!" #Chris Irwin Davis, Ph.D. !Email: [email protected] Phone: (972) 883-3574 Office: ECSS 4.705Chapter 18: Indexing StructuresCS-6360 Database Design!" #Outline• Types of Single-level Ordered Indexes!°Primary Indexes!°Clustering Indexes!°Secondary Indexes!• Multilevel Indexes!• Dynamic Multilevel Indexes Using B-Trees and B+-Trees2!" #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 value3!" #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 4Single-level Ordered IndexesPrimary 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.6Primary Index on the Ordering Key FieldSingle-level Ordered IndexesClustering 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.9A Clustering Index Example• Clusters may begin in the middle of a block"• Clusters span blocks"• No empty block slots"• Bad for dynamic sizeAnother 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 redirectionSingle-level Ordered IndexesSecondary 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 index13Example 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 different sizeExample of a Secondary Index!• Each block of record pointers may be a different size!" #Properties of Index Types18Multi-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 first-level index (primary, secondary, clustering) as long as the first-level index consists of more than one disk block20A 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. 24We can address this problem using…Dynamic Multilevel Indexes Using B-Trees and B+-Trees!" #A Node in a Search Tree with Pointers to Subtrees Below It26!" #Node Detail27!" #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 full28!" #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
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