\begin{frame}
  \frametitle{Snapshot Isolation: Write Skew Anomaly}

  \begin{exampleblock}{Write Skew Anomaly}
    \begin{itemize}
      \item Constraint: $X + Y < 2$
      \item Initially: $X = 0$ and $Y = 0$
    \pause
      \item $T_1:$ $X = X+1$; it sees $X = 1$ and $Y = 0$ and commits
    \pause
      \item $T_2:$ $Y = Y+1$; it sees $X = 0$ and $Y = 1$ and commits
    \pause
      \item $T_1$ and $T_2$ have an empty write intersection (\emph{no conflict}).
    \pause
      \item End result: $X= 1$ and $Y= 1$ \pause \error $X + Y \ge 2$
    \end{itemize}
  \end{exampleblock}
  \pause\medskip
  
  This problem does not occur if 
  this is a \sql{check} constraint comparing values of the same row
  since the finest locking granularity are rows.
  \smallskip
  
  \begin{goal}{}
    Therefore write skew anomalies occur with complex assertions that involve multiple tuples.
  \end{goal}
%   
%   Often not a problem since most databases do not support complex constraints anyway.
\end{frame}

\theme{Optimising Performance}