We explore a new form of view rewrite called view disassembly. The objective is to rewrite views in order to “remove” certain sub-views (or unfoldings) of the view. This becomes pertinent for complex views which may defined over other views and which may involve union. Such complex views arise necessarily in environments as data warehousing and mediation over heterogeneous databases. View disassembly can be used for view and query optimization, preserving data security, making use of cached queries and materialized views, and view maintenance.
We provide computational complexity results of view disassembly. We show that the optimal rewrites for disassembled views is at least NP - hard. However, we provide good news too. We provide an approximation algorithm that has much better run-time behavior. We show a pertinent class of unfoldings for which their removal always results in a simpler disassembled view than the view itself. We also show the complexity to determine when a collection of unfoldings cover the view definition.
KeywordsExtensional Unfolding Query Optimization Simple Unfolding Academic Unit Query Tree
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