> ## Documentation Index
> Fetch the complete documentation index at: https://private-7c7dfe99-revert-104359-revert-104251-parquet-single.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# external_table_* session settings

> ClickHouse session settings in the external_table_* 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.settings](/reference/system-tables/settings) and are autogenerated from [source](https://github.com/ClickHouse/ClickHouse/blob/master/src/Core/Settings.cpp).

<h2 id="external_table_functions_use_nulls">
  external\_table\_functions\_use\_nulls
</h2>

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

Defines how [mysql](/reference/functions/table-functions/mysql), [postgresql](/reference/functions/table-functions/postgresql) and [odbc](/reference/functions/table-functions/odbc) table functions use Nullable columns.

Possible values:

* 0 — The table function explicitly uses Nullable columns.
* 1 — The table function implicitly uses Nullable columns.

**Usage**

If the setting is set to `0`, the table function does not make Nullable columns and inserts default values instead of NULL. This is also applicable for NULL values inside arrays.

<h2 id="external_table_strict_query">
  external\_table\_strict\_query
</h2>

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

If it is set to true, a filter on the columns of an external table (`MySQL`, `PostgreSQL`, `SQLite`, `ODBC`, `JDBC`) that cannot be pushed down to the external database is rejected with an exception instead of being applied locally after the data is fetched.

The check covers the top-level `WHERE` predicate and each conjunct of a top-level `AND`. A `PREWHERE` on the columns of the external table is not a case for this setting: these table engines do not support `PREWHERE`, and such a query is rejected with `ILLEGAL_PREWHERE` regardless of the setting. With the analyzer (the default), the check runs only where a filter could be pushed down at all: when the external table is the only table of the query, on either side of an `INNER JOIN`, or on the preserving side of an outer join (the left side of a `LEFT JOIN`, the right side of a `RIGHT JOIN`). On the non-preserving side of a `LEFT`/`RIGHT JOIN` and on either side of a `FULL JOIN` nothing is pushed down and nothing is checked, so a filter on the columns of the external table is applied locally after the join even in strict mode. Where the check runs, a predicate that references other tables joined in the surrounding query is not pushed down and is excluded from the check, whether it references only the joined side (for example `WHERE r.flag`) or mixes it with the external table inside one non-`AND` expression (for example `WHERE l.id = 1 OR r.flag`); such a predicate keeps its usual ClickHouse evaluation point (`WHERE` after the join, `PREWHERE` before it) and is not rejected. With the old analyzer (`enable_analyzer = 0`) this scoping does not apply: the whole outer filter is checked when the external table is the first table of the join tree, including a predicate on the joined side, and a joined right-hand external table is not checked.