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Update Record in an Indexed-Sequential File
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Deletion of a record, done by setting a tombstone into the record, is also done using this process Equation 3-43 is always appropriate for systems which disallow both the updating of key elds and variable-length records In the general case, the previous version of the record has to be deleted and the new record inserted appropriately The old record is rewritten with the tombstone; the key and pointer elds are kept intact so that the structure of the le is not violated Then TU = TF + TRW + TI = 2TF + 7r + btt in general 3-44 If the cases which permit in-place updates are recognized by the le system, then TU is to be computed based on the mix of in-place and general updates
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An exhaustive search of the le has to be made when the search argument is not the indexed attribute The le may be read serially by following the over ow chains for every block, or if seriality is not required, the entire data area on a cylinder can be read sequentially, followed by sequential reading of the entire over ow area In either case, the index can be ignored unless it contains space-allocation information Most systems provide only the ability to read serially, so that
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TX = TF + (n + o 1)TN (n + o )TN =
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n + o Bfr (r + btt) Bfr
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The assumptions leading to Eq 3-41 are valid here In the alternative case the evaluation would consider the e ective transfer rate, neglecting the delay when skipping from data blocks to over ow blocks Unused over ow blocks will not be read, so that o can be used to estimate the size of the over ow areas read Now TX (n + o ) R t sequential 3-46
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To reorganize the old le, the entire le is read serially and rewritten without the use of over ow areas As a by-product a new index is constructed The prior index can be ignored, since the le is read serially Additional bu ers in memory are needed to collect the new data and index information For the index it is desirable to have at least one bu er for every level The new main le should be at least double-bu ered All outputs can be written sequentially Then
Reorganization of an Indexed-Sequential File
TY =
n + o Bfr R SI (r + btt) + (n + o d) + Bfr t t
3-47
We assume that o new records are in the over ow areas; however, the value of o will be larger at reorganization time than in the cases considered in Eqs 3-33 to 3-46 The value of o will still be less than o, the number of records for which over ow space has been allocated Following the discussion of reorganization in Chap 3-3-2(7), we assume that o = 08 o Such a value is justi ed if the reorganization policy were as follows: Reorganization of a le is to be done the rst night the over ow area exceeds 75% utilization, given that the average daily increase of the over ow area is 10% A simpler assumption that o = o will provide a conservative approximation for the number of over ow records to be processed
122 3-4 THE INDEXED FILE
Basic File-System Organization
Indexed-sequential les only provide one index, but searching for information may have to be done on other attributes than a primary key attribute In a generalized indexed le we permit multiple indexes There may be indexes on any attribute, and perhaps on all attributes A number of changes to the le organization follow from that extension All indexes are treated equally: 1 All indexes are record-anchored 2 The concept of a primary attribute is not retained 3 No sequentiality according to a primary index is maintained No over ow chains can be maintained: 1 Any insertions are placed into the main data le 2 The main le format should make insertion convenient 3 All indexes must be updated to re ect insertions We expand on the trade-o s implied above in more detail throughout this section By givng up the requirement for sequentiality to provide e cient serial access, much exibility is gained In the generalized indexed le the records are accessed only through their indexes There is now no restriction on the placement of a data record, as long as a TID exists in some index that allows the record to be fetched when the goal data from the record is wanted Each index is associated with some attribute The gain in exibility obtained makes this le organization preferable to the indexed-sequential le organization in many applications The actual physical placement and format of records in generalized indexed les can be determined by secondary considerations, as ease of management or reliability Having indexes on more than one attribute greatly increases the availability of the data in information retrieval and advanced processing systems Variable-length records are common in these applications The exibility of generalized indexed les has created a great variety of actual designs The variety of designs has unfortunately also created a diversity of terminology, often quite inconsistent, so that anyone intending to evaluate a speci c approach to indexed les will have to translate terms used in describing such systems into standard concepts We will evaluate again a speci c approach which is becoming increasingly common, based on the use of B-trees for the indexes Figure 3-13 shows schematically three indexes into a Personnel le, for the attributes Names, Professions, and Chronic diseases Each of the variable-length spanned records of the le can be located by giving a name or profession value The third record, in block 2 at position 35, has a eld with the attribute name Chronic diseases and can also be located via that index
Sec 3-4
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