What is MVCC and how does it enable concurrent reads and writes in PostgreSQL?

MVCC (Multi-Version Concurrency Control): PostgreSQL never overwrites rows in place. Instead, each UPDATE creates a NEW version of the row with a new transaction ID (xmin) and marks the old version as dead (xmax = the updating transaction). Every row has xmin (which transaction created it) and xmax (which transaction deleted/updated it, or 0 if current). Read transaction at time T sees rows where