Chapter 11: Indexing and Hashing - Manhattan College
Indexing Database System Concepts, 6th Ed. Silberschatz, Korth and Sudarshan See www.db-book.com for conditions on re-use Basic Concepts Indexing mechanisms used to speed up access to desired data. E.g., author catalog in library Search Key - attribute or set of attributes used to look up records in a file.
An index file consists of records (called index entries) of the form search-key pointer Index files are typically much smaller than the original file Two basic kinds of indices: Ordered indices: search keys are stored in sorted order Hash indices: search keys are distributed uniformly across
buckets using a hash function. Database System Concepts - 6th Edition 11.2 Silberschatz, Korth and Sudarshan Index Evaluation Metrics Access types Access time Insertion time
Deletion time Space overhead Database System Concepts - 6th Edition 11.3 Silberschatz, Korth and Sudarshan Ordered Indices In an ordered index, index entries are stored sorted on the search key value. E.g., author catalog in library.
Primary index: in a sequentially ordered file, the index whose search key specifies the sequential order of the file. Also called clustering index The search key of a primary index is usually but not necessarily the primary key. Secondary index: an index whose search key specifies an order different from the sequential order of the file. Also called non-clustering index.
Database System Concepts - 6th Edition 11.4 Silberschatz, Korth and Sudarshan Dense Index Files Dense index Index record appears for every search-key value in the file. E.g. index on ID attribute of instructor relation Database System Concepts - 6th Edition 11.5 Silberschatz, Korth and Sudarshan Dense Index Files (Cont.)
Dense index on dept_name, with instructor file sorted on dept_name Database System Concepts - 6th Edition 11.6 Silberschatz, Korth and Sudarshan Sparse Index Files Sparse Index: contains index records for only some search-key values. Applicable when records are sequentially ordered on search-key
To locate a record with search-key value K we: Find index record with largest search-key value < K Search file sequentially starting at the record to which the index record points Database System Concepts - 6th Edition 11.7 Silberschatz, Korth and Sudarshan Sparse Index Files (Cont.)
Compared to dense indices: Less space and less maintenance overhead for insertions and deletions. Generally slower than dense index for locating records. Database System Concepts - 6th Edition 11.8 Silberschatz, Korth and Sudarshan Secondary Indices Example Secondary index on salary field of instructor
Index record points to a bucket that contains pointers to all the actual records with that particular search-key value. Secondary indices have to be dense Database System Concepts - 6th Edition 11.9 Silberschatz, Korth and Sudarshan Multilevel Index Database System Concepts - 6th Edition 11.10 Silberschatz, Korth and Sudarshan
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