Data Representation in Database Systems: Fields, Records, Blocks, and Variable-Length Data, Slides of Database Management Systems (DBMS)

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Representing Data Elements

Fields, Records, Blocks Variable-length Data Modifying Records

Overview

• Attributes are represented by sequences of bytes, called fields
• Tuples are represented by collections of fields, called records
• Relations are represented by collections of records, called files
• Files are stored in blocks, using specialized data structures to support efficient modification and querying

Representing Tuples

• For now, assume all attributes (fields) are fixed length.
• Concatenate the fields
• Store the offset of each field in schema

4

0 30 286 287 297

name CHAR(30) 30 bytes

address VARCHAR(255) 256 bytes

gender CHAR(1) 1 byte

birthdate DATE 10 bytes

More on Tuples

• Due to hardware considerations, certain types of data need to start at addresses that are multiples of 4 or 8
• Previous example becomes:

5

0 32 288 292 304

name CHAR(30) 30 bytes

• 2

address VARCHAR(255) 256 bytes

gender CHAR(1) 1 byte

• 3

birthdate DATE 10 bytes

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Packing Records into Blocks

• Start with block header:
  • timestamp of last modification/access
  • offset of each record in the block, etc.
• Follow with sequence of records
• May end with some unused space

7

header record 1 record 2 (^) … record n-1 record n

Representing Addresses

• Often addresses (pointers) are part of records:
  • the application data in object-oriented databases
  • as part of indexes and other data structures supporting the DBMS
• Every data item (block, record, etc.) has two addresses: - database address: address on the disk (typically 8-16 bytes) - memory address, if the item is in memory (typically 4 bytes)

Pointer Swizzling

• When a block is moved from disk into main memory, change all the disk addresses that point to items in this block into main memory addresses.
• Need a bit for each address to indicate if it is a disk address or a memory address.
• Why? Faster to follow memory pointers (only uses a single machine instruction).

Example of Swizzling

11

Block 1

Block 2

Disk Main Memory

read into main memory (^) swizzled

unswizzled

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Automatic Swizzling

• Locating all pointers within a block:
  • refer to the schema, which will indicate where addresses are in the records
  • for index structures, pointers are at known locations
• Update translation table with memory addresses of items in the block
• Update pointers in the block (in memory) with memory addresses, when possible, as obtained from translation table

Unswizzling

• When a block is moved from memory back to disk, all pointers must go back to database (disk) addresses
• Use translation table again
• Important to have an efficient data structure for the translation table

Unpinning a Block

• Consider each item in the block to be unpinned
• Keep in the translation table the places in memory holding swizzled pointers to that item (e.g., with a linked list)
• Unswizzle those pointers: use translation table to replace the memory addresses with database (disk) addresses

Variable Length Data

• Data items with varying size (e.g., if maximum size of a field is large but most of the time the values are small)
• Variable-format records (e.g., NULLs method for representing a hierarchy of entity sets as relations)
• Records that do not fit in a block (e.g., an MPEG of a movie)

Variable Length Fields

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other header info

record length

to var len field 2

var len field 2

var len field 3

fixed len field 2

var len field 1

fixed len field 1

to var len field 3

Variable-Format Records

• Represent by a sequence of tagged fields
• Each tagged field contains
  • name
  • type
  • length, if not deducible from the type
  • value