\begin{frame} \frametitle{Multiversion Concurrency Control: Snapshot Isolation} \begin{alertblock}{} Disadvantages: \begin{itemize} \item versioning requires \emph{space} and \emph{management overhead} \item update transactions still need concurrency control! \end{itemize} \end{alertblock} \begin{alertblock}{} Snapshot isolation does not guarantee serializability! But \begin{itemize} \item the anomalies \begin{itemize} \item dirty read, \item unrepeatable read, \item phantom rows \end{itemize} do not occur. \item however, \alert{write skew} occurs! \end{itemize} \end{alertblock} \pause Snapshot isolation is used in the Oracle SQL Server.\\ (Oracle has no real serializability). \end{frame}