A temporal database contains time-varying data. In a real-time database transactions have deadlines or timing constraints. In this paper we review the substantial research in these two previously separate areas. First we characterize the time domain; then we investigate temporal and real-time data models. We evaluate temporal and real-time query languages along several dimensions. We examine temporal and real-time DBMS implementation. Finally, we summarize major research accomplishments to date and list several unanswered research questions. Index Terms-Object-oriented databases, relational databases, query languages, temporal data models, time-constrained databases, transaction time, user-defined time, valid time.
Present database systems process all the data related to a query before glvmg out responses As a result, the size of the data to be processed becomes excessive for real--constrao~ed envmmnmus A new metbodology 1s needed to cut down systemahcally the hme to process the data ~ovolved m processlog tbe query. To tlus end, we propose to use data samples and construct an appxoxnnate synthehc response to a given query IO thw paper, we coos&r only COUNT(E) type queries, where E IS an art&~ relahooal algebra expression We make no assumphons about the dx#nbutioo of attnbute values and ordermg of tuples m the mput relahoos, and propose wlstent and uobmsed eshmators for arblhary COUNT(E) type quenes We design a samplmg plan based 00 the cluster samplmg method to unprove the uhkahoo of sampled data and to reduce the cost of samplmg We also evaluate the performance of the proposed eshmators
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