1. Physical DataBase Design

2. Conceptual design->logical design->physical design
(ER diagram->relation database->physical design)

3. Physical Database Design
Purpose –of this design is to translate the logical description of data into the technical specifications for storing and retrieving data
Goal – of this phase is to create a design for storing data that will provide adequate performance and ensure database integrity, security and recoverability

4. Physical Design Information
Information needed for physical file and database design includes:
Normalized relations plus size estimates for them
Definitions of each attribute
Descriptions of where and when data are used
entered, retrieved, deleted, updated, and how often
Expectations and requirements for response time, and data security, backup, recovery, retention and integrity
Descriptions of the technologies used to implement the database

5. Physical Design Decisions
During this phase, the decisions are taken on the
Storage Format
Physical record composition
Data arrangement
Indexes
Query optimization and performance tuning

6. Physical Design Decisions
During this phase, the decisions are taken on the
Create base relations
Name
Attributes
Primary key
Foreign key
Alternative key
Indexes
Implement integrity rules
Domain
Enterprise
Referential (no action, cascade, set null, set default, and no check for deleting and updating)
Entity

7. Storage Format
Choosing the storage format of each field (attribute). The DBMS provides some set of data types that can be used for the physical storage of fields in the database
Data Type (format) is chosen to minimize storage space and maximize data integrity

8. Objectives of data type selection
Minimize storage space
Represent all possible values
Improve data integrity
Support all data manipulations
The correct data type should, in minimal space, represent every possible value (but eliminate illegal values) for the associated attribute and can support the required data manipulations (e.g. numerical or string operations)

9. Choosing Data Types CHAR – fixed-length character
VARCHAR – variable-length character (memo)
LONG – large number
NUMBER – positive/negative number
DATE – actual date
BLOB – binary large object (good for graphics, sound clips, etc.)

10. Designing Physical Records
A physical record is a group of fields stored in adjacent memory locations and retrieved together as a unit
The records can be either Fixed Length and variable length

11. Data Storage
The data is stored on memories. The memories can be classified as
Cache memory
Primary memory
(Secondary memory)Disk
Tape

12. The Memory Hierarchy Main Memory = Disk Cache Processor Cache:
access time 10 nano’s
storage capacity 512K
Volatile
storage capacity 256Mb-1Gb
Access time:
nanoseconds
Disk
Tape
Persistent
storage capacity GB storage
speed:
Access time=
10-15 msecs.
1.5 MB/S transfer rate
280 GB typical
capacity
Only sequential access

13. Main Memory Fastest, most expensive (excluding cache)
Today: 512MB are common even on PCs
Many databases could fit in memory
New industry trend: Main Memory Database
E.g TimesTen
Main issue is volatility

14. Secondary Storage Secondary Storage is Disks
They are Slower, cheaper than main memory
It is non volatile in nature, i.e. the data is permanently stored.
The unit of disk I/O = block
Typically 1 block = 4k
A disk block is also called a disk page or simply a page
Blocking factor (bfr) for a file is the average number of records stored in a disk block.

15. The Mechanics of Disk Mechanical characteristics:
Cylinder
Mechanical characteristics:
Rotation speed (5400RPM)
Number of platters (1-30)
Number of tracks (<=10000)
Number of sectors (256/track)
Number of bytes / sector (29=512)
Block size (212=4096)
Spindle
Tracks
Disk head
Sector
Arm movement
Platters
Arm assembly

16. Important Disk Access Characteristics
Block access time = Disk latency + transfer time
Disk latency = seek time + rotational latency
Seek time = time for the head to reach the right track
10ms – 40ms
Rotational latency = rotation time to get to the right sector
Time for one rotation = 10ms
Average rotation latency = 10ms
Transfer time is typically 5-10MB/s
Disks read/write one block at a time (typically 4kB)

17. Representing Data Elements
Relational database elements:
CREATE TABLE Product (
pid INT PRIMARY KEY,
name CHAR(20),
description VARCHAR(200),
maker CHAR(10) REFERENCES Company(name))
A tuple is represented as a record

