font ean 13 para excel The Indexed-Sequential File in Software

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The Indexed-Sequential File
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The multiple-indexed le places updates into the main- le These records must be locatable via the indexes, so that here the indexes are dynamic We use a B-tree index, which trades space for the ability to remain up to date
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Inverted Files A le for which indexes have been created is sometimes referred to as an inverted le This term has its origin in bibliographic indexing, as shown in Fig 3-14 and will be used rarely and carefully in this book Sometimes a copy of a sequential le, when sorted according to another key attribute, has been called an inverted le The term fully inverted generally implies a le where all attributes have indexes, as described in Sec 3-4 The terms inverted index, inverted list, inverted le, and partially inverted le are used inconsistently in the literature, and frequently imply indexing as described in this chapter We simply avoid the adjective inverted in this book
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3-3-2 Structure and Manipulation of Indexed-Sequential Files The indexed-sequential le organization allows, when reading data serially, sequential access to the main record areas of the le, shown conceptially in Fig 3-6a and by example in Fig 3-6 Only some pointer elds, used to handle insertions, must be skipped for serial reading Records which have been inserted are found in a separate le area, called the over ow area An over ow area is similar to the transaction log le used previously, but integrated when we have an indexed-sequential organization The records in this area are located by following a pointer from their predecessor record Serial reading of the combined le proceeds sequentially until a pointer to the over ow le is found, then continues in the over ow le until a NULL pointer is encountered; then reading of the main le is resumed To fetch a speci c record, the index is used
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Basic File-System Organization
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Figure 3-6a
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Components of an indexed sequential le
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Indexes have been implemented in a variety of ways We will consider here a static, multilevel index, using block anchors An alternative, dynamic, index method, called a B-tree, will be presented with the indexed les of Sec 3-3 We discuss here the most prevalent version of the indexed-sequential le Improvements to this scheme are covered in Chap 8-1
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An Index for an Indexed-Sequential File
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The index for an indexed-sequential le is based on the same key attribute used to determine the sequence of the le itself For such a primary index a number of re nements can be applied One of these is indexing only the rst record in every block using block anchors and the other is keeping most of the index on the same cylinder as the data records, cylinder indexes The e ect of having cylinder indexes will be considered in Chap 8-1-1
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Selecting the Key
Block Anchors The bene t of the index is to rapidly access a block of the le Individual records in a block can be found by a search within the block, so that it is not necessary to keep in the index a TID for every record, but only a reference to one record per block The referenced record is called an anchor point, and only the anchor s key value and the block pointer are kept in the index Natural anchor points are based on blocks, tracks, or cylinders In Fig 3-6b the choice of anchor point is the rst record of a block The cost of searching within a le block for a record is small, since the entire block is brought into memory whenever required and can be kept available in a bu er A block will contain a number of records equal to Bfr The number of entries in a block-anchored index is, hence, n/Bfr, and the size of an index entry is V + P
Sec 3-3
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