Filtering, Punctuation, Windows and Synopses

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This chapter addresses some of the problems raised by the high-volume, nonterminating nature of many data streams. We begin by outlining challenges for query processing over such streams, such as outstripping CPU or memory resources, operators that wait for the end of input and unbounded query state. We then consider various techniques for meeting those challenges. Filtering attempts to reduce stream volume in order to save on system resources. Punctuations incorporate semantics on the structure of a stream into the stream itself, and can help unblock query operators and reduce the state they must retain. Windowing modifies a query so that processing takes place on finite subsets of full streams. Synopses are compact, efficiently maintained summaries of data that can provide approximate answers to particular queries.