On Estimating the Cost of Accessing Records in Blocked Database Organizations

The estimation of the cost of processing a query using a particular access path under a given physical organization has important applications in integrated database environments. When records in a file are stored in fixed-length physical blocks in secondary storage, and mechanisms are available whereby a query can be resolved without the accessing of all of the records, an important measure of the cost of using a particular access path is the number of blocks that have to be accessed in referencing the records of interest. In this paper, a general formula is derived for the expected number of blocks on which a random sample of r records from a file containing n records (which may be of arbitrary lengths, and which may extend across block boundaries) will reside. The specialization of this formula to the case of fixedlength records is discussed. An approximation to this formula which is highly accurate for a wide range of parameters and which can be computed very efficiently is also provided.