One of the most abrupt production outages you can experience in PostgreSQL doesn’t come from a hardware failure—it comes from a mathematical boundary.
PostgreSQL handles Multi-Version Concurrency Control (MVCC) using a 32-bit Transaction ID (XID) space. Because there are only ~4.2 billion available IDs, the database treats this space as a circular ring: roughly 2.14 billion transactions represent the "past" (visible data), while the rest represent the "future" (invisible data).
The danger of XID wraparound occurs when your global transaction counter advances too rapidly without "freezing" older rows. If an unfrozen row falls out of that 2.14 billion transaction safe window, its ID mathematically flips into the future. When this happens, completely valid production data suddenly becomes invisible to your queries.
To prevent this silent data corruption, PostgreSQL will forcefully reject new write operations and halt database operations once it gets within 11 million transactions of wraparound.
Usually, Autovacuum hums along in the background to freeze old rows and safely advance the window. However, silent blockers like abandoned logical replication slots, orphaned prepared transactions, or even just a single uncommitted long-running query can quietly stall Autovacuum until it's too late.
For a deeper look at the implementation details, including the exact SQL queries to monitor datfrozenxid age, emergency recovery steps, and an interactive simulation of the XID ring, the full breakdown is here:
