> ## 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.

# prefer_* session settings

> ClickHouse session settings in the prefer_* 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>;
};

export const CloudOnlyBadge = () => {
  return <div className="cloudBadge">
            <div className="cloudIcon">
            <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
                <path fillRule="evenodd" clipRule="evenodd" d="M5.33395 12.6667H12.3739C13.6593 12.6667 14.7073 11.6187 14.7073 10.3334C14.7073 9.04804 13.6593 8.00004 12.3739 8.00004H12.0839V7.33337C12.0839 5.12671 10.2906 3.33337 8.08395 3.33337C6.09928 3.33337 4.45395 4.78537 4.14195 6.68204C2.55728 6.76271 1.29395 8.06204 1.29395 9.66671C1.29395 11.3234 2.63728 12.6667 4.29395 12.6667H5.33395Z" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round" />
            </svg>
        </div>
            {'ClickHouse Cloud only'}
        </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="prefer_column_name_to_alias">
  prefer\_column\_name\_to\_alias
</h2>

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

Enables or disables using the original column names instead of aliases in query expressions and clauses. It especially matters when alias is the same as the column name, see [Expression Aliases](/docs/reference/syntax#notes-on-usage). Enable this setting to make aliases syntax rules in ClickHouse more compatible with most other database engines.

Possible values:

* 0 — The column name is substituted with the alias.
* 1 — The column name is not substituted with the alias.

**Example**

The difference between enabled and disabled:

Query:

```sql theme={null}
SET prefer_column_name_to_alias = 0;
SELECT avg(number) AS number, max(number) FROM numbers(10);
```

Result:

```text theme={null}
Received exception from server (version 21.5.1):
Code: 184. DB::Exception: Received from localhost:9000. DB::Exception: Aggregate function avg(number) is found inside another aggregate function in query: While processing avg(number) AS number.
```

Query:

```sql theme={null}
SET prefer_column_name_to_alias = 1;
SELECT avg(number) AS number, max(number) FROM numbers(10);
```

Result:

```text theme={null}
┌─number─┬─max(number)─┐
│    4.5 │           9 │
└────────┴─────────────┘
```

<h2 id="prefer_external_sort_block_bytes">
  prefer\_external\_sort\_block\_bytes
</h2>

<SettingsInfoBlock type="UInt64" default_value="16744704" />

<VersionHistory rows={[{"id": "row-1","items": [{"label": "24.5"},{"label": "16744704"},{"label": "Prefer maximum block bytes for external sort, reduce the memory usage during merging."}]}]} />

Prefer maximum block bytes for external sort, reduce the memory usage during merging.

<h2 id="prefer_global_in_and_join">
  prefer\_global\_in\_and\_join
</h2>

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

Enables the replacement of `IN`/`JOIN` operators with `GLOBAL IN`/`GLOBAL JOIN`.

Possible values:

* 0 — Disabled. `IN`/`JOIN` operators are not replaced with `GLOBAL IN`/`GLOBAL JOIN`.
* 1 — Enabled. `IN`/`JOIN` operators are replaced with `GLOBAL IN`/`GLOBAL JOIN`.

**Usage**

Although `SET distributed_product_mode=global` can change the queries behavior for the distributed tables, it's not suitable for local tables or tables from external resources. Here is when the `prefer_global_in_and_join` setting comes into play.

For example, we have query serving nodes that contain local tables, which are not suitable for distribution. We need to scatter their data on the fly during distributed processing with the `GLOBAL` keyword — `GLOBAL IN`/`GLOBAL JOIN`.

Another use case of `prefer_global_in_and_join` is accessing tables created by external engines. This setting helps to reduce the number of calls to external sources while joining such tables: only one call per query.

**See also:**

* [Distributed subqueries](/docs/reference/statements/in#distributed-subqueries) for more information on how to use `GLOBAL IN`/`GLOBAL JOIN`

<h2 id="prefer_localhost_replica">
  prefer\_localhost\_replica
</h2>

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

Enables/disables preferable using the localhost replica when processing distributed queries.

Possible values:

* 1 — ClickHouse always sends a query to the localhost replica if it exists.
* 0 — ClickHouse uses the balancing strategy specified by the [load\_balancing](/docs/reference/settings/session-settings/load-balancing#load_balancing) setting.

<Note>
  Disable this setting if you use [max\_parallel\_replicas](/docs/reference/settings/session-settings/max#max_parallel_replicas) without [parallel\_replicas\_custom\_key](/docs/reference/settings/session-settings/parallel-replicas#parallel_replicas_custom_key).
  If [parallel\_replicas\_custom\_key](/docs/reference/settings/session-settings/parallel-replicas#parallel_replicas_custom_key) is set, disable this setting only if it's used on a cluster with multiple shards containing multiple replicas.
  If it's used on a cluster with a single shard and multiple replicas, disabling this setting will have negative effects.
</Note>

<h2 id="prefer_warmed_unmerged_parts_seconds">
  prefer\_warmed\_unmerged\_parts\_seconds
</h2>

<CloudOnlyBadge />

<SettingsInfoBlock type="Int64" default_value="0" />

Only has an effect in ClickHouse Cloud. If a merged part is less than this many seconds old and is not pre-warmed (see [cache\_populated\_by\_fetch](/docs/reference/settings/merge-tree-settings#cache_populated_by_fetch)), but all its source parts are available and pre-warmed, SELECT queries will read from those parts instead. Only for Replicated-/SharedMergeTree. Note that this only checks whether CacheWarmer processed the part; if the part was fetched into cache by something else, it'll still be considered cold until CacheWarmer gets to it; if it was warmed, then evicted from cache, it'll still be considered warm.