FILES OF RECORDS
l A file is a sequence of records, where each record is a collection
of data values (or data items).
l A file descriptor (or file header ) includes information
that describes the file, such as the field names and their data types, and the
addresses of the file blocks on disk.
l Records are stored on disk blocks.
The blocking factor bfr for a file is the (average) number of
file records stored in a disk block.
l A file can have fixed-length records or variable-length records.
Operation on Files
Typical file operations
include:
l OPEN: Readies the file for access, and associates a
pointer that will refer to a current
file record at each point in time.
l FIND: Searches for the first file record that
satisfies a certain condition, and makes it the current file record.
l FINDNEXT: Searches for the next file record
(from the current record) that satisfies a certain condition, and makes it the
current file record.
l READ: Reads the current file record into a program
variable.
l INSERT: Inserts a new record into the file, and makes
it the current file record.
l DELETE: Removes the current file record from the file,
usually by marking the record to indicate that it is no longer valid.
l MODIFY: Changes the values of some fields of the
current file record.
l CLOSE: Terminates access to the file.
l REORGANIZE: Reorganizes the file records. For
example, the records marked deleted are physically removed from the file or a
new organization of the file records is created.
l READ_ORDERED: Read the file blocks in order of a
specific field of the file.
Unordered Files
l Also called a heap or a pile file.
l New records are inserted at the end
of the file.
l To search for a record, a linear
search through the file records is
necessary. This requires reading and searching half the file blocks on the
average, and is hence quite expensive.
l Record insertion is quite efficient.
l Reading the records in order of a
particular field requires sorting the file records.
Ordered Files
l Also called a sequential file.
l File records are kept sorted by the
values of an ordering field.
l Insertion is expensive: records must
be inserted in the correct order. It is common to keep a separate
unordered overflow (or transaction
) file for new records to improve insertion efficiency; this is periodically
merged with the main ordered file.
l A binary search can be used to search for a record on its ordering
field value. This requires reading and searching log2 of the
file blocks on the average, an improvement over linear search.
l Reading the records in order of the
ordering field is quite efficient.
Figure 1 Ordered Files
Hashed files Hashing
for disk files is called External Hashing. To continue read about HASHING or
HASHED FILES visit my blogger site https://myrdbmsnotes.blogspot.com/2020/05/what-is-hashed-files-in-dbms-explain.html
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