Microsoft Windows

Types of File Organization

File organization is a way of organizing the data or records in a file. It does not refer to how files are organized in folders, but how the contents of a file are added and accessed. There are several types of file organization, the most common of them are sequential, relative and indexed. They differ in how easily records can be accessed and the complexity in which records can be organized.

there are four methods of organizing files. They aresequential, line-sequential, indexed-sequential, inverted list and direct or hashed accessorganization.

Sequential Organization

A sequential file contains records organized in the order they were entered. The order of the records is fixed. The records are stored and sorted in physical, contiguous blocks within each block the records are in sequence.

Records in these files can only be read or written sequentially.

Once stored in the file, the record cannot be made shorter, or longer, or deleted. However, the record can be updated if the length does not change. (This is done by replacing the records by creating a new file.) New records will always appear at the end of the file.

If the order of the records in a file is not important, sequential organization will suffice, no matter how many records you may have. Sequential output is also useful for report printing orsequential reads which some programs prefer to do.

Line-Sequential Organization

Line-sequential files are like sequential files, except that the records can contain only characters as data. Line-sequential files are maintained by the native byte stream files of the operating system.

In the COBOL environment, line-sequential files that are created with WRITE statements with the ADVANCING phrase can be directed to a printer as well as to a disk.

Indexed-Sequential Organization

Key searches are improved by this system too. The single-level indexing structure is the simplest one where a file, whose records are pairs, contains a key pointer. This pointer is the position in the data file of the record with the given key. A subset of the records, which are evenly spaced along the data file, is indexed, in order to mark intervals of data records.

This is how a key search is performed: the search key is compared with the index keys to find the highest index key coming in front of the search key, while a linear search is performed from the record that the index key points to, until the search key is matched or until the record pointed to by the next index entry is reached. Regardless of double file access (index + data) required by this sort of search, the access time reduction is significant compared with sequential file searches.

Let's examine, for sake of example, a simple linear search on a 1,000 record sequentially organized file. An average of 500 key comparisons are needed (and this assumes the search keys are uniformly distributed among the data keys). However, using an index evenly spaced with 100 entries, the total number of comparisons is reduced to 50 in the index file plus 50 in the data file: a five to one reduction in the operations count!

Hierarchical extension of this scheme is possible since an index is a sequential file in itself, capable of indexing in turn by another second-level index, and so forth and so on. And the exploit of the hierarchical decomposition of the searches more and more, to decrease the access time will pay increasing dividends in the reduction of processing time. There is however a point when this advantage starts to be reduced by the increased cost of storage and this in turn will increase the index access time.

Hardware for Index-Sequential Organization is usually Disk-based, rather than tape. Records are physically ordered by primary key. And the index gives the physical location of each record. Records can be accessed sequentially or directly, via the index. The index is stored in a file and read into memory at the point when the file is opened. Also, indexes must be maintained.

Life sequential organization the data is stored in physical contiguous box. How ever the difference is in the use of indexes. There are three areas in the disc storage:

§ Primary Area:-Contains file records stored by key or ID numbers.

§ Overflow Area:-Contains records area that cannot be placed in primary area.

§ Index Area:-It contains keys of records and there locations on the disc.

Inverted List

In file organization, this is a file that is indexed on many of the attributes of the data itself. The inverted list method has a single index for each key type. The records are not necessarily stored in a sequence. They are placed in the are data storage area, but indexes are updated for the record keys and location.

Here's an example, in a company file, an index could be maintained for all products, another one might be maintained for product types. Thus, it is faster to search the indexes than every record. These types of file are also known as "inverted indexes." Nevertheless, inverted list filesuse more media space and the storage devices get full quickly with this type of organization. The benefits are apparent immediately because searching is fast. However, updating is much slower.

Content-based queries in text retrieval systems use inverted indexes as their preferred mechanism. Data items in these systems are usually stored compressed which would normally slow the retrieval process, but the compression algorithm will be chosen to support this technique.

When querying a file there are certain circumstances when the query is designed to be modalwhich means that rules are set which require that different information be held in the index. Here's an example of this modality: when phrase querying is undertaken, the particular algorithm requires that offsets to word classifications are held in addition to document numbers.

Direct or Hashed Access

With direct or hashed access a portion of disk space is reserved and a "hashing" algorithm computes the record address. So there is additional space required for this kind of file in the store. Records are placed randomly through out the file. Records are accessed by addresses that specify their disc location. Also, this type of file organization requires a disk storage rather than tape. It has an excellent search retrieval performance, but care must be taken to maintain the indexes. If the indexes become corrupt, what is left may as well go to the bit-bucket, so it is as well to have regular backups of this kind of file just as it is for all stored valuable data!