UUIDv7 was defined in RFC 9562. There are eight versions of UUID, and while they all have a size of 128 bits, the data stored in each version is different. Until I learned about UUIDv7, when I thought of UUIDs, I imagined random data - but that’s version 4 UUID. The version 7 UUID (UUIDv7) has a significant feature: sortability thanks to a timestamp stored at the beginning of the data.

これはPostgreSQL Advent calendar 2024の18日目の記事です。

The well-known implementation of the standard C library is glibc. musl libc is another implementation of the standard C library. It uses the MIT License, and it is known for its simple implementation and small binary size. It is also used in Alpine Linux.

I recently published my hobby project pgroad, a new PostgreSQL Table Access Method (Table AM). pgroad is a PostgreSQL extension that adds a new table format called road to PostgreSQL. ROAD stands for “Read Only Archived Data” and is used to convert existing tables that are accessed infrequently but cannot be dropped into compact, read-only tables.

PostgreSQL’s transaction ID (hereafter XID) is internally represendted as a “monotoronicaly increasing 32-bit unsigned integer value”, so after reaching 2^32-1 (approximately 4 billion) it wraps around back to 0. In PostgreSQL, each transaction that modifies the database such as INSERT, UPDATE and even DDLs is assigned a unique XID. Since the order of XIDs is used to check the visibility of tuples in tables, if XID wraps around after reaching the upper limit, this logic would break.