A materialized view (MV) is a more complex physical design structure than an index since a materialized view may be defined over multiple tables, and can involve selection, projection, join and group by. This richness of structure of MVs makes the problem of selecting materialized views significantly more complex than that of index selection. First, for a given query (and hence workload) the space of materialized views that must be considered is much larger than the space of indexes. For example, MVs on any subset of tables referenced in the query may be relevant. For each such subset many MVs.
