On Two Forms of Structural Recursion

We investigate and compare two forms of recursion on sets for querying nested collections. The rst one is called sri and it corresponds to sequential processing of data. The second one is called sru and it corresponds to data-parallel processing. A uniform rst-order translation from sru into sri was known from previous work. The converse translation is by necessity more di cult and we have obtained three main results concerning it. First, we exhibit a uniform translation of sri queries into sru queries over the nested relational algebra. We observe that this translation maps PTIME algorithms into exponential-space queries. The second result proves that any uniform translation of sri queries into sru queries over the nested relational algebra must map some PTIME queries into exponential-space ones. In fact, in the presence of certain external functions, we provide a PTIME sri query for which any equivalent sru query requires exponential space. Thus, as a mechanism for implementing algor...

• Type: Conference
• Year: 1995
• Where: ICDT
• Authors: Dan Suciu, Limsoon Wong
• Keywords: Database, ICDT 1995, Nested Relational Algebra, Sri Queries, Sru Queries