1. Next:
Data Items
Records
Blocks
Files
Memory
CS 4432
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2. Goal : placing records into blocks
a file
records
assume fixed
length blocks
assume a single file (for now)
CS 4432
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3. Options for storing records in blocks:
(1) separating records
(2) spanned vs. unspanned
(3) mixed record types – clustering
(4) split records
(5) sequencing
(6) indirection
CS 4432
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4. (1) Separating records Block
(a) no need to separate if fixed size records.
(b) or, use special marker
(c) or, give record lengths (or offsets)
- within each record
- in block header R1 R2 R3
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5. (2) Spanned vs. Unspanned
Unspanned: records within one block
block block 2
...
Spanned : records wrap across 2 blocks
block block R1 R2 R3 R4 R5 R1 R2 R3
(a) R3
(b) R4 R5 R6 R7
(a)
CS 4432
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6. Unspanned is much simpler, but may waste space…
Spanned vs. unspanned:
Unspanned is much simpler, but may waste space…
Spanned essential if
record size > block size
CS 4432
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7. Example 106 records each of size 2,050 bytes (fixed)
block size = 4096 bytes
block block 2
2050 bytes wasted bytes wasted 2046 R1 R2
Utiliz = 50% -> ½ of space is wasted
CS 4432
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8. (3) Mixed versus uniform record types
Mixed - records of different types
(e.g., EMPLOYEE, DEPT)
allowed in same block
e.g., a block
EMP e1
DEPT d1
DEPT d2
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9. Why do we want to mix? Answer: CLUSTERING Records that are frequently
accessed together should be
placed into the same block
Problems
Creates variable length records in block
Aim to avoid duplicates (how to cluster?)
Insert/deletes are harder
CS 4432
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10. Example Clustering Q1: select C_NAME, C_CITY, AMOUNT, …
from DEPOSIT, CUSTOMER
where DEPOSIT.C_NAME =
CUSTOMER.C.NAME
a block
layout:
CUSTOMER,NAME=SMITH
DEPOSIT,NAME=SMITH
DEPOSIT,NAME=SMITH
CUSTOMER,NAME=JONES
Question: Good idea or bad idea ?
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11. But if instead Q2 frequent with : Q2: SELECT * FROM CUSTOMER
If Q1 frequent with join on customer and deposit relations, then clustering good
But if instead Q2 frequent with :
Q2: SELECT *
FROM CUSTOMER
then clustering is counter-productive
CS 4432
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12. Compromise: No mixing, but keep related records in same cylinder ...
CS 4432
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13. Recap: Storing records in blocks
(1) Separating records
(2) Spanned vs. Unspanned
(3) Mixed record types - Clustering
(4) Split records
(5) Sequencing
(6) Indirection
CS 4432
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14. Options for storing records in blocks:
(1) separating records
(2) spanned vs. unspanned
(3) mixed record types – clustering
(4) split records
(5) sequencing
(6) indirection
CS 4432
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15. (4) Split records Fixed part in one block Typically for hybrid format
Variable part in
another block
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16. R1 (a) R1 (b) R2 (a) R2 (b) R2 (c) Block with fixed recs.
Block with variable recs.
R1 (a)
R1 (b)
R2 (a)
R2 (b)
R2 (c)
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17. (5) Sequencing Ordering records in file (and block) by some key value
Sequential file ( - sequenced file)
Why sequencing ?
Typically to make it possible to efficiently read records in order
CS 4432
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18. Sequencing Options (a) Next record physically contiguous ...
(b) Linked
What about INSERT/ DELETE ?
Next (R1) R1 R1
Next (R1)
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19. Sequencing Options (c) Overflow area Records in sequence header R1
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20. (6) Indirection Addressing
How does one refer to records?
Problem: Records can be on disk or in (virtual) memory. Need common address, but have different physical locations. Rx
Many options:
Physical Indirect
CS 4432
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21. Purely Physical Addressing
Device ID
E.g., Record Cylinder #
Address = Track #
( ID ) Block #
Offset in block
Block ID
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22. Fully Indirect Addressing
Solution: Record ID (Oracle: ROWID) as global address, maintain a map table.
Map Table
rec ID
r address a
Rec ID
Physical
addr.
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23. What to do : Options inbetween ?
Tradeoff
Physical Indirect
Flexibility Cost
to move records of indirection
(for deletions, insertions) (lookup)
What to do : Options inbetween ?
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24. Ex #1 : Indirection in block
Block Header
A block: Free space R3 R4
R1 R2
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25. Ex. #2 Use logical block #’s. understood by file system
Ex. #2 Use logical block #’s understood by file system instead of direct disk access
REC ID File ID
Block #
Record # or Offset
File ID, Physical
Block # Block ID
File System
Map
CS 4432
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26. Recap: Storing records in blocks
(1) Separating records
(2) Spanned vs. Unspanned
(3) Mixed record types - Clustering
(4) Split records
(5) Sequencing
(6) Indirection
CS 4432
lecture #5