18. Record Formats: Fixed Length
Base address (B)
Address = B+L1+L2
All fields in the record are fixed in length, so the length of the record is fixed. So all records are equal in length 9

19. Record Header To schema length F1 F2 F3 F4 L1 L2 L3 L4 header
timestamp
Need the header because:
The schema may change
for a while new+old may coexist
Records from different relations may coexist 9

20. Variable Length Records
Other header information
header F1 F2 F3 F4 L1 L2 L3 L4
length
Place the fixed fields first: F1, F2
Then the variable length fields: F3, F4 9

21. Records With Referencing Fields
Other header information
header F1 F2 F3 L1 L2 L3
length
E.g. to represent one-many or many-many relationships 9

22. Storing Records in Blocks
Blocks have fixed size (typically 4k)
BLOCK R4 R3 R2 R1

23. Spanning Records Across Blocks
header
block
header R1 R2 R3 R2

24. BLOB Binary large objects Supported by modern database systems
E.g. images, sounds, etc.
Storage: attempt to cluster blocks together

25. Modifications: Insertion
File is unsorted
add it to the end
File is sorted:
Is there space in the right block ?
Yes: we are lucky, store it there
Is there space in a neighboring block ?
Look 1-2 blocks to the left/right, shift records
If anything else fails, create overflow block

26. Overflow Blocks
Blockn-1
Blockn
Blockn+1
Overflow
After a while the file starts being dominated by overflow blocks: time to reorganize

27. Modifications: Deletions
Free space in block, shift records
Maybe be able to eliminate an overflow block

28. Modifications: Updates
If new record is shorter than previous, easy 
If it is longer, need to shift records, create overflow blocks

29. Physical Addresses
Each block and each record have a physical address that consists of:
The disk
The cylinder number
The track number
The block within the track
For records: an offset in the block

30. Logical Addresses Logical address: a string of bytes (10-16)
More flexible: can blocks/records around
But need translation table:
Logical address
Physical address L1 P1 L2 P2 L3 P3

31. Main Memory Address
When the block is read in main memory, it receives a main memory address
Buffer manager has another translation table
Memory address
Logical address M1 L1 M2 L2 M3 L3

32. Physical Design
Interface 1: User request to the DBMS. The user presents a query, the DBMS determines which physical DBs are needed to resolve the query
Interface 2: The DBMS uses an internal model access method to access the data stored in a logical database.
Interface 3: The internal model access methods and OS access methods access the physical records of the database.

33. Physical File Design
A Physical file is a portion of secondary storage (disk space) allocated for the purpose of storing physical records
Pointers - a field of data that can be used to locate a related field or record of data
Access Methods - An operating system algorithm for storing and locating data in secondary storage
Pages - The amount of data read or written in one disk input or output operation

34. Internal Model Access Methods
Many types of access methods:
Physical Sequential
Indexed Sequential
Indexed Random
Direct
Hashed
Differences in
Access Efficiency
Storage Efficiency

35. Physical Sequential
Key values of the physical records are in logical sequence
Main use is for “dump” and “restore”
Access method may be used for storage as well as retrieval
Storage Efficiency is near 100%
Access Efficiency is poor (unless fixed size physical records)

36. Sequential File Organization
A sequential file is one in which the records are stored in sorted order of one or more key fields.

37. Sequential File Organization
Sequential access means that data is accessed in a ordered sequence.
Sequential access is sometimes the only way of accessing the data, for example tape.
Records are usually
stored on tape and
processed one after
the other

38. Sequential file

39. Advantages Simple file design
Very efficient when most of the records must be processed e.g. Payroll
Very efficient if the data has a natural order
Can be stored on inexpensive devices like magnetic tape.

40. Disadvantages
Entire file must be processed even if a single record is to be searched.
Transactions have to be sorted before processing
Overall processing is slow, because you have to go through each record until you get to the one you want!

