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Do As Little As Possible - Oracle

This article, the first of three parts, focuses on the design and creation of applications that use the database. It is excerpted from chapter five of the book Oracle Database 10g DBA Handbook, written by Kevin Loney and Bob Bryla (McGraw-Hill/Osborne, 2005; ISBN: 0072231459).

  1. Developing and Implementing Applications
  2. Do As Little As Possible
  3. In Your Application Design, Strive to Avoid Trips to the Database
  4. Go Atomic
  5. Store Data Efficiently at the Block Level
  6. Test Correctly
  7. Standard Deliverables
  8. Tuning Goals for Queries and Transaction Processing
By: McGraw-Hill/Osborne
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February 09, 2006

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End users do not care, in general, if the underlying database structures are fully normalized to Third Normal Form or if they are laid out in compliance with object-oriented standards. Users want to perform a business process, and the database application should be a tool that helps that business process complete as quickly as possible. The focus of your design should not be the achievement of theoretical design perfection; it should always be on the end user’s ability to do his or her job. Therefore, you should simplify the processes involved at every step in the application.

This can be a difficult point to negotiate with application development teams. If application development teams insist on perfectly normalized data models, DBAs should point out the number of database steps involved in even the simplest transaction. For example, inserts for a complex transaction (such as a line item for an invoice) may involve many code table lookups as well as multiple inserts. For a single user this may not present a problem, but with many concurrent users this design may lead to performance issues or locking issues. From a performance-planning perspective, inserts should involve as few tables as possible, and queries should retrieve data that is already stored in a format that is as close as possible to the final format requested by the users. Fully normalized databases and object-oriented designs tend to require a high number of joins during complex queries. Although you should strive to maintain a manageable data model, the first emphasis should be on the functionality of the application and its ability to meet the business’s performance needs.

In Your Application Design, Strive to Eliminate Logical Reads

In the past, there was a heavy focus on eliminating physical reads—and although this is still a good idea, no physical reads occur unless logical reads require them.

Let’s take a simple example. Select the current time from DUAL. If you select down to the second level, the value will change 86,400 times per day. Yet there are application designers who repeatedly perform this query, executing it millions of times per day. Such a query likely performs few physical reads throughout the day. Therefore, if you are focused solely on tuning the physical I/O, you would likely disregard it. However, it can significantly impact the performance of the application. How? By using the CPU resources available. Each execution of the query will force Oracle to perform work, using processing power to find and return the correct data. As more and more users execute the command repeatedly, you may find that the number of logical reads used by the query exceeds all other queries. In some cases, multiple processors on the server are dedicated to servicing repeated small queries of this sort. If multiple users need to read the same data, you should store it in a table or in a package variable.

Consider the following real-world example. A programmer wanted to implement a pause in a program, forcing it to wait 30 seconds between the completion of two steps. Because the performance of the environment would not be consistent over time, the programmer coded the routine in the following format (shown in pseudo-code):

perform Step 1
select SysDate from DUAL into a StartTime variable
begin loop
select SysDate from DUAL in a CurrentTime variable;
Compare CurrentTime with the StartTime variable value.
If 30 seconds have passed, exit the loop;
    Otherwise repeat the loop, calculating SysDate again.
end loop
Perform Step 2.

Is this a reasonable approach? Absolutely not! It will do what the developer wanted, but at a significant cost to the application. What’s more, there is nothing a database administrator can do to improve its performance. In this case, the cost will not be due to I/O activity—the DUAL table will stay in the instance’s memory area—but rather due to CPU activity. Every time this program is run, by every user, the database will spend 30 seconds consuming as many CPU resources as the system can support. In this particular case the select SysDate from DUAL query accounts for over 40 percent of all the CPU time used by the application. All of that CPU time is wasted. Tuning the database initialization parameters will not solve the problem. Tuning the individual SQL statement will not help; the application design must be revised to eliminate the needless execution of commands. For instance, in this case the developer could have used a sleep command at the operating system level or within a program to enforce the same behavior without the database accesses.

For those who favor tuning based on the buffer cache hit ratio, this database has a hit ratio of almost 100 percent due to the high number of completely unnecessary logical reads without related physical reads. The buffer cache hit ratio compares the number of logical reads to the number of physical reads; if 10 percent of the logical reads require physical reads, the buffer cache hit ratio is 90 percent. Low hit ratios identify databases that perform a high number of physical reads; extremely high hit ratios such as found in this example may identify databases that perform an excessive number of logical reads. You must look beyond the buffer cache hit ratio to the commands that are generating the logical reads and the physical reads.

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