What if an SQL Statement Returned a Database?.
Every SQL statement is limited to return a single, possibly denormalized, table. This design decision has far reaching consequences. (1.) for databases users in terms of slow query performance, long query result transfer times, usability-issues of SQL in web applications and object-relational mappers. In addition, (2.) for database architects it has consequences when designing query optimizers leading to logical (algebraic) join enumeration effort, memory consumption for intermediate result materialization, and physical operator selection effort. So basically, the entire query optimization stack is shaped by that design decision. In this paper, we argue that the single-table limitation should be dropped. We extend the SELECT-clause of SQL by a keyword ‘RESULTDB’ to support returning a result database. Our approach has clear semantics, i.e. our extended SQL returns subsets of all tables with only those tuples that would be part of the traditional (single-table) query result set, however without performing any denormalization through joins. Our SQL-extension is downward compatible. Moreover, we discuss the surprisingly long list of benefits of our approach. First, for database users: far simpler and more readable application code, better query performance, smaller query results, better query result transfer times. Second, for database architects, we present how to leverage existing closed source systems as well as change open source database systems to support our feature. We propose a couple of algorithms to integrate our feature into both closed-source as well as open source database systems. We present an initial experimental study with promising results.

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