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Transcript of Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina,...
![Page 1: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/1.jpg)
Chapter 12 1
Chapter 12:Representing Data Elements
(Slides by Hector Garcia-Molina,http://www-db.stanford.edu/~hector/cs245/
notes.htm)
![Page 2: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/2.jpg)
Chapter 12 2
• How to lay out data on disk• Sec. 12.3 and 15.7 (self-study): How to move data to memory
Topics for today
![Page 3: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/3.jpg)
Chapter 12 3
Issues:
Flexibility Space Utilization
Complexity Performance
![Page 4: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/4.jpg)
Chapter 12 4
To evaluate a given strategy, compute following parameters:-> space used for expected data-> expected time to- fetch record given key- fetch record with next key- insert record- append record- delete record- update record- read all file- reorganize file
![Page 5: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/5.jpg)
Chapter 12 5
What are the data items we want to store?
• a salary• a name• a date• a pictureWhat we have available: Bytes
8bits
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Chapter 12 6
To represent:
• Integer (short): 2 bytese.g., 35 is
00000000 00100011
• Real, floating pointn bits for mantissa, m for exponent….
![Page 7: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/7.jpg)
Chapter 12 7
• Characters various coding schemes suggested,
most popular is ascii
To represent:
Example:A: 1000001a: 11000015: 0110101LF: 0001010
![Page 8: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/8.jpg)
Chapter 12 8
• Booleane.g., TRUE
FALSE
1111 1111
0000 0000
To represent:
• Application specifice.g., RED 1 GREEN 3
BLUE 2 YELLOW 4 …Can we use less than 1
byte/code?Yes, but only if desperate...
![Page 9: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/9.jpg)
Chapter 12 9
• Datese.g.: - Integer, # days since Jan 1, 1900
- 8 characters, YYYYMMDD - 7 characters, YYYYDDD
(not YYMMDD! Why?)• Time
e.g. - Integer, seconds since midnight - characters, HHMMSSFF
To represent:
![Page 10: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/10.jpg)
Chapter 12 10
• String of characters– Null terminated
e.g.,
– Length givene.g.,
- Fixed length
c ta
c ta3
To represent:
![Page 11: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/11.jpg)
Chapter 12 11
• Bag of bits
Length Bits
To represent:
![Page 12: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/12.jpg)
Chapter 12 12
Key Point
• Fixed length items
• Variable length items- usually length given at beginning
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Chapter 12 13
• Type of an item: Tells us how to interpret(plus size if
fixed)
Also
![Page 14: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/14.jpg)
Chapter 12 14
Data Items
Records
Blocks
Files
Memory
Overview
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Chapter 12 15
Record - Collection of related data
items (called FIELDS)E.g.: Employee record:
name field,salary field,date-of-hire field, ...
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Chapter 12 16
Types of records:
• Main choices:– FIXED vs VARIABLE FORMAT– FIXED vs VARIABLE LENGTH
![Page 17: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/17.jpg)
Chapter 12 17
A SCHEMA (not record) containsfollowing information
- # fields- type of each field- order in record- meaning of each field
Fixed format
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Chapter 12 18
Example: fixed format and length
Employee record(1) E#, 2 byte integer(2) E.name, 10 char. Schema(3) Dept, 2 byte code
55 s m i t h 02
83 j o n e s 01
Records
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Chapter 12 19
• Record itself contains format“Self Describing”
Variable format
![Page 20: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/20.jpg)
Chapter 12 20
Example: variable format and length
4I52 4S DROF46
Field name codes could also be strings, i.e. TAGS
# F
ield
s
Cod
e id
enti
fyin
g
field
as
E#
Inte
ger
typ
e
Cod
e f
or
En
am
eS
trin
g t
yp
eLe
ng
th o
f st
r.
![Page 21: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/21.jpg)
Chapter 12 21
Variable format useful for:
• “sparse” records• repeating fields• evolving formats
But may waste space...
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Chapter 12 22
• EXAMPLE: var format record with repeating fields
Employee can have children
3 E_name: Fred Child: SallyChild: Tom
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Chapter 12 23
Note: Repeating fields does not imply- variable format, nor- variable size
John Sailing Chess --
• Key is to allocate maximum number ofrepeating fields (if not used null)
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Chapter 12 24
Many variants betweenfixed - variable format:
Ex. #1: Include record type in record
record type record lengthtells me whatto expect(i.e. points to schema)
5 27 . . . .
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Chapter 12 25
Record header - data at beginning
that describes recordMay contain:
- record type- record length- time stamp- other stuff ...
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Chapter 12 26
Ex #2 of variant between FIXED/VAR format
• Hybrid format– one part is fixed, other variable
E.g.: All employees have E#, name, deptother fields vary.
25 Smith Toy 2 retiredHobby:chess
# of varfields
![Page 27: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/27.jpg)
Chapter 12 27
Other interesting issues:
• Compression– within record - e.g. code selection– collection of records - e.g. find
common patterns
• Encryption
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Chapter 12 28
Next: placing records into blocks
blocks ...
a file
assume fixedlength blocks
assume a single file (for now)
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Chapter 12 29
(1) separating records(2) spanned vs. unspanned(3) mixed record types – clustering(4) split records
Options for storing records in blocks:
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Chapter 12 30
Block
(a) no need to separate - fixed size recs.(b) special marker(c) give record lengths (or offsets)
- within each record- in block header
(1) Separating records
R2R1 R3
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Chapter 12 31
• Unspanned: records must be within one block
block 1 block 2
...
