The Evolution of UUIDs: Why UUIDv7 is Replacing UUIDv4 in Modern Distributed Databases

2. The B-Tree Index Fragmentation Problem

Databases like PostgreSQL and MySQL InnoDB store primary key indexes in balanced B+ Trees. When inserting sequentially incrementing integers (1, 2, 3), new keys are appended neatly to the rightmost leaf page. However, UUIDv4 generates uniformly distributed 122-bit random integers, requiring insertions at random leaf pages. This causes non-stop B-Tree page splits, memory cache thrashing, and up to 400% write amplification.

3. UUIDv7 Anatomy: 48-Bit Timestamp + 74-Bit Entropy

Ratified in RFC 9562, UUIDv7 prepends a 48-bit millisecond Unix timestamp to 74 bits of cryptographically secure random entropy. This achieves time-ordered monotonic insertion while eliminating distributed node coordination requirements.