MVCC¶

Multi-Version Concurrency Control enables concurrent reads and writes.

How MVCC Works¶

Each row has version metadata:

Row:
├── created_txn: 100
├── deleted_txn: NULL (or txn that deleted)
└── data: {...}

Visibility Rules¶

A row is visible to transaction T if:

1. created_txn < T (created before T started)
2. deleted_txn IS NULL OR deleted_txn > T (not deleted before T)

Version Chain¶

Updates create new versions:

Row v3 (current) <- Row v2 <- Row v1 (oldest)

Garbage Collection¶

Old versions are cleaned up when no transaction needs them:

Active transactions: [txn 100, txn 105]
Oldest active: 100
Safe to remove: versions with deleted_txn < 100