UUID

A Universally Unique Identifier (UUID) is a 16-byte value used to identify records. For detailed information about UUIDs, see Wikipedia.

While different UUID variants exist (see here), ClickHouse does not validate that inserted UUIDs conform to a particular variant. UUIDs are internally treated as a sequence of 16 random bytes with 8-4-4-4-12 representation at SQL level.

Example UUID value:

61f0c404-5cb3-11e7-907b-a6006ad3dba0

The default UUID is all-zero. It is used, for example, when a new record is inserted but no value for a UUID column is specified:

00000000-0000-0000-0000-000000000000

Due to historical reasons, UUIDs are sorted by their second half. UUIDs should therefore not be used directly in a primary key, sorting key, or partition key of a table.

Example:

CREATE TABLE tab (uuid UUID) ENGINE = Memory;
INSERT INTO tab SELECT generateUUIDv4() FROM numbers(50);
SELECT * FROM tab ORDER BY uuid;

Result:

┌─uuid─────────────────────────────────┐
│ 36a0b67c-b74a-4640-803b-e44bb4547e3c │
│ 3a00aeb8-2605-4eec-8215-08c0ecb51112 │
│ 3fda7c49-282e-421a-85ab-c5684ef1d350 │
│ 16ab55a7-45f6-44a8-873c-7a0b44346b3e │
│ e3776711-6359-4f22-878d-bf290d052c85 │
│                [...]                 │
│ 9eceda2f-6946-40e3-b725-16f2709ca41a │
│ 03644f74-47ba-4020-b865-be5fd4c8c7ff │
│ ce3bc93d-ab19-4c74-b8cc-737cb9212099 │
│ b7ad6c91-23d6-4b5e-b8e4-a52297490b56 │
│ 06892f64-cc2d-45f3-bf86-f5c5af5768a9 │
└──────────────────────────────────────┘

As a workaround, the UUID can be converted to a type with an intuitive sort order.

Example using conversion to UInt128:

CREATE TABLE tab (uuid UUID) ENGINE = Memory;
INSERT INTO tab SELECT generateUUIDv4() FROM numbers(50);
SELECT * FROM tab ORDER BY toUInt128(uuid);

Result:

┌─uuid─────────────────────────────────┐
│ 018b81cd-aca1-4e9c-9e56-a84a074dc1a8 │
│ 02380033-c96a-438e-913f-a2c67e341def │
│ 057cf435-7044-456a-893b-9183a4475cea │
│ 0a3c1d4c-f57d-44cc-8567-60cb0c46f76e │
│ 0c15bf1c-8633-4414-a084-7017eead9e41 │
│                [...]                 │
│ f808cf05-ea57-4e81-8add-29a195bde63d │
│ f859fb5d-764b-4a33-81e6-9e4239dae083 │
│ fb1b7e37-ab7b-421a-910b-80e60e2bf9eb │
│ fc3174ff-517b-49b5-bfe2-9b369a5c506d │
│ fece9bf6-3832-449a-b058-cd1d70a02c8b │
└──────────────────────────────────────┘

Generating UUIDs

ClickHouse provides the generateUUIDv4 function to generate random UUID version 4 values.

Usage Example

Example 1

This example demonstrates the creation of a table with a UUID column and the insertion of a value into the table.

CREATE TABLE t_uuid (x UUID, y String) ENGINE=TinyLog

INSERT INTO t_uuid SELECT generateUUIDv4(), 'Example 1'

SELECT * FROM t_uuid

Result:

┌────────────────────────────────────x─┬─y─────────┐
│ 417ddc5d-e556-4d27-95dd-a34d84e46a50 │ Example 1 │
└──────────────────────────────────────┴───────────┘

Example 2

In this example, no UUID column value is specified when the record is inserted, i.e. the default UUID value is inserted:

INSERT INTO t_uuid (y) VALUES ('Example 2')

SELECT * FROM t_uuid

┌────────────────────────────────────x─┬─y─────────┐
│ 417ddc5d-e556-4d27-95dd-a34d84e46a50 │ Example 1 │
│ 00000000-0000-0000-0000-000000000000 │ Example 2 │
└──────────────────────────────────────┴───────────┘

Restrictions

The UUID data type only supports functions which String data type also supports (for example, min, max, and count).

The UUID data type is not supported by arithmetic operations (for example, abs) or aggregate functions, such as sum and avg.