Ref: https://blogs.oracle.com/optimizer/sql-plan-management-part-1-of-4-creating-sql-plan-baselines
Part 1:
Introduction
Do you ever experience performance regressions because an execution plan has changed for the worse? If you have, then we have an elegant solution for you called SQL Plan Management (SPM). The next four posts on our blog will cover SPM in detail. Let's begin by reviewing the primary causes for plan changes.
Execution plan changes occur due to various system changes. For example, you might have (manually or automatically) updated statistics for some objects, or changed a few optimizer-related parameters. A more dramatic change is a database upgrade (say from 11g to 12c). All of these changes have the potential to cause new execution plans to be generated for many of your SQL statements. Most new plans are obviously improvements because they are tailored to the new system environment, but some might be worse leading to performance regressions. It is the latter that cause sleepless nights for many DBAs.
DBAs have several options for addressing these regressions. However, what most DBAs want is simple: plans should only change when they will result in performance gains. In other words, the optimizer should not pick bad plans, period.
This first post in our series, describes the concepts of SQL Plan Management and how to create SQL plan baselines. The second part will describe how and when these SQL plan baselines are used. The third part will discuss evolution, the process of adding new and improved plans to SQL plan baselines. Finally, the fourth part will describe user interfaces and interactions with other Oracle objects (like stored outlines).
Execution plan changes occur due to various system changes. For example, you might have (manually or automatically) updated statistics for some objects, or changed a few optimizer-related parameters. A more dramatic change is a database upgrade (say from 11g to 12c). All of these changes have the potential to cause new execution plans to be generated for many of your SQL statements. Most new plans are obviously improvements because they are tailored to the new system environment, but some might be worse leading to performance regressions. It is the latter that cause sleepless nights for many DBAs.
DBAs have several options for addressing these regressions. However, what most DBAs want is simple: plans should only change when they will result in performance gains. In other words, the optimizer should not pick bad plans, period.
This first post in our series, describes the concepts of SQL Plan Management and how to create SQL plan baselines. The second part will describe how and when these SQL plan baselines are used. The third part will discuss evolution, the process of adding new and improved plans to SQL plan baselines. Finally, the fourth part will describe user interfaces and interactions with other Oracle objects (like stored outlines).
SQL Plan Management
SQL Plan Management (SPM) allows database users to maintain stable yet optimal performance for a set of SQL statements. SPM incorporates the positive attributes of plan adaptability and plan stability, while simultaneously avoiding their shortcomings. It has two main objectives:
- prevent performance regressions in the face of database system changes
- offer performance improvements by gracefully adapting to database system changes
A managed SQL statement is one for which SPM has been enabled. SPM can be configured to work automatically or it can be manually controlled either wholly or partially (described later). SPM helps prevent performance regressions by enabling the detection of plan changes for managed SQL statements. For this purpose, SPM maintains, on disk, a plan history consisting of different execution plans generated for each managed SQL statement. An enhanced version of the Oracle optimizer, called SPM aware optimizer, accesses, uses, and manages this information which is stored in a repository called the SQL Management Base (SMB).
The plan history enables the SPM aware optimizer to determine whether the best-cost plan it has produced using the cost-based method is a brand new plan or not. A brand new plan represents a plan change that has potential to cause performance regression. For this reason, the SPM aware optimizer does not choose a brand new best-cost plan. Instead, it chooses from a set of accepted plans. An accepted plan is one that has been either verified to not cause performance regression or designated to have good performance. A set of accepted plans is called a SQL plan baseline, which represents a subset of the plan history.
A brand new plan is added to the plan history as a non-accepted plan. Later, an SPM utility verifies its performance, and keeps it as a non-accepted plan if it will cause a performance regression, or changes it to an accepted plan if it will provide a performance improvement. The plan performance verification process ensures both plan stability and plan adaptability.
The figure below shows the SMB containing the plan history for three SQL statements. Each plan history contains some accepted plans (the SQL plan baseline) and some non-accepted plans.
The plan history enables the SPM aware optimizer to determine whether the best-cost plan it has produced using the cost-based method is a brand new plan or not. A brand new plan represents a plan change that has potential to cause performance regression. For this reason, the SPM aware optimizer does not choose a brand new best-cost plan. Instead, it chooses from a set of accepted plans. An accepted plan is one that has been either verified to not cause performance regression or designated to have good performance. A set of accepted plans is called a SQL plan baseline, which represents a subset of the plan history.
A brand new plan is added to the plan history as a non-accepted plan. Later, an SPM utility verifies its performance, and keeps it as a non-accepted plan if it will cause a performance regression, or changes it to an accepted plan if it will provide a performance improvement. The plan performance verification process ensures both plan stability and plan adaptability.
