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To gain fast random access to records in a le, we can use an index structure Each index structure is associated with a particular search key Just like the index of a book or a library catalog, an ordered index stores the values of the search keys in sorted order, and associates with each search key the records that contain it The records in the indexed le may themselves be stored in some sorted order, just as books in a library are stored according to some attribute such as the Dewey deci-
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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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IV Data Storage and Querying
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Brighton Downtown Downtown Mianus Perryridge Perryridge Perryridge Redwood Round Hill
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Sequential le for account records
mal number A le may have several indices, on different search keys If the le containing the records is sequentially ordered, a primary index is an index whose search key also de nes the sequential order of the le (The term primary index is sometimes used to mean an index on a primary key However, such usage is nonstandard and should be avoided) Primary indices are also called clustering indices The search key of a primary index is usually the primary key, although that is not necessarily so Indices whose search key speci es an order different from the sequential order of the le are called secondary indices, or nonclustering indices
1221 Primary Index
In this section, we assume that all les are ordered sequentially on some search key Such les, with a primary index on the search key, are called index-sequential les They represent one of the oldest index schemes used in database systems They are designed for applications that require both sequential processing of the entire le and random access to individual records Figure 121 shows a sequential le of account records taken from our banking example In the example of Figure 121, the records are stored in search-key order, with branch-name used as the search key
12211 Dense and Sparse Indices
An index record, or index entry, consists of a search-key value, and pointers to one or more records with that value as their search-key value The pointer to a record consists of the identi er of a disk block and an offset within the disk block to identify the record within the block There are two types of ordered indices that we can use: Dense index: An index record appears for every search-key value in the le In a dense primary index, the index record contains the search-key value and a pointer to the rst data record with that search-key value The rest of the records with the same search key-value would be stored sequentially after the
Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
IV Data Storage and Querying
12 Indexing and Hashing
The McGraw Hill Companies, 2001
12
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rst record, since, because the index is a primary one, records are sorted on the same search key Dense index implementations may store a list of pointers to all records with the same search-key value; doing so is not essential for primary indices Sparse index: An index record appears for only some of the search-key values As is true in dense indices, each index record contains a search-key value and a pointer to the rst data record with that search-key value To locate a record, we nd the index entry with the largest search-key value that is less than or equal to the search-key value for which we are looking We start at the record pointed to by that index entry, and follow the pointers in the le until we nd the desired record Figures 122 and 123 show dense and sparse indices, respectively, for the account le Suppose that we are looking up records for the Perryridge branch Using the dense index of Figure 122, we follow the pointer directly to the rst Perryridge record We process this record, and follow the pointer in that record to locate the next record in search-key (branch-name) order We continue processing records until we encounter a record for a branch other than Perryridge If we are using the sparse index (Figure 123), we do not nd an index entry for Perryridge Since the last entry (in alphabetic order) before Perryridge is Mianus, we follow that pointer We then read the account le in sequential order until we nd the rst Perryridge record, and begin processing at that point As we have seen, it is generally faster to locate a record if we have a dense index rather than a sparse index However, sparse indices have advantages over dense indices in that they require less space and they impose less maintenance overhead for insertions and deletions There is a trade-off that the system designer must make between access time and space overhead Although the decision regarding this trade-off depends on the speci c application, a good compromise is to have a sparse index with one index entry per block The reason this design is a good trade-off is that the dominant cost in pro-
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