Chapter 18 Indexing Structures CS 6360 Database Design Chris Irwin Davis Ph D Email cid021000 utdallas edu Phone 972 883 3574 O ce ECSS 4 705 Outline Types of Single level Ordered Indexes Primary Indexes Clustering Indexes Secondary Indexes Multilevel Indexes Dynamic Multilevel Indexes Using B Trees and B Trees 2 Indexes as Access Paths A single level index is an auxiliary file that makes it more efficient to search for a record in the data file In a database the index is usually stored in the same file as the data The index is usually specified on one field of the file although it could be specified on several fields One form of an index is a file of entries field value pointer to record which is ordered by field value 3 Indexes as Access Paths The index is called an access path on the field The index file usually occupies considerably less disk blocks than the data file because its entries are much smaller A binary search on the index yields a pointer to the file record Indexes can also be characterized as dense or sparse A dense index has an index entry for every search key value and hence every record in the data file A sparse or nondense index on the other hand has index entries for only some of the search values 4 Single level Ordered Indexes Primary Index Types of Single Level Indexes Primary Index Defined on an ordered data file The data file is ordered on a key field Includes one index entry for each block in the data file the index entry has the key field value for the first record in the block which is called the block anchor A similar scheme can instead use the last record in a block A primary index is a nondense sparse index since it includes an entry for each disk block of the data file and the keys of its anchor record rather than for every search value 6 Primary Index on the Ordering Key Field Single level Ordered Indexes Clustering Index Types of Single Level Indexes Clustering Index Defined on an ordered data file The data file is ordered on a non key field unlike primary index which requires that the ordering field of the data file have a distinct value for each record Includes one index entry for each distinct value of the field the index entry points to the first data block that contains records with that field value It is another example of nondense index where Insertion and Deletion is relatively straightforward with a clustering index 9 A Clustering Index Example Clusters may begin in the middle of a block Clusters span blocks No empty block slots Bad for dynamic size Another Clustering Index Example No two clusters share the same block space Overflow block per cluster Multiple partially full blocks Accommodates dynamic size Lost performance due to redirection Single level Ordered Indexes Secondary Index Types of Single Level Indexes Secondary Index A secondary index provides a secondary means of accessing a file for which some primary access already exists The secondary index may be on a field which is a candidate key and has a unique value in every record OR a non key with duplicate values The index is an ordered file with two fields The first field is of the same data type as some non ordering field of the data file that is an indexing field The second field is either a block pointer or a record pointer There can be many secondary indexes and hence indexing fields for the same file Includes one entry for each record in the data file hence it is a dense index 13 Example of a Dense Secondary Index The entries are ordered by value of K i so we can perform a binary search Because the records of the data file are not physically ordered by values of the secondary key field we cannot use block anchors i e dense Example of a Dense Secondary Index The entries are ordered by value of K i so we can perform a binary search Because the records of the data file are not physically ordered by values of the secondary key field we cannot use block anchors i e dense Example of a Secondary Index Each block of record pointers may be a di erent size Example of a Secondary Index Each block of record pointers may be a di erent size Properties of Index Types 18 Multi Level Indexes Multi Level Indexes Because a single level index is an ordered file we can create a primary index to the index itself In this case the original index file is called the first level index and the index to the index is called the second level index We can repeat the process creating a third fourth top level until all entries of the top level fit in one disk block A multi level index can be created for any type of firstlevel index primary secondary clustering as long as the first level index consists of more than one disk block 20 A Two Level Primary Index Insertion is handled by some form of overflow file that is merged periodically with the data file The index is recreated during file reorganization A Two Level Primary Index Insertion and Deletion Scalability Flexibility Multi Level Indexes Such a multi level index is a form of search tree However insertion and deletion of new index entries is a severe problem because every level of the index is an ordered file 23 Multi Level Indexes Such a multi level index is a form of search tree However insertion and deletion of new index entries is a severe problem because every level of the index is an ordered file We can address this problem using 24 Dynamic Multilevel Indexes Using B Trees and B Trees A Node in a Search Tree with Pointers to Subtrees Below It 26 Node Detail 27 Dynamic Multilevel Indexes Using B Trees and B Trees Most multi level indexes use B tree or B tree data structures because of the insertion and deletion problem This leaves space in each tree node disk block to allow for new index entries These data structures are variations of search trees that allow efficient insertion and deletion of new search values In B Tree and B Tree data structures each node corresponds to a disk block Each node is kept between half full and completely full 28 Dynamic Multilevel Indexes Using B Trees and B Trees An insertion into a node that is not full is quite efficient If a node is full the insertion causes a split into two nodes Splitting may propagate to other tree levels A deletion is quite efficient if a node does not become less than half full If a deletion causes a node to become less than half full it must be merged with neighboring nodes 29 Di erence between B tree and B tree In a B tree pointers to data records exist at all levels of the tree In a
View Full Document
Unlocking...