The figure below shows the SMB containing the plan history for three SQL statements. Each plan history contains some accepted plans (the SQL plan baseline) and some non-accepted plans.
(Click on the image for a larger view.)
You can create a SQL plan baseline in several ways: using a SQL Tuning Set (STS); from the cursor cache; exporting from one database and importing into another; and automatically for every statement. Let's look at each in turn. The examples in this blog entry use the Oracle Database Sample Schemas so you can try them yourself.
Creating SQL plan baselines from STS
If you are upgrading you might already have an STS containing some or all of your SQL statements. This STS might contain plans that perform satisfactorily. Let's call this STS MY_STS. You can create a SQL plan baseline from this STS as follows:SQL> variable pls number; SQL> exec :pls := dbms_spm.load_plans_from_sqlset(sqlset_name => 'MY_STS', - > basic_filter => 'sql_text like ''select%p.prod_name%''');
This will create SQL plan baselines for all statements that match the specified filter.
Creating SQL plan baselines from cursor cache
You can automatically create SQL plan baselines for any cursor that is currently in the cache as follows:SQL> exec :pls := dbms_spm.load_plans_from_cursor_cache( - > attribute_name => 'SQL_TEXT', - > attribute_value => 'select%p.prod_name%');
This will create SQL plan baselines for all statements whose text matches the specified string. Several overloaded variations of this function allow you to filter on other cursor attributes.
Creating SQL plan baselines using a staging table
If you already have SQL plan baselines (say on an 11g test system), you can export them to another system (a production system for instance).
First, on the test system, create a staging table and pack the SQL plan baselines you want to export:
SQL> exec dbms_spm.create_stgtab_baseline(table_name => 'MY_STGTAB', -
> table_owner => 'SH');
PL/SQL procedure successfully completed.
SQL> exec :pls := dbms_spm.pack_stgtab_baseline( -
> table_name => 'MY_STGTAB', -
> table_owner => 'SH', -
> sql_text => 'select%p.prod_name%');
This will pack all SQL plan baselines for statements that match the specified filter. The staging table, MY_STGTAB, is a regular table that you should export to the production system using Datapump Export.
On the production system, you can now unpack the staging table to create the SQL plan baselines:
SQL> exec :pls := dbms_spm.unpack_stgtab_baseline( - > table_name => 'MY_STGTAB', - > table_owner => 'SH', - > sql_text => 'select%p.prod_name%');
This will unpack the staging table and create SQL plan baselines. Note that the filter for unpacking the staging table is optional and may be different than the one used during packing. This means that you can pack several SQL plan baselines into a staging table and selectively unpack only a subset of them on the target system.
Creating SQL plan baselines automatically
You can create SQL plan baselines for all repeatable statements automatically by setting the parameter optimizer_capture_sql_plan_baselines to TRUE (default is FALSE). The first plan captured for any statement is automatically accepted and becomes part of the SQL plan baseline, so enable this parameter only when you are sure that the default plans are performing well.You can use the automatic plan capture mode when you have upgraded from a previous database version. Set optimizer_features_enable to the earlier version and execute your workload. Every repeatable statement will have its plan captured thus creating SQL plan baselines. You can reset optimizer_features_enable to its default value after you are sure that all statements in your workload have had a chance to execute.
Note that this automatic plan capture occurs only for repeatable statements, that is, statements that are executed at least twice. Statements that are only executed once will not benefit from SQL plan baselines since accepted plans are only used in subsequent hard parses.
The following example shows a plan being captured automatically when the same statement is executed twice:
SQL> alter session set optimizer_capture_sql_plan_baselines = true; Session altered. SQL> var pid number SQL> exec :pid := 100; PL/SQL procedure successfully completed. SQL> select p.prod_name, s.amount_sold, t.calendar_year 2 from sales s, products p, times t 3 where s.prod_id = p.prod_id 4 and s.time_id = t.time_id 5 and p.prod_id < :pid; PROD_NAME AMOUNT_SOLD CALENDAR_YEAR --------- ----------- ------------- ... 9 rows selected. SQL> select p.prod_name, s.amount_sold, t.calendar_year 2 from sales s, products p, times t 3 where s.prod_id = p.prod_id 4 and s.time_id = t.time_id 5 and p.prod_id < :pid; PROD_NAME AMOUNT_SOLD CALENDAR_YEAR --------- ----------- ------------- ... 9 rows selected. SQL> alter session set optimizer_capture_sql_plan_baselines = false; Session altered
Automatic plan capture will not occur for a statement if a stored outline exists for it and is enabled and the parameter use_stored_outlines is TRUE. In this case, turn on incremental capture of plans into an STS using the function capture_cursor_cache_sqlset() in the DBMS_SQLTUNE package. After you have collected the plans for your workload into the STS, manually create SQL plan baselines using the method described earlier. Then, disable the stored outlines or set use_stored_outlines to FALSE. From now on, SPM will manage your workload and stored outlines will not be used for those statements.
