Transaction Manager
2015
are he re! 270 9.1 Query Cost and Selectivity Estimation I The DBMS has a number of alternative implementations available for each (log-ical) algebra operator. I Selecting an implementation for each of the operators in a plan is one of the tasks of the query optimizer. I The applicability of implementations may depend on various physical properties of the argument files (or streams), such as sort orders, availability of indexes, I Among the applicable plans, the optimizer selects the ones with the least ex-pected cost. I The cost of the implementation algorithms is determined by the size of the input arguments. Therefore, it is crucial to know, compute, or estimate the sizes of arguments. I This chapter presents some techniques to measure the quantitative characteris-tics of database files or input/output streams.