This chapter discusses goals for tuning, how to identify high-resource SQL statements, explains what should be collected, and provides tuning suggestions.
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This chapter contains the following sections:
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11.1 Introduction to SQL Tuning
An important facet of database system performance tuning is the tuning of SQL statements. SQL tuning involves three basic steps:
These three steps are repeated until the system performance reaches a satisfactory level or no more statements can be tuned.
11.2 Goals for Tuning
The objective of tuning a system is either to reduce the response time for end users of the system, or to reduce the resources used to process the same work. You can accomplish both of these objectives in several ways:
11.2.1 Reduce the Workload
SQL tuning commonly involves finding more efficient ways to process the same workload. It is possible to change the execution plan of the statement without altering the functionality to reduce the resource consumption.
Two examples of how resource usage can be reduced are:
11.2.2 Balance the Workload
Systems often tend to have peak usage in the daytime when real users are connected to the system, and low usage in the nighttime. If noncritical reports and batch jobs can be scheduled to run in the nighttime and their concurrency during day time reduced, then it frees up resources for the more critical programs in the day.
11.2.3 Parallelize the Workload
Queries that access large amounts of data (typical data warehouse queries) often can be parallelized. This is extremely useful for reducing the response time in low concurrency data warehouse. However, for OLTP environments, which tend to be high concurrency, this can adversely impact other users by increasing the overall resource usage of the program.
11.3 Identifying High-Load SQL
This section describes the steps involved in identifying and gathering data on high-load SQL statements. High-load SQL are poorly-performing, resource-intensive SQL statements that impact the performance of the Oracle database. High-load SQL statements can be identified by:
11.3.1 Identifying Resource-Intensive SQL
The first step in identifying resource-intensive SQL is to categorize the problem you are attempting to fix:
11.3.1.1 Tuning a Specific Program
If you are tuning a specific program (GUI or 3GL), then identifying the SQL to examine is a simple matter of looking at the SQL executed within the program. Oracle Enterprise Manager provides tools for identifying resource intensive SQL statements, generating explain plans, and evaluating SQL performance.
See Also:
If it is not possible to identify the SQL (for example, the SQL is generated dynamically), then use
SQL_TRACE to generate a trace file that contains the SQL executed, then use TKPROF to generate an output file.
The SQL statements in the
TKPROF output file can be ordered by various parameters, such as the execution elapsed time (exeela ), which usually assists in the identification by ordering the SQL statements by elapsed time (with highest elapsed time SQL statements at the top of the file). This makes the job of identifying the poorly performing SQL easier if there are many SQL statements in the file.
See Also:
Chapter 20, 'Using Application Tracing Tools'
11.3.1.2 Tuning an Application / Reducing Load
If your whole application is performing suboptimally, or if you are attempting to reduce the overall CPU or I/O load on the database server, then identifying resource-intensive SQL involves the following steps:
One method to identify which SQL statements are creating the highest load is to compare the resources used by a SQL statement to the total amount of that resource used in the period. For
BUFFER_GETS , divide each SQL statement's BUFFER_GETS by the total number of buffer gets during the period. The total number of buffer gets in the system is available in the V$SYSSTAT table, for the statistic session logical reads.
Similarly, it is possible to apportion the percentage of disk reads a statement performs out of the total disk reads performed by the system by dividing
V$SQL_STATS.DISK_READS by the value for the V$SYSSTAT statistic physical reads. The SQL sections of the Automatic Workload Repository report include this data, so you do not need to perform the percentage calculations manually.
See Also:
Oracle Database Reference for information about dynamic performance views
After you have identified the candidate SQL statements, the next stage is to gather information that is necessary to examine the statements and tune them.
11.3.2 Gathering Data on the SQL Identified
Cant download file from google drive. If you are most concerned with CPU, then examine the top SQL statements that performed the most
BUFFER_GETS during that interval. Otherwise, start with the SQL statement that performed the most DISK_READS .
