Distributed Database Management Systems: Lecture 34

Distributed Database Management Systems: Lecture 34. The main topics covered in this chapter include: query optimization; centralized QO; best access path; join processing; QO in distributed environment; single relation queries: executed according to the best access path;...

Distributed Database Management Systems

Lecture 34

In the previous lecture

• Concluded Data Localization

• Query Optimization

– Components: Search space, cost

model, search strategy

– SS consists of eq query trees

– SSts could be static, dynamic or

randomized

– Cost model sees response and total

times…

–Database statistics are used to

evaluate size of iterm Tables

–Selectivity factor, card, size are

some major figures

• Queries involving Joins

in three steps

1 Determine the possible

ordering of

joins-2 Determine the cost of

each ordering

3 Choose the join ordering

with minimal cost

• Cost model assigns

statistics-• Two major steps in

Optimization Algorithm

– Best access path for

individual relation with pred

– The best join ordering; two

possibilities.

• Example: Select eName

From EMP, ASG, PROJ Where

EMP.eNo = ASG.eNo & PROJ.pNo = ASG.pNo & pName = ‘CAD/CAM’

• EMP has an index on eNo

• ASG has an index on pNo

• PROJ has an index on

pNo and an index on

1- Choose the best access paths to each relation

• EMP: sequential scan (no

2- Determine the best join ordering

–Total 3! orderings are

possible

–Rather than computing for

all, some of them are pruned

–Shown in the tree, next page

(ASG EMP ⋈ ) PROJ ⋈ (PROJ⋈ASG)⋈E

MP

Join Ordering in

Fragmented Queries

Join Ordering

• Two relations: move the

smaller relation to the

• More than 2 relations

– Calculate all possible

costs

– Requires to compute size

of intermediate tables

– Difficult! Lets see why

• Strategy 1:

EMPsite2, site2 computes EMP’= EMP ⋈ ASGsite3 computes EMP’ ⋈ PROJ

• Strategy 2:

ASGsite1, site1 computes EMP’= EMP ⋈ ASGsite3 computes EMP’ ⋈ PROJ

• Strategy 3:

ASGsite3, site3 computes ASG’= PROJ ⋈ ASGsite1 computes EMP ⋈ ASG’

• Strategy 4:

PROJsite2, site2 computes PROJ’= PROJ ⋈ ASGsite1 computes EMP ⋈ PROJ’

• Strategy 5:

EMP, PROJsite2, site2

computes PROJ ⋈ ASG ⋈

Which one to Choose

• We need to know

–Size of operand tables

–Estimate interm tables’ size

• Computing all possibilities

could be lengthy

• Heuristic: Consider only

the size of

tables-Thanks