DirectQuery in SQL Server 2016 Analysis Services whitepaper

DirectQuery transforms the Microsoft SQL Server Analysis Services Tabular model into a metadata layer on top of an external database. For SQL Server 2016, DirectQuery was redesigned for dramatically improved speed and performance, however, it is also now more complex to understand and implement. There are many tradeoffs to consider when deciding when to use DirectQuery versus in memory mode (VertiPaq). Consider using DirectQuery if you have either a small database that is updated frequently or a large database that would not fit in memory

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