Chapter 18 Indexing Structures for Files Copyright 2011 Pearson Education Inc Publishing as Pearson Addison Wesley 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 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 The index is called an access path on the field Copyright 2011 Ramez Elmasri and Shamkant Navathe Indexes as Access Paths cont 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe 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 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe Primary Index on the Ordering Key Field Copyright 2011 Ramez Elmasri and Shamkant Navathe Indexes as Access Paths cont Example Given the following data file EMPLOYEE NAME SSN ADDRESS JOB SAL Suppose that record size R 100 bytes block size B 1024 bytes r 30000 records Then we get blocking factor Bfr B div R 1024 div 100 10 records block number of file blocks b r Bfr 30000 10 3000 blocks For an index on the SSN field assume the field size V SSN 9 bytes assume the record pointer size PR 6 bytes Then index entry size RI VSSN PR 9 6 15 bytes index blocking factor BfrI B div RI 1024 div 15 68 entries block number of index blocks b r BfrI 3000 68 45 blocks binary search needs log2bI log245 6 block accesses 1 This is compared to an average linear search cost of b 2 3000 2 1500 block accesses If the file records are ordered the binary search cost would be log2b log23000 12 block accesses Copyright 2011 Ramez Elmasri and Shamkant Navathe 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe A Clustering Index Example Copyright 2011 Ramez Elmasri and Shamkant Navathe Another Clustering Index Example Copyright 2011 Ramez Elmasri and Shamkant Navathe 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe Example of a Dense Secondary Index Copyright 2011 Ramez Elmasri and Shamkant Navathe Example of a Secondary Index Copyright 2011 Ramez Elmasri and Shamkant Navathe 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe A Two Level Primary Index Copyright 2011 Ramez Elmasri and Shamkant Navathe 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe A Node in a Search Tree with Pointers to Subtrees Below It Copyright 2011 Ramez Elmasri and Shamkant Navathe Copyright 2011 Ramez Elmasri and Shamkant Navathe Dynamic Multilevel Indexes Using BTrees 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe Dynamic Multilevel Indexes Using BTrees and B Trees cont 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 Copyright 2011 Ramez Elmasri and Shamkant Navathe Difference between B tree and B tree In a B tree pointers to data records exist at all levels of the tree In a B tree all pointers to data records exists at the leaf level nodes Copyright 2011 Ramez Elmasri and Shamkant Navathe B tree Structures Copyright 2011 Ramez Elmasri
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