Updates (Contd.)

• Protocol to avoid deadlock involving latches

  • deadlocks involving latches and locks were a major problem in System R and SQL/DS
  • transaction may hold at most two latches at-a-time
  • must never wait for lock while holding latch
    • if both are needed (e.g. Record found after latching page):
    • release latch before requesting lock and then reacquire latch (and recheck conditions in case page has changed inbetween). Optimization: conditional lock request
  • page latch released before updating indices
    • data update and index update may be out of order

Notes: