\begin{frame}
  \frametitle{Isolation Levels via Locking}
  
  Basic idea: use variations of strict 2 PL.
  \begin{goal}{SQL-92 Isolation Levels}
    \begin{itemize}
      \item \emph{\sql{read uncommitted}} (also \sql{dirty read} or \sql{browse})\\[.5ex]
        
        \emph{Only write locks are acquired.} 
        Any row read may be concurrently changed by other transactions.
    \smallskip\pause
    
      \item \emph{\sql{read committed}} (also \sql{cursor stability})\\[.5ex]
        
        \emph{Read locks} are held for as long as the application cursor sits on a particular, \emph{current row}.
        Write locks as usual. % (according to Strict 2 PL).
        Rows may be changed between repeated reads.
    \smallskip\pause
        
    \item \emph{\sql{repeatable read}}\\[.5ex]
      
      Strict 2 PL locking. 
      Nevertheless, a transaction may read \emph{phantom rows} if it executes an aggregation query twice.
    \smallskip\pause

    \item \emph{\sql{serializable}}\\[.5ex]
      
      Strict 2 PL + multi-granularity locking. \emph{No phantom rows}.
    \end{itemize}
  \end{goal}
\end{frame}