> ## 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.

# apply_* session settings

> ClickHouse session settings in the apply_* 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="apply_deleted_mask">
  apply\_deleted\_mask
</h2>

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

Enables filtering out rows deleted with lightweight DELETE. If disabled, a query will be able to read those rows. This is useful for debugging and "undelete" scenarios

<h2 id="apply_mutations_on_fly">
  apply\_mutations\_on\_fly
</h2>

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

If true, mutations (UPDATEs and DELETEs) which are not materialized in data part will be applied on SELECTs.

<h2 id="apply_prewhere_after_final">
  apply\_prewhere\_after\_final
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.12"},{"label": "0"},{"label": "New setting. When enabled, PREWHERE conditions are applied after FINAL processing."}]}]} />

When enabled, PREWHERE conditions are applied after FINAL processing for ReplacingMergeTree and similar engines.
This can be useful when PREWHERE references columns that may have different values across duplicate rows,
and you want FINAL to select the winning row before filtering. When disabled, PREWHERE is applied during reading.
Note: PREWHERE is also deferred, regardless of this setting, whenever a row policy is deferred by
`apply_row_policy_after_final`, because the row policy must be applied before PREWHERE. That happens when the
policy expression is non-deterministic, or reads a column that is not in ORDER BY, or reads an ORDER BY column
whose type is or contains a floating-point type.

Possible values:

* 0 — PREWHERE is applied before FINAL, unless a deferred row policy defers it too (default).
* 1 — PREWHERE is applied after FINAL.

<h2 id="apply_row_policy_after_final">
  apply\_row\_policy\_after\_final
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.10"},{"label": "1"},{"label": "Setting was added as a fix for the post-#87303 regression where row policies and PREWHERE were applied before FINAL. PR #91065 introduced the setting and PR #97279 made the correctness-safe true the default. Recorded as {true, true} so compatibility never reverts to the pre-fix false behavior."}]}]} />

When enabled, row policies are applied after FINAL processing for \*MergeTree tables. (Especially for ReplacingMergeTree)
When the policy is deferred this way, PREWHERE is deferred with it so that the policy is still applied first
(see `apply_prewhere_after_final` for deferring PREWHERE unconditionally).
When disabled, row policies are applied before FINAL, which can cause different results when the policy
filters out rows that should be used for deduplication in ReplacingMergeTree or similar engines.

If the row policy expression is deterministic and depends only on non-floating-point columns in ORDER BY, it will still be
applied before FINAL as an optimization, since such filtering cannot affect the deduplication result. Floating-point columns
or columns containing floating-point (such as `Tuple(Float64, ...)`, `Array(Float64)`, `Nullable(Float64)`, etc.)
are excluded because `-0.0` and `0.0` deduplicate as one key while a policy condition can tell them apart.

Possible values:

* 0 — Row policy is applied before FINAL.
* 1 — Row policy is applied after FINAL (default).

<h2 id="apply_settings_from_server">
  apply\_settings\_from\_server
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.2"},{"label": "1"},{"label": "Client-side code (e.g. INSERT input parsing and query output formatting) will use the same settings as the server, including settings from server config."}]}]} />

Whether the client should accept settings from server.

This only affects operations performed on the client side, in particular parsing the INSERT input data and formatting the query result. Most of query execution happens on the server and is not affected by this setting.

Normally this setting should be set in user profile (users.xml or queries like `ALTER USER`), not through the client (client command line arguments, `SET` query, or `SETTINGS` section of `SELECT` query). Through the client it can be changed to false, but can't be changed to true (because the server won't send the settings if user profile has `apply_settings_from_server = false`).

Note that initially (24.12) there was a server setting (`send_settings_to_client`), but latter it got replaced with this client setting, for better usability.

<h2 id="apply_string_filters_during_scan">
  apply\_string\_filters\_during\_scan
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.10"},{"label": "0"},{"label": "New setting to push down substring search conditions on String columns from PREWHERE into the column scan."}]}]} />

Push down substring search conditions on `String` columns from `PREWHERE` into the column scan.

When a `PREWHERE` condition contains a conjunct that searches for a non-empty substring in a `String` (or `Nullable(String)`) column
(`LIKE`, `position`, `startsWith`, `endsWith`, or equality with a non-empty string), the reader checks every value against this condition
during deserialization and reads non-matching values as empty strings. This makes reading faster and lowers memory usage when the condition
is selective, because the data of non-matching values is not copied into the column. The result of the query does not change,
because the rows with non-matching values are guaranteed to be filtered out by `PREWHERE`, and such conditions never match an empty string.

The filter is disabled adaptively at runtime if it turns out to be non-selective.

Also allows the `WHERE` to `PREWHERE` optimization to move substring search conditions (`LIKE`, `position`, `startsWith`, `endsWith`)
even when they use all queried columns (normally that is pointless, but with this setting the scan itself becomes cheaper).
Equality with a constant string is not moved for this reason: it is still applied during the scan when it is already in `PREWHERE`.

Supported for reading from `MergeTree` tables and from the `Parquet` format.

Note that the estimation of the input bytes collected for the automatic decision about parallel replicas
(`RuntimeDataflowStatisticsInputBytes`) is based on the in-memory size of the read blocks, so it underestimates
the amount of data read from disk when the values are replaced by empty strings.

Possible values:

* 0 — Disabled.
* 1 — Enabled.