Home > Articles > Data > SQL

  • Print
  • + Share This

Understanding the Optimizer and Associated Tools

To see how query variants affect performance, you need two things: information about indexes and a way to see the choices the optimizer made. Most systems provide tools for both these functions, but they vary widely.

Getting Information on Indexes

You'll always be able to find out what indexes are connected to a particular table, through the meta-data system catalogs if nothing else (see "Getting Meta-Data from System Catalogs" in Chapter 7). For example, in Adaptive Server Anywhere, a quick survey of the system catalogs reveals two likely suspects: sysindex and systable. A first try at a query to locate indexes for a particular table might look like this:

Adaptive Server Anywhere
select sysindex.index_name, systable.table_name
from sysindex, systable
where sysindex.table_id = systable.table_id
  and table_name = 'product'

index_name            table_name
===================== =================================
prodix                product
pricex                product

[2 rows]

(See "Writing Queries Using System Catalogs" in Chapter 7 for more examples of generating this kind of code.)

In Adaptive Server Enterprise and MS SQL Server, an easy way to investigate indexes is the sp_helpindex stored procedure (the output from the two RDBMSs is not identical). It tells you the names of the indexes and the columns in each. ("Nonclustered" is a type of Transact-SQL index, with pointers from the bottom row of the index to the data—the index and data are not in the same order. This contrasts to a clustered index, in which the index and data are in the same order.)

Adaptive Server Enterprise
exec sp_helpindex product

index_name index_description                          index_keys  ix_max_rows_per_page
---------- ------------------------------------------ ----------- --------------------
prodix     nonclustered, unique located on default     prodnum           0
pricex     nonclustered located on default             price             0

(2 rows affected)

In many systems, including the Adaptive Server Anywhere on this book's CD, the index information source of choice is a graphical user interface (GUI) tool. Since you may not be much of an ASA user but would like to follow along, here are basic instructions on using ASA Sybase Central for index information (see the help files for details). If you're using another system, the tools and commands will look different but will probably supply very similar information—the name of the index, the columns that make it up, the order in which the columns appear, and the index type.

Open Sybase Central and click Connect under the Tools button (Figure 6–2). Choose Adaptive Server Anywhere. Fill in the forms for the Login and Database tabs for msdpn (Figure 6–3), logging in as DBA with a password of SQL (both must be uppercase) and substituting your msdpn6.db address for C:/SQLBook on the Database file line.

Figure 6-2Figure 6–2. Connecting on Sybase Central

Figure 6-3Figure 6–3. Logging In

After you click OK, you'll see your server (here msdpn, with the terminal icon) listed in the Sybase Central window (Figure 6–4). Click it and then click the database-owner combination you want to use—msdp(DBA)—with the disk icon. You'll see a list of objects (Figure 6–5). Click Tables. From the detailed list of tables and views, click the table name you want (say, product) and then Indexes (Figure 6–6). Pick the index you want. Use the tabs on the window to get details.

Figure 6-4Figure 6–4. Choosing a Database

Figure 6-5Figure 6–5. Listing Objects

Figure 6-6Figure 6–6. Getting Index Details

There is information on the msdpn indexes in "Table Details" in Chapter 1.

Checking the Optimizer

To check your work, you need to know a little about your optimizer, the part of the SQL engine that decides how to process a query. A cost-based optimizer looks at processing options and chooses the one that is "cheapest" in terms of time. If your system is cost-based, you may need to run commands that make sure the statistics on data are current (see Table 6–2). In a rule-based system, the optimizer makes choices based on a set of ranked guidelines.

Most systems support a command or a tool that shows you what the optimizer is doing. In Adaptive Server Anywhere, the software included on the disk, there is one of each.

  • The command is the PLAN function, which takes a query (in quotes) as its argument. For a list of related commands, see Table 6–1.

  • The tool is the Performance Monitor on Sybase Central (an option under Statistics).

If you're following along with the Adaptive Server Anywhere software included on the CD, you may need to send PLAN results to an output file in order to read more than the first line. To do this, end the query with a semicolon and follow it with an OUTPUT TO line naming a file in which to store the query results, and a FORMAT line prescribing the format of columns in the output file. The following examples illustrate this method.

Adaptive Server Anywhere
select plan 
( 'select prodnum, type, price, description from product 
             where prodnum in (1104, 1105, 1106, 1107)' );
output to out.txt
format text

The out.txt file (it could have any name) is located in the directory that holds the ASA database file. When you open the out.txt file, you'll see information on how the query was processed.

Adaptive Server Anywhere
Estimate 1 I/O operations (best of 2 plans considered)
Scan product sequentially
 Estimate getting here 21 times
 For _value_1 in (1104,1105,1106,1107)

If you change the IN phrase to a BETWEEN, the output is different.

Adaptive Server Anywhere
select plan 
( 'select prodnum, type, price, description from product
             where prodnum between 1104 and 1107' );
output to out.txt
format text

Estimate 5 I/O operations
Scan product using unique index prodix
for rows where prodnum is between 1104 and 1107
 Estimate getting here 4 times

Without knowing much about the PLAN messages, you can see that the IN query doesn't use an index—it does a table scan. The BETWEEN query uses the prodix index. The first one goes somewhere 21 times, while the second makes just four trips.

You can get a shorter version of that information by looking at the ASA Interactive SQL Statistics window (Figures 6–7 and 6–8). Notice that the PLAN> line for the two queries is different. The first indicates a sequential scan of the table. The second shows index use. This is parallel to the PLAN results, and easier to read and generate. Figure 6–8 shows just the statistics part of the screen produced by the BETWEEN version of the query. Adaptive Server Anywhere provides more detailed performance information in the Sybase Central Performance Monitor.

Figure 6-7Figure 6–7. Interactive SQL Window


Figure 6-8Figure 6–8. Statistics Pane of the Interactive SQL Window

For a summary of commands that keep tabs on the optimizer, see Table 6–1. You'll need to do some research before you can get much information from the output of any of these commands. You may find additional tools that are used at your site—GUI-based, third party, or home grown.

Table 6-1 Monitoring Performance

ANSI

ASA

ASE

MS SQL Server

Oracle

Informix

 

PLAN ( 'query' )

SET SHOWPLAN ON

SET NOEXEC ON

SET SHOWPLAN _ALL ON

EXPLAIN PLAN

SET EXPLAIN ON


SQL Conventions

Before diving into performance in SQL queries, consider how your code looks. Code is easier to read and understand if you present it consistently. In some systems, reusability of cached code may depend on the various copies being identical. Differences as small as a single space character may be relevant. In addition, training time for new employees is shorter if they can expect consistent patterns. For your sanity, develop coding guidelines. Here are some common suggestions.

  • Start each line with a SQL verb (SELECT, FROM).

       select prodnum, type, price
       from product
       where prodnum between 104 and 107
  • Indent continued lines.

       select prodnum, type, price
       from product
       where prodnum between 104 and 107
         and price >50.00
  • Be consistent in naming tables and columns—don't make some table names singular and others plural, don't use case randomly, and don't call one column pubdate and a related column in another table pub_date.

  • If you use table aliases, stick to the same ones, and don't use nonmnemonic aliases such as a, b, and c for supplier, product, customer.

  • Put in lots of comments: the date, your name, what the query or script is about—everything you'd want to know.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020