In this article, we have seen how to create SQL plan baselines. In the next, we will describe the SPM aware optimizer and how it uses SQL plan baselines.
Part 2:
In Part 1, we saw how you can create SQL plan baselines. After you create a SQL plan baseline for a statement, subsequent executions of that statement will use the SQL plan baseline. From all the plans in the SQL plan baseline, the optimizer will select the one with the best cost in the current environment (including bind values, current statistics, parameters, etc.). The optimizer will also generate the best-cost plan that it would otherwise have used without a SQL plan baseline. However, this best-cost plan will not be used but instead added to the statement's plan history for later verification. In other words, the optimizer will use a known plan from the SQL plan baseline instead of a new and hitherto unknown plan. This guarantees no performance regression.
Let's see this plan selection process in action. First, we create a SQL plan baseline by enabling automatic plan capture and executing the query twice:
Let's see this plan selection process in action. First, we create a SQL plan baseline by enabling automatic plan capture and executing the query twice:
SQL> alter session set optimizer_capture_sql_plan_baselines = true;
Session altered.
SQL> var pid number
SQL> exec :pid := 100;
PL/SQL procedure successfully completed.
SQL> select p.prod_name, s.amount_sold, t.calendar_year
2 from sales s, products p, times t
3 where s.prod_id = p.prod_id
4 and s.time_id = t.time_id
5 and p.prod_id < :pid;
PROD_NAME AMOUNT_SOLD CALENDAR_YEAR
--------- ----------- -------------
...
9 rows selected.
SQL> select * from table(dbms_xplan.display_cursor('bfbr3zrg9d5cc', 0, 'basic note'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------
EXPLAINED SQL STATEMENT:
------------------------
select p.prod_name, s.amount_sold, t.calendar_year from sales s,
products p, times t where s.prod_id = p.prod_id and s.time_id =
t.time_id and p.prod_id < :pid
Plan hash value: 2787970893
----------------------------------------------------------------
| Id | Operation | Name |
----------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS | |
| 2 | NESTED LOOPS | |
| 3 | HASH JOIN | |
| 4 | TABLE ACCESS BY INDEX ROWID | PRODUCTS |
| 5 | INDEX RANGE SCAN | PRODUCTS_PK |
| 6 | PARTITION RANGE ALL | |
| 7 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |
| 8 | BITMAP CONVERSION TO ROWIDS | |
| 9 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX |
| 10 | INDEX UNIQUE SCAN | TIME_PK |
| 11 | TABLE ACCESS BY INDEX ROWID | TIMES |
----------------------------------------------------------------
25 rows selected.
SQL> select p.prod_name, s.amount_sold, t.calendar_year
2 from sales s, products p, times t
3 where s.prod_id = p.prod_id
4 and s.time_id = t.time_id
5 and p.prod_id < :pid;
PROD_NAME AMOUNT_SOLD CALENDAR_YEAR
--------- ----------- -------------
...
9 rows selected.
SQL> alter session set optimizer_capture_sql_plan_baselines = false;
Session altered.
SQL> select sql_text, plan_name, enabled, accepted from dba_sql_plan_baselines;
SQL_TEXT PLAN_NAME ENA ACC
---------------------------------------- ------------------------------ --- ---
select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES
endar_year
from sales s, products p, times t
where s.prod_id = p.prod_id
and s.time_id = t.time_id
and p.prod_id < :pid
We can see that a SQL plan baseline was created for the statement. Suppose the statement is hard parsed again (we do it here by flushing the shared pool). Let's turn off SQL plan management and execute the query with a different bind value:
SQL> exec :pid := 100000;
PL/SQL procedure successfully completed.
SQL> alter system flush shared_pool;
System altered.
SQL> alter session set optimizer_use_sql_plan_baselines = false;
Session altered.
SQL> select p.prod_name, s.amount_sold, t.calendar_year
2 from sales s, products p, times t
3 where s.prod_id = p.prod_id
4 and s.time_id = t.time_id
5 and p.prod_id < :pid;
PROD_NAME AMOUNT_SOLD CALENDAR_YEAR
--------- ----------- -------------
...
960 rows selected.
SQL> select * from table(dbms_xplan.display_cursor('bfbr3zrg9d5cc', 0, 'basic note'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------
EXPLAINED SQL STATEMENT:
------------------------
select p.prod_name, s.amount_sold, t.calendar_year from sales s,
products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and
p.prod_id < :pid
Plan hash value: 2361178149
------------------------------------------
| Id | Operation | Name |
------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN | |
| 2 | HASH JOIN | |
| 3 | PARTITION RANGE ALL| |
| 4 | TABLE ACCESS FULL | SALES |
| 5 | TABLE ACCESS FULL | TIMES |
| 6 | TABLE ACCESS FULL | PRODUCTS |
------------------------------------------
We can see that the optimizer selected a different plan because the new bind value makes the predicate less selective. Let's turn SQL plan management back on and re-execute the query with the same bind value:
SQL> alter session set optimizer_use_sql_plan_baselines = true;
Session altered.
