MySQL supports two different algorithms for views: the MERGE algorithm and the TEMPTABLE algorithm. These two algorithms differ greatly. A view which uses the MERGE algorithm can merge filter conditions into the view query itself. This has significant performance advantages over TEMPTABLE views. A view which uses the TEMPTABLE algorithm will have to compute the entire contents of the view before any filter is applied. Computing the entire contents requires a temporary table and many more rows may be accessed than otherwise would had the filter been merged into the view query.
As long as a view avoids aggregation, DISTINCT, GROUP BY, ORDER BY, etc, then it can use the MERGE algorithm. Unfortunately, this means that views of any significant complexity will almost always use the TEMPTABLE algorithm.
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