CS 4432 lecture #5 1

Data Items

Records

Blocks

Files

Memory

Next:

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Goal : placing records into blocks

blocks ...

a file

assume fixedlength blocks

assume a single file (for now)

records

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(1) separating records(2) spanned vs. unspanned(3) mixed record types – clustering(4) split records(5) sequencing(6) indirection

Options for storing records in blocks:

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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

(1) Separating records

R2R1 R3

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• Unspanned: records within one blockblock 1 block 2

...

• Spanned : records wrap across 2 blocks

block 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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• Unspanned is much simpler, but may sometimes waste space…

• Spanned essential if record size > block size

Spanned vs. unspanned:

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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

• Utiliz = 50% -> ½ of space is wasted

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• Mixed - records of different types(e.g., EMPLOYEE, DEPT)allowed in same block

e.g., a block

(3) Mixed versus uniform record types

EMP e1 DEPT d1 DEPT d2

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Why do we want to mix?

• Answer: CLUSTERING

Records that are frequently accessed together should beplaced into the same block

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Example Clustering

Q1: select C_NAME, C_CITY, AMOUNT, …from DEPOSIT, CUSTOMERwhere DEPOSIT.C_NAME =

CUSTOMER.C.NAME

a blocklayout:

CUSTOMER,NAME=SMITH

DEPOSIT,NAME=SMITH

DEPOSIT,NAME=SMITH

CUSTOMER,NAME=JONES

Question: Good idea or bad idea ?

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• If Q1 frequent with join on customer and deposit relations, then clustering good

• But if instead Q2 frequent with :Q2: SELECT *

FROM CUSTOMERthen clustering is counter-productive

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Mixing of record types on one block?

Problems:

• Creates variable length records in block

• May cause us to store duplicates

• Insert/deletes are harder

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Other Ideas ?

Compromise: No mixing, but keep related

records in same cylinder ...

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(1) Separating records(2) Spanned vs. Unspanned(3) Mixed record types - Clustering(4) Split records(5) Sequencing(6) Indirection

Recap: Storing records in blocks

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(1) separating records(2) spanned vs. unspanned(3) mixed record types – clustering(4) split records(5) sequencing(6) indirection

Options for storing records in blocks:

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Fixed part in one block

Typically forhybrid format

Variable part in another block

(4) Split records

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Block with fixed recs.

R1 (a)R1 (b)

Block with variable recs.

R2 (a)

R2 (b)

R2 (c)

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• Ordering records in file (and thus blocks) by some key value– Sequential file ( sequenced file )

• Why sequencing ?– Typically maked it possible to efficiently read

records in order

(5) Sequencing

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Sequencing Options

(a) Next record physically contiguous

...

(b) Linked

What about INSERT/ DELETE ?

Next (R1)R1

R1 Next (R1)

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(c) Overflow area

Recordsin sequence

R1

R2

R3

R4

R5

Sequencing Options

header

R2.1

R1.3

R4.7

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• How does one refer to records?

• Problem: Records can be on disk or in (virtual) memory. Need common address, but have different physical locations.

(6) Indirection Addressing

Rx

Many options: Physical Indirect

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Purely Physical Addressing

Device ID E.g., Record Cylinder #

Address = Track #( ID ) Block #

Offset in block

Block ID

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Fully Indirect Addressing

Solution: Record ID (Oracle: ROWID) as global address, maintain a map table.

Map Tablerec ID r address

a

Physicaladdr.Rec ID

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Tradeoff

Flexibility Costto move records of indirection(for deletions, insertions) (lookup)

What to do : Options inbetween ?

Physical Indirect

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Ex #1 : Indirection in block

Block Header

A block: Free

space

R3

R4

R1 R2

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Ex. #2 Use logical block #’s understood by file system

instead of direct disk accessREC ID File ID

Block # Record # or Offset

File ID, PhysicalBlock # Block ID

File System Map

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(1) Separating records(2) Spanned vs. Unspanned(3) Mixed record types - Clustering(4) Split records(5) Sequencing(6) Indirection

Recap: Storing records in blocks