SQL> select p.prod_name, s.amount_sold, t.calendar_year
2 from sales s, products p, times t
3 where s.prod_id = p.prod_id
4 and s.time_id = t.time_id
5 and p.prod_id < :pid;
PROD_NAME AMOUNT_SOLD CALENDAR_YEAR
--------- ----------- -------------
...
960 rows selected.
SQL> select * from table(dbms_xplan.display_cursor('bfbr3zrg9d5cc', 0, 'basic note'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------
EXPLAINED SQL STATEMENT:
------------------------
select p.prod_name, s.amount_sold, t.calendar_year from sales s,
products p, times t where s.prod_id = p.prod_id and s.time_id =
t.time_id and p.prod_id < :pid
Plan hash value: 2787970893
----------------------------------------------------------------
| Id | Operation | Name |
----------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS | |
| 2 | NESTED LOOPS | |
| 3 | HASH JOIN | |
| 4 | TABLE ACCESS BY INDEX ROWID | PRODUCTS |
| 5 | INDEX RANGE SCAN | PRODUCTS_PK |
| 6 | PARTITION RANGE ALL | |
| 7 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |
| 8 | BITMAP CONVERSION TO ROWIDS | |
| 9 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX |
| 10 | INDEX UNIQUE SCAN | TIME_PK |
| 11 | TABLE ACCESS BY INDEX ROWID | TIMES |
----------------------------------------------------------------
Note
-----
- SQL plan baseline SYS_SQL_PLAN_fcc170b0a62d0f4d used for this statement
The note at the bottom tells you that the optimizer is using the SQL plan baseline. In other words, we can see that the optimizer used an accepted plan in the SQL plan baseline in favor of a new plan. In fact, we can also check that the optimizer inserted the new plan into the statement's plan history:
SQL> select sql_text, plan_name, enabled, accepted from dba_sql_plan_baselines; SQL_TEXT PLAN_NAME ENA ACC ---------------------------------------- ------------------------------ --- --- select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b08cbcb825 YES NO endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid
The 'NO' value for the accepted column implies that the new plan is in the plan history but is not available for use until it is verified to be a good plan. The optimizer will continue to use an accepted plan until new plans are verified and added to the SQL plan baseline. If there is more than one plan in the SQL plan baseline, the optimizer will use the one with the best cost under the then-current conditions (statistics, bind values, parameter settings and so on).
When you create a SQL plan baseline for a SQL statement, the SPM aware optimizer thus guarantees that no new plans will be used other than the ones in the SQL plan baseline. This prevents unexpected plan changes that sometimes lead to performance regressions.
Preventing new plans from being used is fine, but what if the new plans are in fact better? In Part 3, we will describe how new and improved plans are added to a SQL plan baseline.
Part 3:
Evolving SQL Plan Baselines
In the example in Part 2, we saw that the optimizer used an accepted plan instead of a brand new plan. The statement has two plans in its plan history, but only one is accepted and thus in the SQL plan baseline:
SQL> select sql_text, plan_name, enabled, accepted from dba_sql_plan_baselines; SQL_TEXT PLAN_NAME ENA ACC ---------------------------------------- ------------------------------ --- --- select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b08cbcb825 YES NO endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid
Non-accepted plans can be verified by executing the evolve_sql_plan_baseline function. This function will execute the non-accepted plan and compare its performance to the best accepted plan. The execution is performed using the conditions (e.g., bind values, parameters, etc.) in effect at the time the non-accepted plan was added to the plan history. If the non-accepted plan's performance is better, the function will make it accepted, thus adding it to the SQL plan baseline. Let's see what happens when we execute this function:
SQL> var report clob; SQL> exec :report := dbms_spm.evolve_sql_plan_baseline(); PL/SQL procedure successfully completed. SQL> print :report REPORT ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- Evolve SQL Plan Baseline Report ------------------------------------------------------------------------------- Inputs: ------- SQL_HANDLE = PLAN_NAME = TIME_LIMIT = DBMS_SPM.AUTO_LIMIT VERIFY = YES COMMIT = YES Plan: SYS_SQL_PLAN_fcc170b08cbcb825 ----------------------------------- Plan was verified: Time used .1 seconds. Passed performance criterion: Compound improvement ratio >= 10.13 Plan was changed to an accepted plan. Baseline Plan Test Plan Improv. Ratio ------------- --------- ------------- Execution Status: COMPLETE COMPLETE Rows Processed: 960 960 Elapsed Time(ms): 19 15 1.27 CPU Time(ms): 18 15 1.2 Buffer Gets: 1188 116 10.24 Disk Reads: 0 0 Direct Writes: 0 0 Fetches: 0 0 Executions: 1 1 ------------------------------------------------------------------------------- Report Summary ------------------------------------------------------------------------------- Number of SQL plan baselines verified: 1.
