Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 1Chapter 12: Indexing and HashingChapter 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 AccessDatabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke 2Basic ConceptsBasic 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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 pointerDatabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke 3Index Evaluation MetricsIndex 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 overheadIndexing techniques evaluated on basis of:Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 4Ordered IndicesOrdered 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 5Dense Index FilesDense Index Files Dense index — Index record appears for every search-key value in the file.Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 6Sparse Index FilesSparse 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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 7Example of Sparse Index FilesExample of Sparse Index FilesDatabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke 8Multilevel IndexMultilevel 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 9Multilevel Index (Cont.)Multilevel Index (Cont.)Database Management Systems 3ed, R. Ramakrishnan and J. Gehrke 10Index Update: DeletionIndex 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 11Index Update: InsertionIndex 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 algorithmsDatabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke 12Secondary IndicesSecondary 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 13Secondary Index on Secondary Index on balancebalancefield of field of accountaccountDatabase Management Systems 3ed, R. Ramakrishnan and J. Gehrke 14Primary and Secondary IndicesPrimary 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 diskDatabase Management Systems 3ed, R.