Materialized Views are typically used in warehouses to increase the speed of queries on very large databases. Queries often involve joins between tables and/or agregations such as SUM. The fact of building a Materialized View containing a table join and/or agregates, allows the optimizer to perform query rewrite operations that improve speed of the query as a result of use the Materialized View instead of perform expensive join operations. Remember that all the join operations and/or aggregations are precalculated at the Materialized View create time.
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