11.3.2.1 Information to Gather During Tuning
The tuning process begins by determining the structure of the underlying tables and indexes. The information gathered includes the following:
11.4 Automatic SQL Tuning Features
Because the manual SQL tuning process poses many challenges to the application developer, the SQL tuning process has been automated by the automatic SQL Tuning manageability features. Theses features have been designed to work equally well for OLTP and Data Warehouse type applications. See Chapter 12, 'Automatic SQL Tuning'.
ADDM
Automatic Database Diagnostic Monitor (ADDM) analyzes the information collected by the AWR for possible performance problems with the Oracle database, including high-load SQL statements. See 'Automatic Database Diagnostic Monitor'.
SQL Tuning Advisor
SQL Tuning Advisor allows a quick and efficient technique for optimizing SQL statements without modifying any statements. https://newworld110.weebly.com/blog/auto-tune-8-download-for-pc. See 'SQL Tuning Advisor'.
SQL Tuning Sets
When multiple SQL statements are used as input to ADDM or SQL Tuning Advisor, a SQL Tuning Set (STS) is constructed and stored. The STS includes the set of SQL statements along with their associated execution context and basic execution statistics. See 'SQL Tuning Sets'.
SQLAccess Advisor
In addition to the SQL Tuning Advisor, Oracle provides the SQLAccess Advisor that provides advice on materialized views, indexes, and materialized view logs. The SQLAccess Advisor helps you achieve your performance goals by recommending the proper set of materialized views, materialized view logs, and indexes for a given workload. In general, as the number of materialized views and indexes and the space allocated to them is increased, query performance improves. The SQLAccess Advisor considers the trade-offs between space usage and query performance and recommends the most cost-effective configuration of new and existing materialized views and indexes.
To access the SQLAccess Advisor through Oracle Enterprise Manager Database Control:
See 'Using the SQL Access Advisor'.
11.5 Developing Efficient SQL Statements
This section describes ways you can improve SQL statement efficiency:
11.5.1 Verifying Optimizer Statistics
The query optimizer uses statistics gathered on tables and indexes when determining the optimal execution plan. If these statistics have not been gathered, or if the statistics are no longer representative of the data stored within the database, then the optimizer does not have sufficient information to generate the best plan.
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11.5.2 Reviewing the Execution Plan
When tuning (or writing) a SQL statement in an OLTP environment, the goal is to drive from the table that has the most selective filter. This means that there are fewer rows passed to the next step. If the next step is a join, then this means that fewer rows are joined. Check to see whether the access paths are optimal.
When examining the optimizer execution plan, look for the following:
If any of these conditions are not optimal, then consider restructuring the SQL statement or the indexes available on the tables.
11.5.3 Restructuring the SQL Statements
Often, rewriting an inefficient SQL statement is easier than modifying it. If you understand the purpose of a given statement, then you might be able to quickly and easily write a new statement that meets the requirement.
11.5.3.1 Compose Predicates Using AND and =
To improve SQL efficiency, use equijoins whenever possible. Statements that perform equijoins on untransformed column values are the easiest to tune.
11.5.3.2 Avoid Transformed Columns in the WHERE Clause
Use untransformed column values. For example, use:
rather than:
Do not use SQL functions in predicate clauses or
WHERE clauses. Any expression using a column, such as a function having the column as its argument, causes the optimizer to ignore the possibility of using an index on that column, even a unique index, unless there is a function-based index defined that can be used.
Avoid mixed-mode expressions, and beware of implicit type conversions. When you want to use an index on the
VARCHAR2 column charcol , but the WHERE clause looks like this:
where
numexpr is an expression of number type (for example, 1, USERENV ('SESSIONID '), numcol , numcol +0,.), Oracle translates that expression into:
Avoid the following kinds of complex expressions:
These expressions prevent the optimizer from assigning valid cardinality or selectivity estimates and can in turn affect the overall plan and the join method.
Add the predicate versus using
NVL () technique.
