1. CS186 Final Review
Query Optimization

2. Overview Query  Relational Algebra  Tree What is a query plan? ?
Implementation choices at each operator
Order of operators
What is a query plan?
Tree of R.A. ops (and some others) with choice of algorithm for each op.
Reserves
Sailors
sid=sid
bid=100
rating > 5
sname ?

3. Join Joins (Chapter 14.4) (Page-oriented) Nested loops join
Blocked nested loops:
simple, exploits buffer pages
Indexed nested loops:
best if 1 relation small and one indexed
Sort/Merge Join
sort cost + merge cost (M+N  M*N)
Hash Join
partition cost (2(M+N)) + match cost (M+N)

4. Query  Tree Rating from 1 to 10 100 boats |S| = 500, |R|=1000
SELECT S.sname
FROM Reserves R, Sailors S
WHERE R.sid=S.sid AND
R.bid=100 AND S.rating>5
Rating from 1 to 10
100 boats
|S| = 500, |R|=1000
Sailors
Reserves
sid=sid
bid=100
rating > 5
sname
(Page-Oriented
Nested loops)
(On-the-fly)
Reserves
Sailors
sid=sid
bid=100
sname
(Page-Oriented
Nested loops)
(On-the-fly)
rating>5
(Scan &
Write to
temp T2)

5. Query  Tree Left-deep plans Relational Algebra Equivalences
Fully pipelined plans: Intermediate results not written to temporary files.
All left-deep trees are fully pipelined? (SM join?)
Relational Algebra Equivalences
Chapter 15.3
Cost estimation
Reduction Factors (also called “selectivity”)
values are uniformly distributed
histogram

6. Choice of Algorithm for Operator
Rating from 1 to 10, 100 boats
|S| = 500, |R|=1000
Sailors
Reserves
sid=sid
rating > 5
sname
(Page-Oriented
Nested loops)
(On-the-fly)
bid=100
(Scan &
Write to
temp T2)
Reserves
Sailors
sid=sid
bid=100
sname
(On-the-fly)
rating > 5
(Scan;
write to
temp T1)
temp T2)
(Sort-Merge Join)
4250 IOs
3560 IOs

7. Choice of Algorithm for Operator
What affect choices?
# Buffer Pages
Statistic of the relation
Index?
Clustered
Unclustered: access each tuple  1 I/O

8. Conclusion What you should know?
Given a SQL query, how to give the best (good) query plan?
Best plan for accessing each relation
Enumeration to get trees (left-deep join)
Select algorithm for each operator
Cost estimation
# of Buffer?
Index