How InnoDB Uses a Hybrid MVCC Approach Across MySQL Isolation Levels

MySQL's default storage engine, InnoDB, uses a hybrid approach to concurrency control — applying MVCC only to read operations while relying entirely on locking for updates, inserts, and deletes. Unlike PostgreSQL, which stores full row versions in the same heap structure, InnoDB stores only the delta (the difference between versions) in a separate structure called the undo log. When a transaction reads a row, InnoDB builds a snapshot of in-flight transactions at the first SQL statement and keeps it frozen for the transaction's lifetime to determine row visibility. For write operations, InnoDB acquires the appropriate lock and fetches the latest row version directly, bypassing the version chain altogether. Repeatable Read is InnoDB's default isolation level and serves as the foundation from which other isolation levels differ only in minor ways.