1. Variable Length Data and Records
Dhivyakrishnan
CS 257
ID: 105

2. Agenda Records with Variable Length Fields
Records with Repeating Fields
Variable Format Records
Records that do not fit in a block
BLOBS

3. Example Fig 1 : Movie star record with four fields name address gender
birth date
Fig 1 : Movie star record with four fields

4. Records with Variable Fields
An effective way to represent variable length records is as follows
Fixed length fields are stored before the variable length fields
Record header contains
Length of the record
Pointers to the beginning of all variable
length fields except the first one.

5. Records with Variable Length Fields
header information
record length
to address
gender
birth date
name
address
Figure 2 : A Movie Star record with name and address implemented as variable length character strings

6. Records with Repeating Fields
Records contains variable number of occurrences of a field F
All occurrences of field F are grouped together and the record
header contains a pointer to the first occurrence of field F
The repeating field F is of fixed length L bytes
Locating an occurrence of field F within the record
Add offset for the field F which are the integer multiples of L starting with 0 , L ,2L,3L and so on to locate

7. Records with Repeating Fields
other header information
record length
to address
to movie pointers
name
address
pointers to movies
Figure 3 : A record with a repeating group of references to movies

8. Records with Repeating Fields
record header information
to name
length of name
to address
length of address
to movie references
number of references
address
name
Figure 4 : Storing variable-length fields separately from the record

9. Records with Repeating Fields
Advantage
Keeping the record itself fixed length allows record to be searched more efficiently, minimizes the overhead in the block headers, and allows records to be moved within or among the blocks with minimum effort.
Disadvantage
Storing variable length components on another block increases the number of disk I/O’s needed to examine all components of a record.

10. Records with Repeating Fields
A compromise strategy is to allocate a fixed portion of the record for the repeating fields
If the number of repeating fields is lesser than
allocated space, then there will be some unused space
If the number of repeating fields is greater than
allocated space, then extra fields are stored in a
different location and
Pointer to that location and count of additional
occurrences is stored in the record

11. Variable Format Records
Records that do not have fixed schema
Variable format records are represented by sequence of
tagged fields
Each of the tagged fields consist of information
Attribute or field name
Type of the field
Length of the field
Value of the field
Why use tagged fields
Information – Integration applications
Records with a very flexible schema

12. Variable Format Records
code for name
code for restaurant owned
code for string type
code for string type
length
length N S 14
Clint Eastwood R S 16
Hog’s Breath Inn
Fig 5 : A record with tagged fields

13. Records that do not fit in a block
When the length of a record is greater than block size ,then
then record is divided and placed into two or more blocks
Portion of the record in each block is referred to as a
RECORD FRAGMENT
Record with two or more fragments is called
SPANNED RECORD
Record that do not cross a block boundary is called
UNSPANNED RECORD

14. Spanned Records
Spanned records require the following extra header information
A bit indicates whether it is fragment or not
A bit indicates whether it is first or last fragment of
a record
Pointers to the next or previous fragment for the
same record

15. Records that do not fit in a block
block header
record header
record
2 - b
record
2 - a
record 1
record 3
block 1
block 2
Figure 6 : Storing spanned records across blocks

16. BLOBS Large binary objects are called BLOBS
e.g. : audio files, video files
Storage of BLOBS
Retrieval of BLOBS

17. END