Function registerDataTypeFixedString

src/DataTypes/DataTypeFixedString.cpp:80–192

void registerDataTypeFixedString(DataTypeFactory & factory)
{
    factory.registerDataType("FixedString", create, DataTypeFactory::Case::Sensitive,
        Documentation{
            .description = R"DOCS_MD(
A fixed-length string of `N` bytes (neither characters nor code points).

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

```sql
<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:

- The 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](../../sql-reference/data-types/uuid.md) 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.

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

```sql


INSERT INTO FixedStringTable VALUES ('a'), ('ab'), ('');
```

```sql
SELECT
    name,
    toTypeName(name),
    length(name),
    empty(name)
FROM FixedStringTable;
```

```text
┌─name─┬─toTypeName(name)─┬─length(name)─┬─empty(name)─┐
│ a    │ FixedString(2)   │            2 │           0 │
│ ab   │ FixedString(2)   │            2 │           0 │
│      │ FixedString(2)   │            2 │           1 │
└──────┴──────────────────┴──────────────┴─────────────┘
```

Note that the length of the `FixedString(N)` value is constant. The [length](/sql-reference/functions/array-functions#length) function returns `N` even if the `FixedString(N)` value is filled only with null bytes, but the [empty](/sql-reference/functions/array-functions#empty) function returns `1` in this case.