Transcript CS257_13.7_207_Madhuri

Madhuri Gollu
Id: 207
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Records with Variable Length Fields
Records with Repeating Fields
Variable Format Records
Records that do not fit in a block
BLOBs
Column Stores
name
0
address
30
gender
286
287
birth date
297
Fig 1 : Movie star record with four fields
An effective way to represent variable length
records is as follows
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Fixed length fields are Kept ahead of the
variable length fields
 Record
header contains
• Length of the record
• Pointers to the beginning of all variable
length fields except the first one.
Records with Variable Length
Fields
header information
record length
to address
gender
birth date
name
address
Fig 2 : A Movie Star record with name and address
implemented as variable length character strings
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Records contains variable number of occurrences of a
field F, but the field itself is of fixed length.
All occurrences of field F are grouped together and the
record header contains a pointer to the first occurrence
of field F
L bytes are devoted to one instance of field F
Locating an occurrence of field F within the record
• Add to the offset for the field F which are the integer
multiples of L starting with 0 , L ,2L,3L and so on to
locate
• We stop whenever we reach the offset of the field F or
end of the record.
Records with Repeating Fields
other header information
record length
to address
to movie pointers
name
address
pointers to movies
Fig3 : A record with a repeating group of references to movies
Records with Repeating Fields
record header
information
to name
length of name
to address
length of address
to movie references
number of references
Record
address
Additional space
name
Fig4 : Storing variable-length fields separately from the record
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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.
A compromise strategy is to allocate a fixed-length
portion of the record enough space for repeating fields
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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
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Records that do not have fixed schema
Variable format records are represented by
sequence of tagged fields, which has value of field
preceded by information of this field
Each of the tagged fields consist of information
• Attribute or field name
• Type of the field
• Length of the field
Variable Format Records
code for name
code for restaurant owned
code for string type
code for string type
length
N
S
14
Clint Eastwood
length
R
S
16
Hog’s Breath Inn
Fig 5 : A record with tagged fields
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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
SPANNED RECORD
Record that do not cross a block boundary is called
UNSPANNED RECORD
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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
Records that do not fit in a block
block header
record header
record 1
block 1
record
2-a
record
2-b
record 3
block 2
Fig 6 : Storing spanned records across blocks
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Binary Large Objects are called BLOBs
Example : audio files, video files
Storage of BLOBs
It must be stored as sequence of blocks
Retrieval of BLOBs
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Store each column as a record.
A relation may occupy more than a single
block
We keep the values in each column in the
same order , then we can reconstruct the
relation from the column records.