Chapter 12:Indexing and Hashing Basic Concepts Ordered Indices B+-Tree Index Files B-Tree Index Files Static Hashing Dynamic Hashing Comparison of Ordered Indexing and Hashing Index Definition in SQL Multiple-Key Access Database System Concepts-5th Edition,Oct 4,2006 12.2 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 12.2 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Chapter 12: Indexing and Hashing Basic Concepts Ordered Indices B+-Tree Index Files B-Tree Index Files Static Hashing Dynamic Hashing Comparison of Ordered Indexing and Hashing Index Definition in SQL Multiple-Key Access
Basic Concepts Indexing mechanisms used to speed up access to desired data. E.g.,author catalog in library Search Key -attribute to 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“nash function”. Database System Concepts-5th Edition,Oct 4,2006 12.3 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 12.3 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Basic Concepts Indexing mechanisms used to speed up access to desired data. E.g., author catalog in library Search Key - attribute to set of attributes used to look up records in a file. An index file consists of records (called index entries) of the form 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”. search-key pointer
Index Evaluation Metrics Access types supported efficiently.E.g., records with a specified value in the attribute or records with an attribute value falling in a specified range of values. Access time Insertion time Deletion time Space overhead Database System Concepts-5th Edition,Oct 4,2006 12.4 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 12.4 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Index Evaluation Metrics Access types supported efficiently. E.g., records with a specified value in the attribute or records with an attribute value falling in a specified range of values. Access time Insertion time Deletion time Space overhead
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. Index-sequential file:ordered sequential file with a primary index. Database System Concepts-5th Edition,Oct 4,2006 12.5 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 12.5 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 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. Index-sequential file: ordered sequential file with a primary index
Dense Index Files Dense index-Index record appears for every search-key value in the file. Brighton A-217 Brighton 750 Downtown A-101 Downtown 500 Mianus A-110 Downtown 600 Perryridge A-215 Mianus 700 Redwood A-102 Perryridge 400 Round Hill A-201 Perryridge 900 A-218 Perryridge 700 A-222 Redwood 700 A-305 Round Hill 350 Database System Concepts-5th Edition,Oct 4,2006 12.6 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 12.6 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Dense Index Files Dense index — Index record appears for every search-key value in the file
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 Brighton A-217 Brighton 750 Mianus A-101 Downtown 500 Redwood A-110 Downtown 600 A-215 Mianus 700 A-102 Perryridge 400 A-201 Perryridge 900 A-218 Perryridge 700 A-222 Redwood 700 A-305 Round Hill 350 Database System Concepts-5th Edition,Oct 4,2006 12.7 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 12.7 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 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
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. Good tradeoff:sparse index with an index entry for every block in file, corresponding to least search-key value in the block. data block 0 data block 1 Database System Concepts-5th Edition,Oct 4,2006 12.8 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 12.8 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 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. Good tradeoff: sparse index with an index entry for every block in file, corresponding to least search-key value in the block
Multilevel Index If primary index does not fit in memory,access becomes expensive Solution:treat primary index kept on disk as a sequential file and construct a sparse index on it. outer index-a sparse index of primary index inner index-the primary index file If even outer index is too large to fit in main memory,yet another level of index can be created,and so on. Indices at all levels must be updated on insertion or deletion from the file. Database System Concepts-5th Edition,Oct 4,2006 12.9 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 12.9 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Multilevel Index If primary index does not fit in memory, access becomes expensive. Solution: treat primary index kept on disk as a sequential file and construct a sparse index on it. outer index – a sparse index of primary index inner index – the primary index file If even outer index is too large to fit in main memory, yet another level of index can be created, and so on. Indices at all levels must be updated on insertion or deletion from the file
Multilevel Index (Cont.) index data block 0 block 0 index data block 1 block 1 outer index inner index Database System Concepts-5th Edition,Oct 4,2006 12.10 @Silberschatz,Korth and Sudarshan
Database System Concepts - 5 12.10 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Multilevel Index (Cont.)
Index Update:Deletion If deleted record was the only record in the file with its particular search- key value,the search-key is deleted from the index also. Single-level index deletion: Dense indices-deletion of search-key:similar to file record deletion Sparse indices- if an entry for the search key exists in the index,it is deleted by replacing the entry in the index with the next search-key value in the file (in search-key order). If the next search-key value already has an index entry,the entry is deleted instead of being replaced. Brighton A-217 Brighton 750 Mianus A-101 Downtown 500 Redwood A-110 Downtown 600 A-215 Mianus 700 A-102 Perryridge 400 A-201 Perryridge 900 A-218 Perryridge 700 A-222 Redwood 700 A-305 Round Hill 350 Database System Concepts-5th Edition,Oct 4,2006 12.11 ©Silberschat乜，Korth and Sudarshan
Database System Concepts - 5 12.11 ©Silberschatz, Korth and Sudarshan th Edition, Oct 4, 2006 Index Update: Deletion If deleted record was the only record in the file with its particular searchkey value, the search-key is deleted from the index also. Single-level index deletion: Dense indices – deletion of search-key:similar to file record deletion. Sparse indices – if an entry for the search key exists in the index, it is deleted by replacing the entry in the index with the next search-key value in the file (in search-key order). If the next search-key value already has an index entry, the entry is deleted instead of being replaced