MVCC in Databases Quiz
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Questions and Answers

What is the purpose of Multiversion Concurrency Control (MVCC) in database management systems?

  • To make all readers wait until the writer is done, known as a read-write lock
  • To ensure that readers will not see a half-written or inconsistent piece of data
  • To provide concurrent access to the database by keeping multiple copies of each data item (correct)
  • To create contention especially between long read transactions and update transactions

    • What property provides guarantees in the concurrent accesses to data?

  • Contention
  • Concurrency
  • Isolation (correct)
  • Read-write lock

    • What is the downside of using locks for concurrency control in database management systems?

  • Locks make all readers wait until the writer is done, known as a read-write lock
  • Locks create contention especially between long read transactions and update transactions (correct)
  • Locks ensure that readers will not see a half-written or inconsistent piece of data
  • Locks do not have any downsides

    • How does MVCC ensure concurrent access to the database without creating contention?

    <p>By keeping multiple copies of each data item, allowing each user to see a snapshot of the database at a particular instant in time</p> Signup and view all the answers

    What is the most common isolation level implemented with MVCC?

    <p>Snapshot isolation</p> Signup and view all the answers

    Which database still uses locks despite implementing MVCC?

    <p>Oracle</p> Signup and view all the answers

    What is the challenge introduced by MVCC in terms of data management?

    <p>Removing obsolete versions of data</p> Signup and view all the answers

    What enables recreation of older versions of data in some databases using MVCC?

    <p>Transaction log</p> Signup and view all the answers

    What does MVCC use to achieve transactional consistency?

    <p>Timestamps and incrementing transaction IDs</p> Signup and view all the answers

    What happens when a transaction wants to write to an object in MVCC?

    <p>The transaction is aborted and restarted</p> Signup and view all the answers

    What is the drawback of MVCC?

    <p>High cost of storing multiple versions of objects</p> Signup and view all the answers

    What is MVCC adept at implementing?

    <p>Optimistic concurrency control</p> Signup and view all the answers

    Who described multiversion concurrency control in a 1981 paper?

    <p>Phil Bernstein</p> Signup and view all the answers

    In which database software was MVCC first featured in 1984?

    <p>VAX Rdb/ELN</p> Signup and view all the answers

    Who created the second commercially successful database featuring MVCC?

    <p>Jim Starkey</p> Signup and view all the answers

    What did MVCC allow originally, according to a 1978 dissertation?

    <p>Lock-free snapshot isolation reads</p> Signup and view all the answers

    Study Notes

    Multiversion Concurrency Control (MVCC) in Databases

    • MVCC in a database ensures that changes made by a writer are not visible to other users until the changes are committed, creating multiple versions of the data item.
    • Snapshot isolation is the most common isolation level implemented with MVCC, providing point-in-time consistent views for transactions.
    • MVCC allows read and write transactions to be isolated from each other without the need for locking, although some MVCC databases, like Oracle, still use locks.
    • MVCC introduces the challenge of removing obsolete versions of data, which may involve periodic sweeping and deleting of obsolete versions, as seen in PostgreSQL's VACUUM FREEZE process.
    • Some databases use a storage block split into two parts: the data part keeps the last committed version, and the undo log enables recreation of older versions of data.
    • MVCC uses timestamps and incrementing transaction IDs to achieve transactional consistency, ensuring that a transaction never has to wait to read a database object.
    • When a transaction wants to write to an object, it must have a timestamp that is not earlier than the object's current read timestamp, or the transaction is aborted and restarted.
    • The drawback of MVCC is the cost of storing multiple versions of objects in the database, but it ensures that reads are never blocked, important for workloads involving mostly reading values from the database.
    • MVCC is adept at implementing true snapshot isolation, which other methods of concurrency control may do incompletely or with high performance costs.
    • Multiversion concurrency control was described in detail in a 1981 paper by Phil Bernstein and Nathan Goodman, and was first featured in the 1984 VAX Rdb/ELN database software by Digital Equipment Corporation.
    • MVCC was also featured in the second commercially successful database, InterBase, created by Jim Starkey, who also worked on VAX Rdb/ELN.
    • MVCC allows snapshot isolation reads without any locks, and it was originally described in a 1978 dissertation by David P. Reed.

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    Description

    Test your knowledge of Multiversion Concurrency Control (MVCC) in databases with this quiz. Explore the key concepts, challenges, and implementations of MVCC, including snapshot isolation, storage block splitting, transactional consistency, and the history of MVCC in database systems.

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