THE PILE A form of file organization where data are collected inthe same order they arrived This organization simply accumulate mass of data andsave it Each field is self-describing, includes a field name and avalue. Length of each field is determined as shown Implicitly indicated by delimiters Explicitly included as subfield or Done by default for that particular field type Records may have different fields and there is no visiblestructure. Hence, record access is through exhaustivesearch where all record or the entire file is examined This type of file uses space efficiently and is updatedeasily. However, retrieval of a single record could betedious
THE PILE ORGANIZATIONVariable-length records DelimiterDelimiterDelimiterDelimiterDelimiterDelimiterVariable-length recordsVariable-length recordsVariable-length recordsVariable-length records
THE SEQUENTIAL FILE Most common form of file structure Fixed format are used for records and records are of thesame length Field names and lengths are attributes of the file One particular field (usually first field) is referred to asthe key field :- Uniquely identifies the record Records are stored in key sequence Alphabetical order or numerical order depending on the key field It is the simplest structure and easy to implement in anydevice. However, in the worst case scenario, accessinga single record takes a long time.
THE SEQUENTIAL FILE ORGANIZATION12…….n-1nKey Field Attributes
THE SEQUENTIAL FILE ORGANIZATION To enable a sequential form of records, newrecords are placed in a log file or transaction file.Then, a batch update is performed to merge the logfile with the master file to produce a new file withthe correct key sequence1 2 n-1 n…RecordTerminators
THE INDEXED SEQUENTIAL FILE A file management system that allows records to beaccessed either sequentially (in the order they wereentered) or randomly (with an index) A secondary set of hash tables known as indexes iscreated that contains pointers to the main file In indexed sequential file, records are organized insequence based on key fields Each file has an index to support random search Overflow file is added such as each record inoverflow file is located by following a pointer fromits predecessor record in main file
DESCRIPTION As an example:- Firstly, a single level of indexing is used. Hence, theindex is a simple sequential file. Each record in the index file consists of:- Key field(same as key field in the main file) Pointer to the main file To find a specific field, the index file is searched formatching key values Then, the pointer indicates the record having thematching key values as depicted by figure below This reduces the average search length
There is an addition to the organization where each record inthe main file contains an additional field which is a pointer tothe overflow file When a new record is added, it is added into the overflow file.The record in main file which is immediately before the newrecord in a logical sequence to be inserted is updated tocontain a pointer to the new record in the overflow file However, if the record before the new record in logicalsequence is itself in the overflow file, then the pointer in thatrecord is updated The processing of entire file sequentially involves processingof records of main files sequentially until the pointer to theoverflow file is found, then accessing is continued in theoverflow file until a null pointer is encountered Secondly, we visualize the organization using multiple levelsof indexes The lowest level of index points to the main file, whereas theindex files sitting on the level above points to the index filebelow it Hence, the efficiency in access is greatly increased andaverage length of search is greatly reduced as conveyed infigure below
THE INDEXED FILE Uses multiple indexes for different key fields whichmay be the subject of a search Record accessibility are through their indexes May contain an exhaustive index that contains oneentry for every record in the main file. The indexitself is organized in the sequential form for ease ofsearching May contain a partial index. It contains entries torecords where the desired field exists
THE INDEXED FILE ORGANIZATIONPrimary fileExhaustive PartialExhaustiveIndex AttributesPointerIndicator
THE DIRECT/HASHED FILE This file management system that access directly anyblock of a known address. A key field is required for each record. Uses hashing on the key value. No concept of sequential ordering implemented. Such examples are:- Directories Pricing table Schedules Name lists It is used where rapid access is required, where fixed-length records are used and where records areaccessed one-at-a-time. The concept of hashing can be shown as below.
HASHING METHODS Direct method The key is the address a key value might be between 1-100. The address of a certainrecords will be the same as the key. Subtraction method Subtract certain amount of numbers from the key a key value might start with 1001 and ends with 1100. A simplehashing function could subtract 1000 from the key to determinethe address. Modulo-division method Key value is divided by the size of records. the remainder becomes the address of the key. Digit-extraction method Selected digits are extracted from the key As an example:- a field has 10 digits. The hashing function onlyextracts the first 2 digits and the last digit and use them asaddress.