow_experimental_ytsaurus_dictionary_source">
  allow\_experimental\_ytsaurus\_dictionary\_source
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.8"},{"label": "0"},{"label": "New setting."}]}]} />

Experimental dictionary source for integration with YTsaurus.

<h2 id="allow_experimental_ytsaurus_table_engine">
  allow\_experimental\_ytsaurus\_table\_engine
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.8"},{"label": "0"},{"label": "New setting."}]}]} />

Experimental table engine for integration with YTsaurus.

<h2 id="allow_experimental_ytsaurus_table_function">
  allow\_experimental\_ytsaurus\_table\_function
</h2>

<ExperimentalBadge />

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.8"},{"label": "0"},{"label": "New setting."}]}]} />

Experimental table engine for integration with YTsaurus.