1. Variable Length Data and Records

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

3. Example of a record
name
address
gender
birth date

4. Records with Variable Length Fields
Simple and Effective way to represent variable length records is as follows –
1. Fixed length fields are kept ahead of the variable length records.
2. A header is put in front of the of the record.
3. Record header contains
Length of the record
Pointers to the beginning of all variable length fields except the first one.

5. Example Record with name and address as variable length field.
header information
record length
to address
gender
birth date
name
address
Record with name and address as variable length field.

6. Records with repeating fields
Repeating fields simply means fields of the same length L.
All occurrences of Field F grouped together.
Pointer pointing to the first field F is put in the header.
Based on the length L the starting offset of any repeating field can be obtained.

7. Example of a record with Repeating Fields
other header information
record length
to address
to movie pointers
name
address
Movie star record with “movies” as the repeating field.
pointers to movies

8. Alternative representation
Record is of fixed length
Variable length fields stored on a separate block.
The record itself keeps track of -
1. Pointers to the place where each repeating field begins, and
2. Either how many repetitions there are, or where the repetitions end.

9. Storing variable length fields separately from the record.

10. Variable Format Records
Records that do not have fixed schema
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

11. 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

12. Records that do not fit in a block
When the length of a record is greater than block size ,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 a SPANNED RECORD
Record that do not cross a block boundary is called UNSPANNED RECORD

13. 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

14. Spanned Records block header record header record 2 - b record 2 - a