• Spannedblock 1 block 2
...
(2) Spanned vs. Unspanned
R1 R2
R1
R3 R4 R5
R2 R3(a)
R3(b) R6R5R4 R7
(a)
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Chapter 12 32
need indication need indication
of partial record of continuation
“pointer” to rest (+ from where?)
R1 R2 R3(a)
R3(b) R6R5R4 R7
(a)
With spanned records:
![Page 33: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/33.jpg)
Chapter 12 33
• Unspanned is much simpler, but may waste space…
• Spanned essential if record size > block size
Spanned vs. unspanned:
![Page 34: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/34.jpg)
Chapter 12 34
Example
106 recordseach of size 2,050 bytes (fixed)block size = 4096 bytes
block 1 block 2
2050 bytes wasted 2046 2050 bytes wasted 2046
R1 R2
• Total wasted = 2 x 109 Utiliz = 50%• Total space = 4 x 109
![Page 35: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/35.jpg)
Chapter 12 35
• Mixed - records of different types(e.g. EMPLOYEE, DEPT)allowed in same block
e.g., a block
(3) Mixed record types
EMP e1 DEPT d1 DEPT d2
![Page 36: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/36.jpg)
Chapter 12 36
Why do we want to mix?Answer: CLUSTERING
Records that are frequently accessed together should bein the same block
![Page 37: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/37.jpg)
Chapter 12 37
Compromise:
No mixing, but keep relatedrecords in same cylinder ...
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Chapter 12 38
Example
Q1: select A#, C_NAME, C_CITY, …from DEPOSIT, CUSTOMERwhere DEPOSIT.C_NAME =
CUSTOMER.C.NAME
a blockCUSTOMER,NAME=SMITH
DEPOSIT,NAME=SMITH
DEPOSIT,NAME=SMITH
![Page 39: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/39.jpg)
Chapter 12 39
• If Q1 frequent, clustering good• But if Q2 frequent
Q2: SELECT * FROM CUSTOMER
CLUSTERING IS COUNTER PRODUCTIVE
![Page 40: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/40.jpg)
Chapter 12 40
Fixed part in one block
Typically forhybrid format
Variable part in another block
(4) Split records
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Chapter 12 41
Block with fixed recs.
R1 (a)R1 (b)
Block with variable recs.
R2 (a)
R2 (b)
R2 (c)
This block alsohas fixed recs.
![Page 42: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/42.jpg)
Chapter 12 42
Question
What is difference between- Split records- Simply using two different
record types?
![Page 43: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/43.jpg)
Chapter 12 43
(1) Separating records(2) Spanned vs. Unspanned(3) Mixed record types - Clustering(4) Split records
Options for storing records in blocks
![Page 44: Chapter 121 Chapter 12: Representing Data Elements (Slides by Hector Garcia-Molina, hector/cs245/notes.htm)](https://reader035.fdocuments.in/reader035/viewer/2022062408/56649ed35503460f94be33f6/html5/thumbnails/44.jpg)
Chapter 12 44
(1) Insertion/Deletion(2) Buffer Management(3) Comparison of Schemes
Other Topics
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Chapter 12 45
Block
Deletion
Rx
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Chapter 12 46
Options:
(a) Immediately reclaim space(b) Mark deleted
– May need chain of deleted records
(for re-use)– Need a way to mark:
• special characters
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Chapter 12 47
As usual, many tradeoffs...
• How expensive is to move valid record to free space for immediate reclaim?
• How much space is wasted?– e.g., deleted records, free space
chains,...
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Chapter 12 48
Dangling pointers
Concern with deletions
R1 ?
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Chapter 12 49
Solution #1: Do not worry
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Chapter 12 50
E.g., Leave “MARK” in map or old location
Solution #2: Tombstones
• Physical IDs
A block
This space This space cannever re-used be re-used
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Chapter 12 51
• Logical IDs
ID LOC
7788
map
Never reuseID 7788 nor
space in map...
E.g., Leave “MARK” in map or old location
Solution #2: Tombstones
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Chapter 12 52
Easy case: records not in sequence Insert new record at end of
file or in deleted slot If records are variable size,
not as easy...
Insert
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Chapter 12 53
Hard case: records in sequence If free space “close by”, not too bad... Or use overflow idea...
Insert
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Chapter 12 54
Interesting problems:
• How much free space to leave in each block, track, cylinder?
• How often do I reorganize file + overflow?
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Chapter 12 55
• There are 10,000,000 ways to organize my data on disk…
Which is right for me?
Comparison
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Chapter 12 56
Issues:
Flexibility Space Utilization
Complexity Performance
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Chapter 12 57
To evaluate a given strategy, compute following parameters:-> space used for expected data-> expected time to- fetch record given key- fetch record with next key- insert record- append record- delete record- update record- read all file- reorganize file
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Chapter 12 58
Example
How would you design Megatron 3000 storage system? (for a relational DB, low end)– Variable length records?– Spanned?– What data types?– Fixed format?– How to handle deletions?
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Chapter 12 59
• How to lay out data on disk
Data Items
Records
Blocks
Files
Memory
DBMS
Summary
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Chapter 12 60
How to find a record quickly,given a key
Next