The plan verification report shows that the new plan's performance was better and so it was made accepted and became part of the SQL plan baseline. We can confirm it by looking in the dba_sql_plan_baselines view:
SQL> select sql_text, plan_name, enabled, accepted from dba_sql_plan_baselines; SQL_TEXT PLAN_NAME ENA ACC ---------------------------------------- ------------------------------ --- --- select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b08cbcb825 YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid
The SQL plan baseline now has two accepted plans: SYS_SQL_PLAN_fcc170b08cbcb825 is now accepted.
You can either execute the evolve_sql_plan_baseline() function manually or schedule it to run automatically in a maintenance window.
You can either execute the evolve_sql_plan_baseline() function manually or schedule it to run automatically in a maintenance window.
Another way of evolving a SQL plan baseline is to use the SQL Tuning Advisor. Instead of executing evolve_sql_plan_baseline, suppose we start from the original state where we have one accepted and one non-accepted plan:
SQL> select sql_text, plan_name, enabled, accepted from dba_sql_plan_baselines; SQL_TEXT PLAN_NAME ENA ACC ---------------------------------------- ------------------------------ --- --- select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b08cbcb825 YES NO endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid
You can execute the SQL Tuning Advisor on the cursor in the cursor cache:
SQL> var tname varchar2(30); SQL> exec :tname := dbms_sqltune.create_tuning_task(sql_id => 'bfbr3zrg9d5cc'); PL/SQL procedure successfully completed. SQL> exec dbms_sqltune.execute_tuning_task(task_name => :tname); PL/SQL procedure successfully completed. SQL> select dbms_sqltune.report_tuning_task(:tname, 'TEXT', 'BASIC') FROM dual; DBMS_SQLTUNE.REPORT_TUNING_TASK(:TNAME,'TEXT','BASIC') ------------------------------------------------------------------------------- GENERAL INFORMATION SECTION ------------------------------------------------------------------------------- Tuning Task Name : TASK_505 Tuning Task Owner : SH Workload Type : Single SQL Statement Scope : COMPREHENSIVE Time Limit(seconds): 1800 Completion Status : COMPLETED Started at : 11/11/2008 16:43:12 Completed at : 11/11/2008 16:43:13 ------------------------------------------------------------------------------- Schema Name: SH SQL ID : bfbr3zrg9d5cc SQL Text : select p.prod_name, s.amount_sold, t.calendar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid ------------------------------------------------------------------------------- FINDINGS SECTION (1 finding) ------------------------------------------------------------------------------- 1- A potentially better execution plan was found for this statement. ------------------------------------------------------------------------------- EXPLAIN PLANS SECTION ------------------------------------------------------------------------------- 1- Original With Adjusted Cost ------------------------------ Plan hash value: 2787970893 ---------------------------------------------------------------- | Id | Operation | Name | ---------------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | NESTED LOOPS | | | 2 | NESTED LOOPS | | | 3 | HASH JOIN | | | 4 | TABLE ACCESS BY INDEX ROWID | PRODUCTS | | 5 | INDEX RANGE SCAN | PRODUCTS_PK | | 6 | PARTITION RANGE ALL | | | 7 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES | | 8 | BITMAP CONVERSION TO ROWIDS | | | 9 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX | | 10 | INDEX UNIQUE SCAN | TIME_PK | | 11 | TABLE ACCESS BY INDEX ROWID | TIMES | ---------------------------------------------------------------- 2- Original With Adjusted Cost ------------------------------ Plan hash value: 2787970893 ---------------------------------------------------------------- | Id | Operation | Name | ---------------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | NESTED LOOPS | | | 2 | NESTED LOOPS | | | 3 | HASH JOIN | | | 4 | TABLE ACCESS BY INDEX ROWID | PRODUCTS | | 5 | INDEX RANGE SCAN | PRODUCTS_PK | | 6 | PARTITION RANGE ALL | | | 7 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES | | 8 | BITMAP CONVERSION TO ROWIDS | | | 9 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX | | 10 | INDEX UNIQUE SCAN | TIME_PK | | 11 | TABLE ACCESS BY INDEX ROWID | TIMES | ---------------------------------------------------------------- 3- Using SQL Profile -------------------- Plan hash value: 2361178149 ------------------------------------------ | Id | Operation | Name | ------------------------------------------ | 0 | SELECT