ClickHouse/docs/en/data_types/fixedstring.md

FixedString

A fixed-length string of N bytes (neither characters nor code points).

To declare a column of FixedString type, use the following syntax:

<column_name> FixedString(N)

Where N is a natural number.

The FixedString type is efficient when data has the length of precisely N bytes. In all other cases, it is likely to reduce efficiency.

Examples of the values that can be efficiently stored in FixedString-typed columns:

• Binary representation of IP addresses (FixedString(16) for IPv6).
• Language codes (ru_RU, en_US ... ).
• Currency codes (USD, RUB ... ).
• Binary representation of hashes (FixedString(16) for MD5, FixedString(32) for SHA256).

To store UUID values, use the UUID data type.

When inserting the data, ClickHouse:

• Complements a string with null bytes if the string contains fewer than N bytes.
• Throws the Too large value for FixedString(N) exception if the string contains more than N bytes.

When selecting the data, ClickHouse does not remove the null bytes at the end of the string. If you use the WHERE clause, you should add null bytes manually to match the FixedString value. The following example illustrates how to use the WHERE clause with FixedString.

Let's consider the following table with the single FixedString(2) column:

┌─name──┐
│ b     │
└───────┘

The query SELECT * FROM FixedStringTable WHERE a = 'b' does not return any data as a result. We should complement the filter pattern with null bytes.

SELECT * FROM FixedStringTable
WHERE a = 'b\0'

┌─a─┐
│ b │
└───┘

This behavior differs from MySQL behavior for the CHAR type (where strings are padded with spaces, and the spaces are removed for output).

Note that the length of the FixedString(N) value is constant. The length function returns N even if the FixedString(N) value is filled only with null bytes, but the empty function returns 1 in this case.

Original article