PostgreSQL MVCC, Byte by Byte

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You run SELECT * FROM orders in one psql session and see 50 million rows. A colleague in another session runs the same query at the same moment and sees 49,999,999. Neither of you is wrong, and neither is seeing stale data. You are both reading the same 8KB heap pages, the same bytes on disk. This is the promise of PostgreSQL's MVCC (Multi-Version Concurrency Control), and it's the…

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