Distributed Database Management Systems: Lecture 32

Distributed Database Management Systems: Lecture 32. The main topics covered in this chapter include: final phase of QD; next phase of query optimization: data localization; idempotency of unary Ops; commuting selection with projection;...

Distributed Database Management Systems

Lecture 32

In the previous lecture

• Query Processing

• Query Decomposition

• Its Different Phases.

3- Idempotency of unary Ops

• Many equivalence query

trees can be generated

• Comparing all such trees

to select best is not

feasible

• Heuristic is applied

1 Separation of Unary Ops

2 Unary ops on the same

relation grouped together

3 Unary ops commuted with

binary ops

4 Binary ops are ordered

ASN PROJ EMP

x

⋈ pNo^eNo

(pName = ‘CAD/CAM’)^ ( dur = 12 v dur = 24)^ eName ’Saleem’

eName

PROJ ASG EMP

pName = ‘CAD/CAM’ dur=12 v dur = 24 eName != ‘Saleem’

pNo, eName eName

• This concludes Query

• Now we move to the

second phase of Query Optimization; Data

Localization of DD

phase transform into

local ones (fragments)

Reduction During Data

Localization

Example Schema

• EMP(eNo, eName, title)

• Horizontal Fragmentation

• EMP1 = eNo ≤ ‘E3’ (EMP)

• EMP2 = ’E3’<eNo ≤ ‘E6’ (EMP)

• EMP3 = eNo > ‘E6’ (EMP)

• Reduction with Selection

• Rule 1:

pi (Rj) = Ø if x in R ∀ j: (pi(x) ^ pj(x))

• That is, there exist

conflicting predicates

• Select * from EMP where

Reduction on Join

• Distributing joins over

unions and avoiding

unnecessary joins

• (R1UR2) ⋈ R3= (R1 ⋈

R3) U (R2 ⋈ R3)

• Rule2:

• Ri⋈ Rj = Ø if x in R ∀ iand y in R ∀ j: (pi(x) ^

pj(x))

• Useless joins can be

determined viewing the join predicates

• Remember! Reduced query

is not always better We

have to be

watchful-• Parallel Execution

• ASG1 = eNo ≤ ‘E3’ (ASG)ASG1 =

• ASG2 = ’eNo > ‘E3’ (ASG).ASG2 =

• Select eName

From EMP, ASG

Where EMP.eNo = ASG eNo.

• We already know about

• EMP1 = eNo, eName (EMP)

• EMP2 = eNo, title (EMP)

Rule3: D,K(Ri) is

useless if the set of

projection attributes D

is not in A‘.

• Example

• ASG1: ASG ⋉ ENO EMP1

• ASG2: ASG ⋉ ENO EMP2

• Query

• SELECT * FROM

EMP, ASG WHERE

ASG.eNo = EMP.eNo AND EMP.title =

"Mech Eng."

⋈eNo

U

title = ‘Mech Eng.’

Pushing Selection Down

⋈eNo

⋈eNo

U

title = ‘Mech Eng.’

Union Moved Up

title = ‘Mech Eng.’

⋈eNo

title = ‘Mech Eng.’

Optimal Reduced Query

⋈eNo

Thanks