For example:
Also:
When you need to use SQL functions on filters or join predicates, do not use them on the columns on which you want to have an index; rather, use them on the opposite side of the predicate, as in the following statement:
rather than
See Also:
Chapter 15, 'Using Indexes and Clusters' for more information on function-based indexes
11.5.3.3 Write Separate SQL Statements for Specific Tasks
SQL is not a procedural language. Using one piece of SQL to do many different things usually results in a less-than-optimal result for each task. If you want SQL to accomplish different things, then write various statements, rather than writing one statement to do different things depending on the parameters you give it.
Note:
Oracle Forms and Reports are powerful development tools that allow application logic to be coded using PL/SQL (triggers or program units). This helps reduce the complexity of SQL by allowing complex logic to be handled in the Forms or Reports. You can also invoke a server side PL/SQL package that performs the few SQL statements in place of a single large complex SQL statement. Because the package is a server-side unit, there are no issues surrounding client to database round-trips and network traffic.
It is always better to write separate SQL statements for different tasks, but if you must use one SQL statement, then you can make a very complex statement slightly less complex by using the
UNION ALL operator.
Optimization (determining the execution plan) takes place before the database knows what values will be substituted into the query. An execution plan cannot, therefore, depend on what those values are. For example:
Written as shown, the database cannot use an index on the
somecolumn column, because the expression involving that column uses the same column on both sides of the BETWEEN .
This is not a problem if there is some other highly selective, indexable condition you can use to access the driving table. Often, however, this is not the case. Frequently, you might want to use an index on a condition like that shown but need to know the values of :
loval , and so on, in advance. With this information, you can rule out the ALL case, which should not use the index.
If you want to use the index whenever real values are given for :
loval and :hival (if you expect narrow ranges, even ranges where :loval often equals :hival ), then you can rewrite the example in the following logically equivalent form:
If you run
EXPLAIN PLAN on the new query, then you seem to get both a desirable and an undesirable execution plan. However, the first condition the database evaluates for either half of the UNION ALL is the combined condition on whether :hival and :loval are ALL . The database evaluates this condition before actually getting any rows from the execution plan for that part of the query.
When the condition comes back false for one part of the
UNION ALL query, that part is not evaluated further. Only the part of the execution plan that is optimum for the values provided is actually carried out. Because the final conditions on :hival and :loval are guaranteed to be mutually exclusive, only one half of the UNION ALL actually returns rows. (The ALL in UNION ALL is logically valid because of this exclusivity. It allows the plan to be carried out without an expensive sort to rule out duplicate rows for the two halves of the query.)
11.5.3.4 Use of EXISTS versus IN for Subqueries
In certain circumstances, it is better to use
IN rather than EXISTS . In general, if the selective predicate is in the subquery, then use IN . If the selective predicate is in the parent query, then use EXISTS .
Note:
This discussion is most applicable in an OLTP environment, where the access paths either to the parent SQL or subquery are through indexed columns with high selectivity. In a DSS environment, there can be low selectivity in the parent SQL or subquery, and there might not be any indexes on the join columns. In a DSS environment, consider using semijoins for the EXISTS case.
Sometimes, Oracle can rewrite a subquery when used with an
IN clause to take advantage of selectivity specified in the subquery. This is most beneficial when the most selective filter appears in the subquery and there are indexes on the join columns. Conversely, using EXISTS is beneficial when the most selective filter is in the parent query. This allows the selective predicates in the parent query to be applied before filtering the rows against the EXISTS criteria.
Note:
You should verify the optimizer cost of the statement with the actual number of resources used (BUFFER_GETS , DISK_READS , CPU_TIME from V$SQL STATS or V$SQLAREA ). Situations such as data skew (without the use of histograms) can adversely affect the optimizer's estimated cost for an operation.
'Example 1: Using IN - Selective Filters in the Subquery' and 'Example 2: Using EXISTS - Selective Predicate in the Parent' are two examples that demonstrate the benefits of
IN and EXISTS . Both examples use the same schema with the following characteristics:
11.5.3.4.1 Example 1: Using IN - Selective Filters in the Subquery
This example demonstrates how rewriting a query to use
IN can improve performance. This query identifies all employees who have placed orders on behalf of customer 144.
