Thursday, November 1, 2007

Query Tuning Frequently Asked Questions


This document records a number of Frequently Asked Questions pertaining to the tuning of SQL statememts.

Scope & Application

For all DBAs

Query Tuning FAQ

Why is an index not used?
Why is a particular query slow?
Why is a particular query slower than it used to be?
Why is a particular query slower since upgrading?
Why does a particular query's runtime vary?
Why does the execution plan for a particular query change?
Why does a particular query's runtime vary with bind variables rather than literals?
Which optimizer is a query using?
Can Optimizer Parameters be hinted?
Why is partition elimination not occuring?
What is the importance of Global Statistics?
How to Gather Information for Query Tuning Problems
How to Gather Optimizer Statistics
How to Gather Access Path Information for Queries
How to Store and Apply Query Outlines
How to Gather Trace for Query Tuning Issues
Why is an index not used?
The following note is a comprehensive discussion of a number of reasons why indexes may not be selected for certain queries:
Note 67522.1 Diagnosing Why a Query is Not Using an Index and Why it is Using a Full Table Scan?

Why is a particular query slow?
If a new query has been created that is slow on 10g and above, then Oracle Support advises users to utilise the 10g SQL Access Advisor to suggest some suitable modifications.

Oracle10g Database Performance Tuning Guide
10g Release 2 (10.2) Part Number B14211-01
Chapter 17 SQL Access Advisor

Alternatively, various tuning articles are available to assist with manual tuning on all releases:

Note 372431.1 TROUBLESHOOTING: Tuning a New Query
Note 179668.1 TROUBLESHOOTING: Tuning Slow Running Queries

Why is a particular query slower than it used to be?
There are a number of factors that can change a query's response time. If a query's performance has changed then it follows that something in the environment has changed to initiate this. There could me more data involved, incomplete or inaccurate statistics, i/o performance changes, access path changes among others. The following articles address various query tuning issues:

Note 179668.1 TROUBLESHOOTING: Tuning Slow Running Queries
Note 163563.1 TROUBLESHOOTING: Advanced Query Tuning

Occasionally, changes in Oracle software through upgrading or patching can cause execution sub-optimal execution plans.
See the section: " Why is this query slower since upgrading?" below for help with this kind of problem.
Why is a particular query slower since upgrading?
As Oracle continues to develop the Database Software, changes are introduced in the optimizer that are designed to give better performance. Occasionally, changes that provides improved performance for many, can have an adverse effect for a small number of others. New versions can also require different approaches in system management to maintain, or achieve, better perfomance.

Bug fixes or security patches may also mean a previous fast execution plan is no longer possible. For example, queries may have being using a bugged access method that could cause problems or security issues in specific circumstances. In resolving the bug, and closing the loophole, the access path that was previously used is no longer available.

The following notes give guidance on what to do when upgrading to ensure good performance is maintained and what to do if a performance regression is encountered:

Note 160089.1 TROUBLESHOOTING: Server Upgrade Results in Slow Query Performance

Note 258167.1 Upgrading from 8.1.X to 9.X - Potential Query Tuning Related Issues
Note 258945.1 Upgrading from 8.1.X to 9.X - Subquery Issues - Diagnosing and Resolving
Note 258946.1 Upgrading from 8.1.X to 9.X - View Issues - Diagnosing and Resolving
Note 259126.1 Upgrading from 8.1.X to 9.X - Btree Bitmap Plan Issues - Diagnosing and Resolving

Note 295819.1 Upgrading from 9i to 10g - Potential Query Tuning Related Issues

Note 223806.1 Query with unchanged execution plan is slower after database upgrade

Why does a particular query's runtime vary?
Given the same inputs, a statement will always return the same outputs. For performance to vary, one of the inputs must have changed. There are a very large number of variables that can affect the performance of a query.
Note 372431.1 TROUBLESHOOTING: Tuning a New Query
Note 179668.1 TROUBLESHOOTING: Tuning Slow Running Queries

Why does the execution plan for a particular query change?
The explain plan for a query can change if one of the factors used by the CBO has changed. These can include:

Initialisation parameters
Object statistics
System statistics
Predicate values

You can use the following script to compare differences that might be causing the execution plan to change. Run the script for each case and compare the outputs.
Note 215187.1 SQLTXPLAIN.SQL - Enhanced Explain Plan and related diagnostic info for one SQL statement:

If explain plans change despite no differences being evident, then it is possible that you are hitting an issue discussed in the following article:
Note 338113.1 Plans can change despite no stats being regathered

@ This could mean that UNPUBLISHED Bug 4567767 is being encountered.
Why does a particular query's runtime vary with bind variables rather than literals?
For details on this issue see:
Note 401068.1 Possible Poor Runtime Performance for Bind Variables when Compared with Literal Values

Which optimizer is a query using?
Often it is important to be able to determine which optimizer is being used by a query. The following article helps with that:

Note 66484.1 Which Optimizer is Being Used ?

Can Optimizer Parameters be hinted?
In 10g Release 2, optimizer parameters can be set with a hint. See:

Note 377333.1 OPT_PARAM Hint in 10g R2

Why is partition elimination not occuring?
Partition elimination or Pruning occurs when the predicates and join order for a query provide sufficient information to allow a set of partitions to be eliminated from the result set without accessing the data. Details of different pruning methods can be found in the following articles:
Note 179518.1 Partition Pruning and Joins
Note 166118.1 Partition Pruning/Elimination
Note 209070.1Partition Pruning based on Joins to Partitioning Criteria Stored in Dimension Tables

Please also see the Why are Global Statistics Required? section.
What is the importance of Global Statistics?
It is important to collect Global Statistics as well as partition level statistics when gathering statistics against partitioned tables. Oracle Corporation recommends setting the GRANULARITY parameter to AUTO to gather both types of partition statistics.
Note 236935.1 Global statistics - An Explanation

Oracle(R) Database Performance Tuning Guide
10g Release 2 (10.2)
Part Number B14211-01
Chapter 14 Managing Optimizer Statistics
Section Statistics on Partitioned Objects

How to Gather Information for Query Tuning Problems
Note 68735.1 Diagnostics for Query Tuning Problems

How to Gather Optimizer Statistics
Note 114671.1 Gathering Statistics for the Cost Based Optimizer

How to Gather Access Path Information for Queries
Note 235530.1 Recommended Method for Obtaining a Formatted Explain Plan
Note 46234.1 Interpreting Explain plan

How to Store and Apply Query Outlines
Note 67536.1 Stored Outline Quick Reference
Note 102311.1 How to Move Stored Outlines for One Application from One Database to Another

How to Gather Trace for Query Tuning Issues
Note 376442.1 Recommended Methods for Obtaining 10046 trace for Tuning

@ Note 225598.1 How to Obtain Tracing of Optimizer Computations (EVENT 10053)
@ Note 242489.1 Transferring Optimizer Statistics to Support

Note 199083.1 Query Tuning Overview
Note 402983.1 Database Performance FAQ
@ Note 399630.1 INCOMPLETE TOPICS to be added .


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