STATEMENT | | | 1 | HASH JOIN | | | 2 | HASH JOIN | | | 3 | PARTITION RANGE ALL| | | 4 | TABLE ACCESS FULL | SALES | | 5 | TABLE ACCESS FULL | TIMES | | 6 | TABLE ACCESS FULL | PRODUCTS | ------------------------------------------ ------------------------------------------------------------------------------- SQL> exec dbms_sqltune.accept_sql_profile(task_name => :tname); PL/SQL procedure successfully completed. SQL> select sql_text, plan_name, enabled, accepted from dba_sql_plan_baselines; SQL_TEXT PLAN_NAME ENA ACC ---------------------------------------- ------------------------------ --- --- select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b08cbcb825 YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid select p.prod_name, s.amount_sold, t.cal SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid SQL> select sql_text, type, status from dba_sql_profiles; SQL_TEXT TYPE STATUS ---------------------------------------- ------- -------- select p.prod_name, s.amount_sold, t.cal MANUAL ENABLED endar_year from sales s, products p, times t where s.prod_id = p.prod_id and s.time_id = t.time_id and p.prod_id < :pid
What we see here is that SQL Tuning Advisor found a tuned plan (that coincidentally happened to be the non-accepted plan in our plan history). When we accepted the recommended SQL profile, the SQL Tuning Advisor created a SQL profile and also changed the non-accepted plan to accepted status, thus evolving the SQL plan baseline to two plans.
Note that the SQL Tuning Advisor may also find a completely new tuned plan, one that is not in the plan history. If you then accept the recommended SQL profile, the SQL Tuning Advisor will create a SQL profile and also add the tuned plan to the SQL plan baseline.
Thus, you can evolve a SQL plan baseline either by executing the evolve_sql_plan_baseline function or by using the SQL Tuning Advisor. New and provably better plans will be added by either of these methods to the SQL plan baseline.
Part 4:
User Interfaces and Other Features
In the first three parts of this article, we have seen how SQL plan baselines are created, used and evolved. In this final installment, we will show some user interfaces, describe the interaction of SPM with other features and answer some of your questions.
DBMS_SPM package
A new package, DBMS_SPM, allows you to manage plan histories. We have already seen in previous examples how you can use it to create and evolve SQL plan baselines. Other management functions include changing attributes (like enabled status and plan name) of plans or dropping plans. You need the ADMINISTER SQL MANAGEMENT OBJECT privilege to execute this package. Viewing the plan history Regardless of how a plan history is created, you can view details about the various plans in the view DBA_SQL_PLAN_BASELINES. At the end of Part 3 of this blog, we saw that the SQL statement had two accepted plans:
SQL> select sql_text, sql_handle, plan_name, enabled, accepted 2 from dba_sql_plan_baselines; SQL_TEXT SQL_HANDLE PLAN_NAME ENA ACC ------------------------ ------------------------ ----------------------------- --- --- select p.prod_name, s.am SYS_SQL_4bf04d85fcc170b0 SYS_SQL_PLAN_fcc170b08cbcb825 YES YES ount_sold, t.calendar_ye ar from sales s, products p , times t where s.prod_id = p.prod _id and s.time_id = t.time _id and p.prod_id < :pid select p.prod_name, s.am SYS_SQL_4bf04d85fcc170b0 SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES ount_sold, t.calendar_ye ar from sales s, products p , times t where s.prod_id = p.prod _id and s.time_id = t.time _id and p.prod_id < :pid
The SQL handle is a unique identifier for each SQL statement that you can use when managing your plan history using the DBMS_SPM package. Creating an accepted plan by modifying the SQL text Some of you may be manually tuning SQL statements by adding hints or otherwise modifying the SQL text. If you enable automatic capture of SQL plans and then execute this statement, you will be creating a SQL plan baseline for this modified statement. What you most likely want, however, is to add this plan to the plan history of the original SQL statement. Here's how you can do this using the above SQL statement as an example. Let's modify the SQL statement, execute it and look at the plan:
SQL> var pid number
SQL> exec :pid := 100;
PL/SQL procedure successfully completed.
SQL> select /*+ leading(t) */ p.prod_name, s.amount_sold, t.calendar_year
2 from sales s, products p, times t
3 where s.prod_id = p.prod_id
4 and s.time_id = t.time_id
5 and p.prod_id < :pid;
PROD_NAME AMOUNT_SOLD CALENDAR_YEAR
--------- ----------- -------------
...
9 rows selected.
