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 Management Systems 3ed R Ramakrishnan and J Gehrke 1 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 Management Systems 3ed R Ramakrishnan and J Gehrke 2 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 Indexing techniques evaluated on basis of Access time Insertion time Deletion time Space overhead Database Management Systems 3ed R Ramakrishnan and J Gehrke 3 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 Not 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 Wrong Again Index sequential file ordered sequential file with a primary index Database Management Systems 3ed R Ramakrishnan and J Gehrke 4 Dense Index Files Dense index Index record appears for every search key value in the file Database Management Systems 3ed R Ramakrishnan and J Gehrke 5 Sparse Index Files Sparse Index contains index records for only some search key values Applicable when records are sequentially ordered on searchkey 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 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 6 Example of Sparse Index Files Database Management Systems 3ed R Ramakrishnan and J Gehrke 7 Multilevel Index If primary index does not fit in memory access becomes expensive To reduce number of disk accesses to index records 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 Management Systems 3ed R Ramakrishnan and J Gehrke 8 Multilevel Index Cont Database Management Systems 3ed R Ramakrishnan and J Gehrke 9 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 is 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 Database Management Systems 3ed R Ramakrishnan and J Gehrke 10 Index Update Insertion Single level index insertion Perform a lookup using the search key value appearing in the record to be inserted Dense indices if the search key value does not appear in the index insert it Sparse indices if index stores an entry for each block of the file no change needs to be made to the index unless a new block is created In this case the first search key value appearing in the new block is inserted into the index Multilevel insertion as well as deletion algorithms are simple extensions of the single level algorithms Database Management Systems 3ed R Ramakrishnan and J Gehrke 11 Secondary Indices Frequently one wants to find all the records whose values in a certain field which is not the search key of the primary index satisfy some condition Example 1 In the account database stored sequentially by account number we may want to find all accounts in a particular branch Example 2 as above but where we want to find all accounts with a specified balance or range of balances We can have a secondary index with an index record for each search key value index record points to a bucket that contains pointers to all the actual records with that particular search key value Database Management Systems 3ed R Ramakrishnan and J Gehrke 12 Secondary Index on balance field of account Database Management Systems 3ed R Ramakrishnan and J Gehrke 13 Primary and Secondary Indices Secondary indices have to be dense Indices offer substantial benefits when searching for records When a file is modified every index on the file must be updated Updating indices imposes overhead on database modification Sequential scan using primary index is efficient but a sequential scan using a secondary index is expensive each record access may fetch a new block from disk Database Management Systems 3ed R Ramakrishnan and J Gehrke 14 B Tree Index Files B tree indices are an alternative to indexed sequential files Disadvantage of indexed sequential files performance degrades as file grows since many overflow blocks get created Periodic reorganization of entire file is required Advantage of B tree index files automatically reorganizes itself with small local changes in the face of insertions and deletions Reorganization of entire file is not required to maintain performance Disadvantage of B trees extra insertion and deletion overhead space overhead Advantages of B trees outweigh disadvantages and they are used extensively Database Management Systems 3ed R Ramakrishnan and J Gehrke 15 B Tree Index Files Cont A B tree is a rooted tree