Multi-Version Concurrency Control (MVCC) and Tuple Storage: Postgres Heap vs InnoDB Index-Organized Undo Logs, Snapshot Visibility, Vacuuming, and Transaction ID Wraparound

Deep End Era. Engineering with Sankalp · Oct 29, 2026 · 1h 4m

A forty-gigabyte telemetry table in Stockholm has silently swollen to six hundred gigabytes on disk, read queries are mysteriously writing megabytes of Write-Ahead Log per second, and the PostgreSQL log is counting down ten million transactions to a hard safety shutdown. To understand why, we open up the slotted page and compare the two great schools of Multi-Version Concurrency Control—PostgreSQL's append-only heap versus MySQL InnoDB's index-organized undo logs—before walking through snapshot visibility, vacuuming, and 32-bit transaction ID wraparound by ear.

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