SQL> select * from table(dbms_xplan.display_cursor('b17wnz4y8bqv1', 0, 'basic note'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------
EXPLAINED SQL STATEMENT:
------------------------
select /*+ leading(t) */ p.prod_name, s.amount_sold, t.calendar_year
from sales s, products p, times t where s.prod_id = p.prod_id and
s.time_id = t.time_id and p.prod_id < :pid
Plan hash value: 2290436051
---------------------------------------------------------------
| Id | Operation | Name |
---------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN | |
| 2 | HASH JOIN | |
| 3 | TABLE ACCESS FULL | TIMES |
| 4 | PARTITION RANGE ALL | |
| 5 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |
| 6 | BITMAP CONVERSION TO ROWIDS | |
| 7 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX |
| 8 | TABLE ACCESS BY INDEX ROWID | PRODUCTS |
| 9 | INDEX RANGE SCAN | PRODUCTS_PK |
---------------------------------------------------------------
We can now create a new accepted plan for the original SQL statement by associating the modified statement's plan to the original statement's sql handle (obtained from DBA_SQL_PLAN_BASELINES):
SQL> var pls number SQL> exec :pls := dbms_spm.load_plans_from_cursor_cache( - > sql_id => 'b17wnz4y8bqv1', - > plan_hash_value => 2290436051, - > sql_handle => 'SYS_SQL_4bf04d85fcc170b0');
If the original SQL statement does not already have a plan history (and thus no SQL handle), another version of load_plans_from_cursor_cache allows you to specify the original statement's text. To confirm that we now have three accepted plans for our SQL statement, let's check in DBA_SQL_PLAN_BASELINES:
SQL> select sql_text, sql_handle, plan_name, enabled, accepted 2 from dba_sql_plan_baselines; SQL_TEXT SQL_HANDLE PLAN_NAME ENA ACC ------------------------ ------------------------ ----------------------------- --- --- select p.prod_name, s.am SYS_SQL_4bf04d85fcc170b0 SYS_SQL_PLAN_fcc170b0888547d3 YES YES ount_sold, t.calendar_ye ar from sales s, products p , times t where s.prod_id = p.prod _id and s.time_id = t.time _id and p.prod_id < :pid select p.prod_name, s.am SYS_SQL_4bf04d85fcc170b0 SYS_SQL_PLAN_fcc170b08cbcb825 YES YES ount_sold, t.calendar_ye ar from sales s, products p , times t where s.prod_id = p.prod _id and s.time_id = t.time _id and p.prod_id < :pid select p.prod_name, s.am SYS_SQL_4bf04d85fcc170b0 SYS_SQL_PLAN_fcc170b0a62d0f4d YES YES ount_sold, t.calendar_ye ar from sales s, products p , times t where s.prod_id = p.prod _id and s.time_id = t.time _id and p.prod_id < :pid
Displaying plans
When the optimizer uses an accepted plan for a SQL statement, you can see it in the plan table (for explain) or V$SQL_PLAN (for shared cursors). Let's explain the SQL statement above and display its plan:
SQL> explain plan for
2 select p.prod_name, s.amount_sold, t.calendar_year
3 from sales s, products p, times t
4 where s.prod_id = p.prod_id
5 and s.time_id = t.time_id
6 and p.prod_id < :pid;
Explained.
SQL> select * from table(dbms_xplan.display('plan_table', null, 'basic note'));
PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------
Plan hash value: 2787970893
----------------------------------------------------------------
| Id | Operation | Name |
----------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS | |
| 2 | NESTED LOOPS | |
| 3 | HASH JOIN | |
| 4 | TABLE ACCESS BY INDEX ROWID | PRODUCTS |
| 5 | INDEX RANGE SCAN | PRODUCTS_PK |
| 6 | PARTITION RANGE ALL | |
| 7 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |
| 8 | BITMAP CONVERSION TO ROWIDS | |
| 9 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX |
| 10 | INDEX UNIQUE SCAN | TIME_PK |
| 11 | TABLE ACCESS BY INDEX ROWID | TIMES |
----------------------------------------------------------------
Note
-----
- SQL plan baseline "SYS_SQL_PLAN_fcc170b0a62d0f4d" used for this statement
The note at the bottom tells you that the optimizer used an accepted plan. A plan history might have multiple plans. You can see one of the accepted plans if the optimizer selects it for execution. But what if you want to display some or all of the other plans? You can do this using the display_sql_plan_baseline function in the DBMS_XPLAN package. Using the above example, here's how you can display the plan for all plans in the plan history.
