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

# flat dictionary layout

> Store a dictionary in memory as flat arrays.

With the `flat` layout, the dictionary is completely stored in memory in the form of flat arrays.
The amount of memory used is proportional to the size of the largest key (in space used).

<Tip>
  This layout type provides the best performance among all available methods of storing a dictionary.
</Tip>

The dictionary key has the [UInt64](/reference/data-types/int-uint) type and the value is limited to `max_array_size` (by default — 500,000).
If a larger key is discovered when creating the dictionary, ClickHouse throws an exception and does not create the dictionary.
The initial size of dictionary flat arrays are controlled by the `initial_array_size` setting (by default — 1024).

All types of sources are supported.
When updating the dictionary, data (from a file or from a table) is read in its entirety.

Configuration example:

<Tabs>
  <Tab title="DDL">
    ```sql theme={null}
    LAYOUT(FLAT(INITIAL_ARRAY_SIZE 50000 MAX_ARRAY_SIZE 5000000))
    ```
  </Tab>

  <Tab title="Configuration file">
    ```xml theme={null}
    <layout>
      <flat>
        <initial_array_size>50000</initial_array_size>
        <max_array_size>5000000</max_array_size>
      </flat>
    </layout>
    ```
  </Tab>
</Tabs>

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