OVERVIEW OF MAJOR COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 59

SELECT *
FROM CUSTOMER
WHERE CUSTOMER_ZONE = 'North' and
CUST_BALANCE > 10000

You submit the query for execution and printing of the customer list for the VP.
What happens in the DBMS when your query is presented to it for execution? The
database engine coordinates the services of the query processor to go through
different steps and produce the results for the query. Figure 2-10 shows the steps of
the query execution.
Note the processing of the query through the following key steps:

• Database engine initiates the execution.
• Data dictionary determines the data elements needed.
• Security module checks data access authorizations.
• Query processor verifies language syntax.
• Query optimizer subcomponent examines the query and creates an optimal
execution plan.
• Query processor works with the database engine to execute the optimal plan.
• Database engine interfaces with the operating system to retrieve the requested
data.
• Query processor presents the data.

When you write and submit a query for execution, the query component takes
over and produces the result set. Apart from using direct queries, you also request
information through other means. You may use a form or screen to initiate the data
request. Data retrieval may also be through applications. The query processor com-
ponent is involved when you make data requests through other such methods.

Initiate query execution

Determine data elements

Customer
Data

DATABASE
ENGINE

Verify security authorization

QUERY PROCESSOR

DATA
REPOSITORY

Verify query language syntax

Query Optimizer

Security Module

Prepare execution plan

Structure of
Customer data

I/O Module

Execute query plan

Security Authori-
zations

DBMS

Retrieve requested data

DATA
DICTIONARY

Present query result

Figure 2-10 Query processor: query execution.