> ## 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.

# write_* MergeTree table settings

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

<h2 id="write_final_mark">
  write\_final\_mark
</h2>

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

Obsolete setting, does nothing.

<h2 id="write_marks_for_substreams_in_compact_parts">
  write\_marks\_for\_substreams\_in\_compact\_parts
</h2>

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

<VersionHistory rows={[{"id": "row-1","items": [{"label": "25.8"},{"label": "1"},{"label": "Enable writing marks for substreams in compact parts by default"}]}, {"id": "row-2","items": [{"label": "25.5"},{"label": "0"},{"label": "New setting"}]}]} />

Enables writing marks per each substream instead of per each column in Compact parts.
It allows to read individual subcolumns from the data part efficiently.

For example, column `t Tuple(a String, b UInt32, c Array(Nullable(UInt32)))` is serialized in the next substreams:

* `t.a` for String data of tuple element `a`
* `t.b` for UInt32 data of tuple element `b`
* `t.c.size0` for array sizes of tuple element `c`
* `t.c.null` for null map of nested array elements of tuple element `c`
* `t.c` for UInt32 data pf nested array elements of tuple element `c`

When this setting is enabled, we will write a mark for each of these 5 substreams, which means that we will be able to read
the data of each individual substream from the granule separately if needed. For example, if we want to read the subcolumn `t.c` we will read only data of
substreams `t.c.size0`, `t.c.null` and `t.c` and won't read data from substreams `t.a` and `t.b`. When this setting is disabled,
we will write a mark only for top-level column `t`, which means that we will always read the whole column data from the granule, even if we need only data of some substreams.