File Organization and Storage Structures
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1 File Organization and Storage Structures o Storage of data File Organization and Storage Structures Primary Storage = Main Memory Fast Volatile Expensive Secondary Storage = Files in disks or tapes Non-Volatile Secondary Storage is preferred for storing data File Organization and Storage Structures - 1 File Organization and Storage Structures - 2 Basic Concepts Logical Record Vs Physical Record o Information are stored in data files o Each file is a sequence of records o Each record consists of one or more fields Sno Lname Position NIN SL21 White Manager WK440211B SG37 Beech Snr Asst WL432514C SG14 Ford Deputy WL220658D Bno B5 o Logical record Eg. The record of a staff (SG37). A record o Physical record The unit of transfer between disk and primary storage. A page, A block Generally, a physical record consists of more than one logical record File Organization and Storage Structures - 3 File Organization and Storage Structures - 4
2 Logical Record Vs Physical Record File Organization & Access Method o File Organization means the physical arrangement Sno SL21 SG37 Lname White Beech Position Manager Snr Asst NIN WK440211B WL432514C Bno B5 Page 1 of data in a file into records and pages on secondary storage Eg. Ordered files, indexed sequential file etc. SG14 SA9 SG5 SL41 Ford Howe Brand Lee Deputy Assistant Manager Assistant WL220658D WM532187D WK588932E WA290573K B7 B5 2 o Access Method means the steps involved in storing and retrieving records from a file. Eg. Using an indexed access method to retrieve a record from an indexed sequntial file. File Organization and Storage Structures - 5 File Organization and Storage Structures - 6 Heap Files o Heap files are files of unordered records. o Quick insertion (no particular ordering) When a new record is created, it is put in the last page of the file if there is sufficient space. Otherwise a new page is added to the file. o Slow retrieval (only allow linear search) reading pages from the file until a required record is found. Ordered Files o Ordered Files: Records are sorted on field(s) => Key o Allow Binary Searching Suppose one page stores one record. To search for SG37, search the middle page (6/2 = 3) first. We find that SG37 does not exist in this page(sg14). Then, since SG37 is greater than SG14, we search the middle page within the lower half of the file, and so on. o To delete a record, the record is marked as deleted. Space is reclaimed during periodical reoganization. File Organization and Storage Structures - 7 File Organization and Storage Structures - 8
3 Ordered Files o Inserting a record If the appropriate page is full, may have to reorganize the whole file => Time consuming Solution: use a temporary unsorted file (transaction file). Merge to the sorted file periodically. o Rarely used unless come with an index => Indexed Sequential File o Both Heap Files and Ordered Files are also called Sequential Files. o are also called Hash Files or Random Files o No need to write records sequentially o Use a hash function to calculate the number of the page (bucket) which a record should be located o Eg., use the division-remainder calculation method that, bucket_no = Record_key mod 3 File Organization and Storage Structures - 9 File Organization and Storage Structures - 10 Open Addressing o Upon a collision, the system performs a linear search to find the first available slot. o Problem: If a new record SG41 is created, which bucket to go? o Collision Management Open addressing, Unchained overflow, Chained overflow, Multiple hashing o When last bucket has been searched, starts from the first bucket. o SL41 will be inserted to: Bucket 1 File Organization and Storage Structures - 11 File Organization and Storage Structures - 12
4 Unchained Overflow o An overflow area is maintained for collisions. o SL41 will be inserted to: Bucket 3 Chained Overflow o Each bucket has a synonym pointer o Value of the synonym pointer: Zero: no collision occurred Non-zero: the overflow bucket used File Organization and Storage Structures - 13 File Organization and Storage Structures - 14 Multiple Hashing o Upon collision, apply a second hashing function to produce a new hash address in an overflow area. Limitation (of Hashing) Inappropriate for some retrievals: based on pattern matching eg. Find all students with ID like 98xxxxxx. Involving ranges of values eg. Find all students from to Based on a field other than the hash field File Organization and Storage Structures - 15 File Organization and Storage Structures - 16
5 Indexes Index: A data structure that allows particular records in a file to be located more quickly ~ Index in a book An index can be sparse or dense: Sparse: record for only some of the search key values (eg. Staff Ids: CS001, EE001, MA001). Applicable to ordered data files only. Dense: record for every search key value. (eg. Staff Ids: CS001, CS002,.. CS089, EE001, EE002,..) Indexes TERMINOLOGY Data file: a file containing the logical records Index file: a file containing the index records Indexing field: the field used to order the index records in the index file Key: One or more fields which can uniquely identify a record (eg. No 2 students have the same student ID). File Organization and Storage Structures - 17 File Organization and Storage Structures - 18 Indexes TYPES OF INDEXES Primary Index: An index ordered in the same way as the data file, which is sequentially ordered according to a key. (The indexing field is equal to this key.) Secondary Index: An index that is defined on a nonordering field of the data file. (The indexing field need not contain unique values). A data file can associate with at most one primary index plus several secondary indexes. Indexed Sequential Files What are Indexed Sequential Files? = A sorted data file with a primary index Advantage of an Indexed Sequential File Allows both sequential processing and individual record retrieval through the index. Structure of an Indexed Sequential File o A primary storage area o A separate index or indexes o An overflow area File Organization and Storage Structures - 19 File Organization and Storage Structures - 20
6 In B + -Tree, data or indexes are stored in a hierarchy of nodes o B => Balanced o Consistent access time (for each access, same number of nodes are searched) Point to data File Organization and Storage Structures - 21 TERMINOLOGY Degree (Order) : The maximum number of children allowed per parent. Depth : The maximum number of levels between the root node and a leaf node in the tree. File Organization and Storage Structures - 22 In practice, each node in the tree is actually a page, so we can store many pointers and keys. Eg. For a page size of 4KB, the B + -Tree can be of order 512. Access time depends more ofen upon depth than on breadth => Shallow trees are preferred. RULES o The root (if not a leaf node) must have at least 2 children o For a tree of order n, each node (except root and leaf) must have between n/2 and n pointers and children. If n/2 is not an integer, the result is rounded up. File Organization and Storage Structures - 23 RULES (Cont d): o For a tree or order n, the number of key values in a leaf node must be between (n-1)/2 and (n-1) pointers and children. If (n-1)/2 is not an integer, the result is rounded up. o The number of key values contained in a nonleaf node is 1 less than the number of pointers. o The tree must always be balanced: every path from the root node to a leaf must have the same length. o Leaf nodes are linked in order of key values. File Organization and Storage Structures - 24
7 Balancing can be costly to maintain. Example: Example: Adding SG14 Adding SA9 File Organization and Storage Structures - 25 File Organization and Storage Structures - 26 Example: Adding SA9 Summary o Basic concepts (Files, Records, Fields) o Primary storage vs secondary storage o Logical record vs physical record o File Organization (and access methods) Heap files Ordered Files (Binary Search) (Hashing) Indexes Indexed Sequential Files B + - Trees File Organization and Storage Structures - 27 File Organization and Storage Structures - 28
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