Optimization and Dataflow Algorithms for Nested Tree Queries

The SQL language allows useIs to express queries that have nested subqueries in them. Optimization of nested queries has received considerable attention over the last few years. Most of the previous optimization work has assumed that at most one block is nested within any given block. The solutions presented iu the literature for the general case (where an arbitrary number of blocks are nested within a block) have either been incorrect or have dealt with a restricted sub& of queries. The two main contributions of this paper are:. (1) optimization strategies for queries that have an arbitrary number of blocks nested within any given block, and (2) a new dataflow algoritbm for the execution of nested queries, involving one or more outer joins, in a multi-processor environment such as the one found in GAMMA. The new algorithm cuts down on message and CPU costs over conventional datiow algorithms.