QueryOptimization_Siao

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Query Optimization

CS 157B

Ch. 14

Mien Siao


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Outline Introduction

Steps in Cost-based query optimization- Query

Flow Projection Example

Query Interaction in DBMS

Cost-based query Optimization: Algebraic

Expressions


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Introduction What is Query Optimization?

Suppose you were given a chance tovisit 15 pre-selected different citiesin Europe. The only constraint wouldbe Time

-> Would you have a plan to visitthe cities in any order?


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Plan:

-> Place the 15 cities in different groups

based on their proximity to each other.-> Start with one group and move on tothe next group.

Important point made over here is thatyou would have visited the cities in amore organized manner, and the Timeconstraint mentioned earlier would have

been dealt with efficiently.


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Starting with System-R, most of thecommercial DBMSs use cost-based

optimizers.

The estimation should be accurateand easy. Another important point is

the need for being logicallyconsistent because the least costplan will always be consistently low.


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Steps in a Cost-based query

optimization

1. Parsing

2. Transformation

3. Implementation

4. Plan selection based on costestimates


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Query Flow

Parser

Optimizer

CodeGenerator/Interpreter

Processor

SQL


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Query Parser Verify validity of the SQLstatement. Translate query into an internalstructure using relational calculus.

Query Optimizer Find the best expressionfrom various different algebraic expressions.Criteria used is Cheapness

Code Generator/Interpreter Make calls forthe Query processor as a result of the work doneby the optimizer.

Query Processor Execute the calls obtainedfrom the code generator.


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Cost of physical plans includes processortime and communication time. The mostimportant factor to consider is disk I/Os

because it is the most time consumingaction.

Some other costs associated are:- Operations (joins, unions,

intersections).- The order of operations.Why?


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Joins, unions, and intersections areassociative and commutative.

- Management of storage ofarguments and passing of it.

Factors mentioned above should belimited and minimized when creatingthe best physical plan.


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We can fit 5 tuples into 1 block- 5 tuples * 190 bytes/tuple = 950 bytes

can fit into 1 block- For 20,000 tuples, we would require

4,000 blocks (20,000 / 5 tuples per block= 4,000

With a projection resulting in elimination ofcolumn c (150 bytes), we could estimatethat each tuple would decrease to 40bytes (190 150 bytes)


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Now, the new estimate will be 25 tuples in1 block.

- 25 tuples * 40 bytes/tuple = 1000 byteswill be able to fit into 1 block

- With 20,000 tuples, the new estimate is800 blocks (20,000 tuples / 25 tuples per

block = 800 blocks)

Result is reduction by a factor of 5


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Query interaction in DBMS How does a query interact with a

DBMS?

- Interactive users

- Embedded queries in programswritten in C, C++, etc.

What is the difference betweenthese two ?


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Interactive Users:

- When there is an interactive userquery, the query goes through theQuery Parser, Query Optimizer,Code Generator, and Query

Processor each time.


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- In an embedded query, the callsgenerated by the code generator are

stored in the database. Each timethe query is reached within theprogram at run-time, the QueryProcessor invokes the stored calls in

the database.- Optimization is independent in

embedded queries.


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Cost-based query Optimization:

Algebraic ExpressionsIf we had the following query-

SELECT p.pname, d.dname

FROM Patients p, Doctors d

WHERE p.doctor = d.dname

AND d.dgender = M


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projection

filter

join

Scan (Patients) Scan (Doctors)


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Cost-based query Optimization :

Transformationprojection projection

filter join

join

Scan (Patients) Scan (Doctors) Scan(Patients) Scan(Doctors)


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Implementationprojection projection

filter hash join

natural join filter

Scan(Patients) Scan(Doctors) Scan(Patients) Scan(Doctors)


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Plan selection based on costsprojection projection

filter hash join

natural join filter

Scan(Patients) Scan(Doctors) Scan(Patients) Scan(Doctors)

Estimated Costs= 100ms

Estimated Costs= 50ms

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