Question 1

What is a UUID?

Accepted Answer

A UUID (Universally Unique Identifier) is a 128-bit label used to uniquely identify objects in computer systems. Written as 32 hexadecimal digits in 8-4-4-4-12 format (e.g., 550e8400-e29b-41d4-a716-446655440000), they are statistically guaranteed to be unique across all space and time without a central registry.

Question 2

What is the difference between UUID v1, v4, and v7?

Accepted Answer

UUID v1 is timestamp-based using your MAC address — sortable but exposes machine info. UUID v4 is fully random — the most widely used version, ideal for database keys and tokens. UUID v7 (RFC 9562, 2024) combines a millisecond timestamp prefix with random bits — sortable like v1 but private like v4, making it the best choice for new database primary keys.

Question 3

Is UUID v4 truly unique?

Accepted Answer

UUID v4 has 122 bits of randomness, giving 5.3×10³⁶ possible values. The probability of generating a duplicate is astronomically small — you would need to generate 1 billion UUIDs per second for 85 years to have a 50% chance of a collision. In practice, UUID v4 is safe to treat as globally unique.

Question 4

Why should I use UUID v7 for database primary keys?

Accepted Answer

UUID v7 encodes a millisecond-precision Unix timestamp in the most significant bits, making UUIDs naturally sortable by creation time. This dramatically improves B-tree index performance in PostgreSQL, MySQL, and other databases, because new records are always inserted at the end of the index rather than at random positions (which causes page splits and fragmentation).

Question 5

What is the difference between UUID and GUID?

Accepted Answer

GUID (Globally Unique Identifier) is Microsoft's name for UUID. They are the same 128-bit format. Microsoft sometimes displays GUIDs with uppercase letters and curly braces {550E8400-E29B-41D4-A716-446655440000}, while UUIDs are typically lowercase without braces, but they represent the same value.

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