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Add a warning about the UUID sort order

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Robert Schulze 2024-04-30 11:16:06 +00:00
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docs/en/sql-reference/data-types/uuid.md
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@ -8,7 +8,8 @@ sidebar_label: UUID
A Universally Unique Identifier (UUID) is a 16-byte value used to identify records. For detailed information about UUIDs, see [Wikipedia](https://en.wikipedia.org/wiki/Universally_unique_identifier).
While different UUID variants exist (see [here](https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis)), 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](https://en.wikipedia.org/wiki/Universally_unique_identifier#Textual_representation) at SQL level.
While different UUID variants exist (see [here](https://datatracker.ietf.org/doc/html/draft-ietf-uuidrev-rfc4122bis)), 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](https://en.wikipedia.org/wiki/Universally_unique_identifier#Textual_representation) at SQL level.
Example UUID value:
@ -22,6 +23,46 @@ The default UUID is all-zero. It is used, for example, when a new record is inse
00000000-0000-0000-0000-000000000000
```
Due to historical reasons, UUIDs are sorted by their second half (which is unintuitive).
UUIDs should therefore not be used in an primary key (or sorting key) of a table, or as partition key.
Example:
``` sql
CREATE TABLE tab (uuid UUID) ENGINE = Memory;
INSERT INTO tab SELECT generateUUIDv4() FROM numbers(50);
SELECT * FROM tab ORDER BY uuid;
```
Result:
``` text
┌─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 │
│ 1be30226-57b2-4739-88ec-5e3d490090f2 │
│ f65853a9-4375-4f0e-8b96-906ff622ed3c │
│ d5a0c7a6-79c6-4107-8bb8-df85915edcb7 │
│ 258e6068-17d1-4a1a-8be3-ed2ceb21815c │
│ 04b0f6a9-1f7b-4a42-8bfc-62f37b8a32b8 │
│ 9924f0d9-9c16-43a9-8f08-0944ab495aed │
│ 6720dc14-4eab-4e3e-8f0c-10c4ae8d2673 │
│ 5ddadb52-0452-4f5d-9030-c3f969af93a4 │
│ [...] │
│ 2dde30e6-59a1-48f8-b260-eb37921185b6 │
│ d5402a1b-77b3-4897-b288-29edf5c3ed12 │
│ 01843939-3ba7-4fea-b2aa-45f9a6f1e057 │
│ 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 │
└──────────────────────────────────────┘
```
## Generating UUIDs
ClickHouse provides the [generateUUIDv4](../../sql-reference/functions/uuid-functions.md) function to generate random UUID version 4 values.