Nested Queries in Object Bases

Many declarative query languages for object oriented oo databases allow nested subqueries This paper contains the rst algebra which is capable of handling arbitrary nested queries and the rst complete classi cation of oo nested queries and the according unnesting strategies For unnesting a two phase approach is used The rst phase called dependency based optimization transforms queries at the query language level in order to treat common subexpressions and independent subqueries more e ciently The trans formed queries are translated to nested algebraic expressions These entail nested loop evaluation which may be very ine cient Hence the second phase unnests nested algebraic expressions to allow for more e cient evaluation The paper also discusses the di erences between unnesting in the relational and unnesting in the oo context Since the rst phase is rather simple the paper concentrates on the second phase