The following SQL statement uses
EXISTS :
Notes:
The following plan output is the execution plan (from
V$SQL_PLAN ) for the preceding statement. The plan requires a full table scan of the employees table, returning many rows. Each of these rows is then filtered against the orders table (through an index).
Rewriting the statement using
IN results in significantly fewer resources used.
The SQL statement using
IN :
Note:
The following plan output is the execution plan (from
V$SQL_PLAN ) for the preceding statement. The optimizer rewrites the subquery into a view, which is then joined through a unique index to the employees table. This results in a significantly better plan, because the view (that is, subquery) has a selective predicate, thus returning only a few employee_ids . These employee_ids are then used to access the employees table through the unique index.
11.5.3.4.2 Example 2: Using EXISTS - Selective Predicate in the ParentAuto Select Fresno
This example demonstrates how rewriting a query to use
EXISTS can improve performance. This query identifies all employees from department 80 who are sales reps who have placed orders.
The following SQL statement uses
IN :
Note:
The following plan output is the execution plan (from
V$SQL_PLAN ) for the preceding statement. The SQL statement was rewritten by the optimizer to use a view on the orders table, which requires sorting the data to return all unique employee_ids existing in the orders table. Because there is no predicate, many employee_ids are returned. The large list of resulting employee_ids are then used to access the employees table through the unique index.
The following SQL statement uses
EXISTS :
Note:
The following plan output is the execution plan (from
V$SQL_PLAN ) for the preceding statement. The cost of the plan is reduced by rewriting the SQL statement to use an EXISTS . This plan is more effective, because two indexes are used to satisfy the predicates in the parent query, thus returning only a few employee_ids . The employee_ids are then used to access the orders table through an index.
Note:
An even more efficient approach is to have a concatenated index on department_id and job_id . This eliminates the need to access two indexes and reduces the resources used.
11.5.4 Controlling the Access Path and Join Order with Hints
You can influence the optimizer's choices by setting the optimizer approach and goal, and by gathering representative statistics for the query optimizer. Sometimes, the application designer, who has more information about a particular application's data than is available to the optimizer, can choose a more effective way to execute a SQL statement. You can use hints in SQL statements to instruct the optimizer about how the statement should be executed.
Hints, such as /*+
FULL */ control access paths. For example:
See Also:
Chapter 13, 'The Query Optimizer' and Chapter 16, 'Using Optimizer Hints'
Join order can have a significant effect on performance. The main objective of SQL tuning is to avoid performing unnecessary work to access rows that do not affect the result. This leads to three general rules:
The following example shows how to tune join order effectively:
11.5.4.1 Use Caution When Managing Views
Be careful when joining views, when performing outer joins to views, and when reusing an existing view for a new purpose.
11.5.4.1.1 Use Caution When Joining Complex Views
Joins to complex views are not recommended, particularly joins from one complex view to another. Often this results in the entire view being instantiated, and then the query is run against the view data.
For example, the following statement creates a view that lists employees and departments:
The following query finds employees in a specified state:
In the following plan table output, note that the
emp_dept view is instantiated:
11.5.4.1.2 Do Not Recycle Views
Beware of writing a view for one purpose and then using it for other purposes to which it might be ill-suited. Querying from a view requires all tables from the view to be accessed for the data to be returned. Before reusing a view, determine whether all tables in the view need to be accessed to return the data. If not, then do not use the view. Instead, use the base table(s), or if necessary, define a new view. The goal is to refer to the minimum number of tables and views necessary to return the required data.
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Consider the following example:
The entire view is first instantiated by performing a join of the
employees and departments tables and then aggregating the data. However, you can obtain department_name and department_id directly from the departments table. It is inefficient to obtain this information by querying the emp_dept view.
