\begin{frame}
  \frametitle{Joins}
  
  \begin{goal}{}
    A good DBMS will use a \emph{better evaluation algorithm}
    (depending on the condition $C$).
    \begin{itemize}
      \item This is the task of the \emph{query optimiser.}
    \end{itemize}
  \end{goal}
  \pause
  \begin{exampleblock}{}
  For example, if $C$ contains the join condition 
  \begin{tcenter}
    \sql{S.sid = R.sid}
  \end{tcenter}
  then the DBMS might execute the query efficiently by:
  \begin{itemize}
    \item loop over the row in \sql{Results},
    \item find matching \sql{Students} row via an \emph{index} on \sql{Students.sid}
  \end{itemize}
  \end{exampleblock}
  \remark{DBMS typically create an index over the key attributes.}
  \pause
  
  \begin{goal}{}
    For understanding the \emph{semantics} of a query,
    the simple nested \textbf{foreach algorithm} suffices!
    \smallskip

    The query optimiser may use any algorithm that produces the
    \emph{exact same output} (except possibly the tuple order).
  \end{goal}
  \bigskip
\end{frame}