> ## 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.

# table_* MergeTree table settings

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

<h2 id="table_disk">
  table\_disk
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.2"},{"label": "0"},{"label": "New setting"}]}]} />

This is table disk: the path/endpoint points to the table data, not the database data.
Supported for object-storage disks whose metadata lives on the object storage itself
(s3\_plain, s3\_plain\_rewritable, web, web\_index) and their cached variants. Encrypted
variants are supported only over the writable s3\_plain / s3\_plain\_rewritable disks, not
over the read-only web / web\_index disks.
The parts already present under the path are trusted to match the columns and the sorting
key this table declares. Nothing records the sorting key a part was written with, so a
table declaring a different one over the same parts returns wrong results or an error.

<h2 id="table_readonly">
  table\_readonly
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "26.3"},{"label": "0"},{"label": "New setting to mark table as read-only, preventing inserts and modifications"}]}]} />

If set to true, the table is in read-only mode.

All foreground operations that would modify the table are rejected: inserts, mutations, `OPTIMIZE`, and the data-mutating partition commands
(`ATTACH`/`MOVE`/`DROP`/`DROP DETACHED`/`FETCH`/`REPLACE PARTITION`, as well as `MOVE PARTITION ... TO TABLE` targeting this table). Operations
that do not modify the table's data, such as `FREEZE`/`UNFREEZE` and `FORGET PARTITION`, remain allowed.

Background work that modifies table data is not scheduled: regular merges, TTL merges (`DELETE`/`MOVE`/recompression), recompression merges, background mutations,
and background part moves are all suppressed. As a consequence, a table with a TTL no longer reclaims or moves its expired data while this setting
is enabled. Cleanup is stopped, waiting for an active cleanup iteration to finish. The asynchronous loading of outdated (inactive) parts that a
writable table performs after start is suspended if it is still pending: no further part is loaded, including the loads that were already queued
but had not started, and the parts that remain unloaded are loaded once the setting is turned off again. The background workers are disabled
before the `ALTER` that enables the setting commits it, so no cleanup or part load starts on a table that is already durably read-only. Other
operations already in progress, including the loading of the parts that had already started, may finish.

The in-memory statistics cache still refreshes periodically. Set `refresh_statistics_interval = 0` to disable this background task too.
Streaming reads (`SELECT ... STREAM`) keep working: the background job that serves their subscriptions only reads parts and runs on read-only tables as well.

The setting can always be toggled back with `ALTER TABLE ... MODIFY SETTING table_readonly = 0` (or `RESET SETTING`). The background workers
that a read-only table never started are started at that point, so merges, mutations, moves, TTL, and cleanup resume without a server restart.
Outdated (inactive) parts are loaded before cleanup can remove empty parts that cover them. The table stays read-only for concurrent queries for
the whole duration of that `ALTER`: it accepts writes again only once the statement returned, not already when its metadata was committed.
This setting is not supported for `ReplicatedMergeTree`.