11.5.4.1.3 Use Caution When Unnesting Subqueries
Subquery unnesting merges the body of the subquery into the body of the statement that contains it, allowing the optimizer to consider them together when evaluating access paths and joins.
See Also:
Oracle Database Data Warehousing Guide for an explanation of the dangers with subquery unnesting
11.5.4.1.4 Use Caution When Performing Outer Joins to Views
In the case of an outer join to a multi-table view, the query optimizer (in Release 8.1.6 and later) can drive from an outer join column, if an equality predicate is defined on it.
An outer join within a view is problematic because the performance implications of the outer join are not visible.
11.5.4.2 Store Intermediate Results
Intermediate, or staging, tables are quite common in relational database systems, because they temporarily store some intermediate results. In many applications they are useful, but Oracle requires additional resources to create them. Always consider whether the benefit they could bring is more than the cost to create them. Avoid staging tables when the information is not reused multiple times.
Some additional considerations:
11.5.5 Restructuring the Indexes
Often, there is a beneficial impact on performance by restructuring indexes. This can involve the following:
Do not use indexes as a panacea. Application developers sometimes think that performance will improve if they create more indexes. If a single programmer creates an appropriate index, then this might indeed improve the application's performance. However, if 50 programmers each create an index, then application performance will probably be hampered.
11.5.6 Modifying or Disabling Triggers and Constraints
Using triggers consumes system resources. If you use too many triggers, then you can find that performance is adversely affected and you might need to modify or disable them.
11.5.7 Restructuring the Data
After restructuring the indexes and the statement, you can consider restructuring the data.
Ast Auto Select Tuning Reviews11.5.8 Maintaining Execution Plans Over Time
You can maintain the existing execution plan of SQL statements over time either using stored statistics or stored SQL execution plans. Storing optimizer statistics for tables will apply to all SQL statements that refer to those tables. Storing an execution plan (that is, plan stability) maintains the plan for a single SQL statement. If both statistics and a stored plan are available for a SQL statement, then the optimizer uses the stored plan.
See Also:
11.5.9 Visiting Data as Few Times as PossibleAst Auto Select Tuning Tool
Applications should try to access each row only once. This reduces network traffic and reduces database load. Consider doing the following:
11.5.9.1 Combine Multiples Scans with CASE Statements
Often, it is necessary to calculate different aggregates on various sets of tables. Usually, this is done with multiple scans on the table, but it is easy to calculate all the aggregates with one single scan. Eliminating n-1 scans can greatly improve performance.
Combining multiple scans into one scan can be done by moving the
WHERE condition of each scan into a CASE statement, which filters the data for the aggregation. For each aggregation, there could be another column that retrieves the data.
The following example asks for the count of all employees who earn less then 2000, between 2000 and 4000, and more than 4000 each month. This can be done with three separate queries:
However, it is more efficient to run the entire query in a single statement. Each number is calculated as one column. The count uses a filter with the
CASE statement to count only the rows where the condition is valid. For example:
This is a very simple example. The ranges could be overlapping, the functions for the aggregates could be different, and so on.
11.5.9.2 Use DML with RETURNING Clause
When appropriate, use
INSERT , UPDATE , or DELETE . RETURNING to select and modify data with a single call. This technique improves performance by reducing the number of calls to the database.
See Also:
Oracle Database SQL Language Reference for syntax on the INSERT , UPDATE , and DELETE statements
11.5.9.3 Modify All the Data Needed in One Statement
When possible, use array processing. This means that an array of bind variable values is passed to Oracle for repeated execution. This is appropriate for iterative processes in which multiple rows of a set are subject to the same operation.
For example:
Alternatively, you could define a cascading constraint on
orders . In the previous example, one SELECT and nDELETE s are executed. When a user issues the DELETE on orders DELETE FROM orders WHERE order_id = :id , the database automatically deletes the positions with a single DELETE statement.
See Also:
Oracle Database Administrator's Guide or Oracle Database Heterogeneous Connectivity User's Guide
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