41. Sequential File Organization
A collection of records
Stored in key sequence
Adding/deleting record requires making new file (so that the sequence is maintained)
Used as master files

42. Indexed Sequential
Key values of the physical records are in logical sequence
Access method may be used for storage and retrieval
Index of key values is maintained with entries for the highest key values per block(s)
Access Efficiency depends on the levels of index, storage allocated for index, number of database records, and amount of overflow
Storage Efficiency depends on size of index and volatility of database

43. Indexed sequential file
Each record of a file has a key field which uniquely identifies that record.
An index consists of keys and addresses, just like an index in a book:
The pages in a book are stored sequentially, so you can read through it page by page OR
You can look up the page you want
in the index and flick straight to it

44. Indexed sequential file
An indexed sequential file is a sequential file (i.e. sorted into order of a key field) which has an index.
A full index to a file is one in which there is an entry for every record.
Because each record has an index, we can access individual records directly, without having to scroll through all the other records first.

45. Indexed sequential file
Indexed sequential files are important for applications where data needs to be accessed.....
sequentially , one record after another OR
randomly using the index.

46. An example of an Indexed Sequential file
A company may store details about its employees as an indexed sequential file. Sometimes the file is accessed....
sequentially. For example when the whole of the file is processed to produce pay slips at the end of the month.

47. An example of an Indexed Sequential file
Sometimes the file is accessed....
randomly. Maybe an employee changes address, or a female employee gets married and changes her surname.

48. Indexed sequential file
An indexed sequential file can only be stored on a random access device e.g. magnetic disc or CD.
This is because we need a device that will allow us direct access
to random files, rather
than the sequential
access that
magnetic tape allows.

49. Advantages
Provides flexibility for users who need both type of access with the same file
Faster than
sequential

50. Disadvantages
Extra storage space for the index is required, just like in a book: your text book would be 372 pages without the index (go on, check!) but is 380 pages with the index.

51. Index Sequential Data File Block 1 Adams Becker Block 2 Block 3 Getta
Address
Block
Number 1 2 3 …
Actual
Value
Dumpling
Harty
Texaci
...
Adams
Becker
Getta
Mobile
Sunoci

52. Indexed Sequential: Two Levels
Address 7 8 9 …
Key
Value
385
678
805
001
003 .
150
705
710
785
251
455
480
536
605
610
791 1 2 3 4 5 6

53. Indexed Random
Key values of the physical records are not necessarily in logical sequence
Index may be stored and accessed with Indexed Sequential Access Method
Index has an entry for every data base record. These are in ascending order. The index keys are in logical sequence. Database records are not necessarily in ascending sequence.
Access method may be used for storage and retrieval

54. Indexed Random Address Block Number 2 1 3 Actual Value Adams Becker
Dumpling
Getta
Harty

55. Btree F | | P | | Z | R | | S | | Z | H | | L | | P | B | | D | | F |
Devils
Aces
Boilers
Cars
Minors
Panthers
Seminoles
Flyers
Hawkeyes
Hoosiers

56. Direct (Random) File Organization
Records are read directly from or written on to the file.
The records are stored at known address.
The address is calculated by applying a mathematical function to
the key field.

57. Direct
Key values of the physical records are not necessarily in logical sequence
There is a one-to-one correspondence between a record key and the physical address of the record
May be used for storage and retrieval
No duplicate keys permitted

58. Hashing
A bucket is a unit of storage containing one or more records (a bucket is typically a disk block).
In a hash file organization we obtain the bucket of a record directly from its search-key value using a hash function.
Hash function h is a function from the set of all search-key values K to the set of all bucket addresses B.

59. Hashing Organization
Hash function is used to locate records for access, insertion as well as deletion.
Records with different search-key values may be mapped to the same bucket; thus entire bucket has to be searched sequentially to locate a record.

60. EXAMPLE 2 records/bucket1 2 3
INSERT:
h(a) = 1
h(b) = 2
h(c) = 1
h(d) = 0 d a c b e
h(e) = 1

61. EXAMPLE: deletion Delete: e f c a 1 2 b d 3 c d e f g maybe move1 2 3 a d b d c c e
maybe move
“g” up f g