eth-summaries/semester4/dmdb/parts/04_query-processing/03_aggregation.tex

And again, we have the option to do this via sorting or hashing:
\begin{itemize}
    \item \bi{Sorting}: Sort on the attribute. Then scan the sorted tuples, computing running aggregate (such as max/min, average, etc).
          When a new group is encountered, then we output the aggregate.
          Alternatively, we can already compute the aggregates on the last step of sorting (the merge) to save some time.
          The limiting factor then becomes the sorting.
    \item \bi{Hashing}: We hash the attribute and now each hash table entry is a group of all records with this value of the attribute.
          For each one of these groups, we compute the aggregate.
          If the table is too large for memory, we use a two-step approach, as before
\end{itemize}