Madhuri Gollu Id: 207

      Records with Variable Length Fields Records with Repeating Fields Variable Format Records Records that do not fit in a block BLOBs Column Stores

0 name 30 address gender birth date 286 287 297 Fig 1 : Movie star record with four fields

An effective way to represent variable length records is as follows  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

 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 name Additional space 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.

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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  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

 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 string type length code for restaurant owned code for string type length N S 14 Clint Eastwood R S 16 Hog’s Breath Inn Fig 5 : A record with tagged fields

 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

 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 record 2 - a record 2 - b record 3 block 1 block 2 Fig 6 : Storing spanned records across blocks

 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

 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.