Query Processing and Query
Optimization
CS 157B
Dennis Le
Weishan Wang
What are we going to cover
1.
2.
3.
4.
5.
6.
Processing Selection Queries
Processing Projection Queries and
Eliminating Duplicates
Processing Join Queries
Query Plans and Query Optimization for
Complex Relational Expression
Query Optimization and Database
Analysis in Oracle8
Summary
1. How database processes queries
1. Processing Selection Queries

A Query Plan is a particular strategy for reading
the indexes and files to process a query
Query plan for query 1
select * from Customer
where accountId = 101
Query 5 selection based on a hash
table index
select * from Customer
where zipcode = 32306
2. Processing Projection Queries
and Eliminating Duplicates

Suppose we use the following query to
eliminate the duplicates
select distinct lastName, firstName from Customer
The DBMS has two ways of doing this
which are based on sorting and hashing
 The sorting method sorts the projected
records and writes the nonduplicates
 The hashing method partitions the file
into buckets then sorts within buckets to
eliminate duplicates

3. Processing Join Queries
Suppose we have following three tables:
Customer, Rental and Movie
 B: number of Blocks, R: number of rows
 The following example shows the same
cost of data blocks reads for both plans

Two plans have the same cost
Improvement
- block nested loops join
Use the method readBlock() in the outer
loop to read the outer table into an array
 The inner loop reads the inner file(array)
 Reduce the cost to Bc + Bc x Br

Improvement
- indexed nested loops join
Reduce the reads by using index
 If one of the attributes is indexed, we
should take advantage of it
 Use indexed read  readByAccountId()
 Two plans have different costs because of
the one-to-many relationship

Improvement
- indexed nested loops join
Improvement
- indexed nested loops join
Each rental has a customer but not every
customer has a rental
 Plan-1 (Rental inner)reads every
customer and every rental once, but plan2 (Rental outer)reads some customers
many times and some customer no times
 Which plan to use is depending on the
size of the file(table).
 Chose plan-1 if the customer file is
smaller

Improvement
- sort-merge join
Sort both tables and merge the result
 The cost will depend on the cost of
sorting(typically on the order of NlogN
for N records)

Improvement
- hash join
Create a hash table for each join table
 The cost of the hash join depends on the
size of the two hash tables(Hc and Hr)
 The total cost is 2(Bc+Br+Hc+Hr)

4. Query Plans and Query Optimization
for Complex Relational Expression

Query Optimization Process:
1. Enumerate the query plans
2. Estimate the costs of the plans
3. Choose the best plan for the query
4. Query Plans and Query Optimization
for Complex Relational Expression
4. Query Plans and Query Optimization
for Complex Relational Expression
4. Query Plans and Query Optimization
for Complex Relational Expression
4. Query Plans and Query Optimization
for Complex Relational Expression

Estimating costs (Query Optimizer)
- number of records
- size of records in each table
- location and types of indexes
- ordering of files
- memory available for disk cache
- etc…
5. Query Optimization and
Database Analysis in Oracle8

Tables that keep information about the
DataBase

Analyze Statement
5. Query Optimization and
Database Analysis in Oracle8
6. Summary
Principles and Practices of Query
Processing
 Techniques in optimizing Query Processes
 Process of Query Optimization
 Commercial products implements these
principles

7. References

“Principles of Database Systems with
internet and JAVA applications,” by Greg
Riccardi