SQL> select *
2 from table(dbms_xplan.display_sql_plan_baseline(
3 sql_handle => 'SYS_SQL_4bf04d85fcc170b0', format => 'basic'));
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
SQL handle: SYS_SQL_4bf04d85fcc170b0
SQL text: select p.prod_name, s.amount_sold, t.calendar_year from sales s,
products p, times t where s.prod_id = p.prod_id and s.time_id =
t.time_id and p.prod_id < :pid
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
Plan name: SYS_SQL_PLAN_fcc170b0888547d3
Enabled: YES Fixed: NO Accepted: YES Origin: MANUAL-LOAD
--------------------------------------------------------------------------------
Plan hash value: 2290436051
---------------------------------------------------------------
| Id | Operation | Name |
---------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN | |
| 2 | HASH JOIN | |
| 3 | TABLE ACCESS FULL | TIMES |
| 4 | PARTITION RANGE ALL | |
| 5 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |
| 6 | BITMAP CONVERSION TO ROWIDS | |
| 7 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX |
| 8 | TABLE ACCESS BY INDEX ROWID | PRODUCTS |
| 9 | INDEX RANGE SCAN | PRODUCTS_PK |
---------------------------------------------------------------
--------------------------------------------------------------------------------
Plan name: SYS_SQL_PLAN_fcc170b08cbcb825
Enabled: YES Fixed: NO Accepted: YES Origin: AUTO-CAPTURE
--------------------------------------------------------------------------------
Plan hash value: 2361178149
------------------------------------------
| Id | Operation | Name |
------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | HASH JOIN | |
| 2 | HASH JOIN | |
| 3 | PARTITION RANGE ALL| |
| 4 | TABLE ACCESS FULL | SALES |
| 5 | TABLE ACCESS FULL | TIMES |
| 6 | TABLE ACCESS FULL | PRODUCTS |
------------------------------------------
--------------------------------------------------------------------------------
Plan name: SYS_SQL_PLAN_fcc170b0a62d0f4d
Enabled: YES Fixed: NO Accepted: YES Origin: AUTO-CAPTURE
--------------------------------------------------------------------------------
Plan hash value: 2787970893
----------------------------------------------------------------
| Id | Operation | Name |
----------------------------------------------------------------
| 0 | SELECT STATEMENT | |
| 1 | NESTED LOOPS | |
| 2 | NESTED LOOPS | |
| 3 | HASH JOIN | |
| 4 | TABLE ACCESS BY INDEX ROWID | PRODUCTS |
| 5 | INDEX RANGE SCAN | PRODUCTS_PK |
| 6 | PARTITION RANGE ALL | |
| 7 | TABLE ACCESS BY LOCAL INDEX ROWID| SALES |
| 8 | BITMAP CONVERSION TO ROWIDS | |
| 9 | BITMAP INDEX RANGE SCAN | SALES_PROD_BIX |
| 10 | INDEX UNIQUE SCAN | TIME_PK |
| 11 | TABLE ACCESS BY INDEX ROWID | TIMES |
----------------------------------------------------------------
Parameters
Two parameters allow you to control SPM. The first, optimizer_capture_sql_plan_baselines, which is FALSE by default, allows you to automatically capture plans. SPM will start managing every repeatable SQL statement that is executed and will create a plan history for it. The first plan that is captured will beautomatically accepted. Subsequent plans for these statements will not be accepted until they are evolved. The second parameter, optimizer_use_sql_plan_baselines, is TRUE by default. It allows the SPM aware optimizer to use the SQL plan baseline if available when compiling a SQL statement. If you set this parameter to FALSE, the SPM aware optimizer will be disabled and you will get the regular cost-based optimizer which will select the best plan based on estimated cost.
SPM and SQL profiles
A SQL statement can have both a SQL profile and a SQL plan baseline. Such a case was described in Part 3 where we evolved a SQL plan baseline by accepting a SQL profile. In this case, the SPM aware optimizer will use both the SQL profile and the SQL plan baseline. The SQL profile contains additional information that helps the optimizer to accurately cost each accepted plan and select the best one. The SPM aware optimizer may choose a different accepted plan when a SQL profile is present than when it is not.
SPM and Stored Outlines
It is possible for a SQL statement to have a stored outline as well as a SQL plan baseline. If a stored outline exists for a SQL statement and is enabled for use, then the optimizer will use it. If you are using stored outlines, you can test SPM by creating SQL plan baselines and disabling the stored outlines. If you are satisfied with SPM, you can either drop the stored outlines or leave them disabled. If SPM doesn't work for you (and we would love to know why), you can re-enable the stored outlines. If you are using stored outlines, be aware of their limitations: You can only have one stored outline at a time for a given SQL statement. This may be fine in some cases, but a single plan is not necessarily the best when the statement is executed under varying conditions (e.g., bind values). The second limitation is related to the first. Stored outlines do not allow for evolution. That is, even if a better plan exists, the stored outline will continue to be used, potentially degrading your system's performance. To get the better plan, you have to manually drop the current stored outline and generate a new one. If an access path (e.g., an index) used in a stored outline is dropped or otherwise becomes unusable, the partial stored outline will continue to be used with the potential of a much worse plan.
One question that readers have is what we plan to do with the stored outlines feature. Stored outlines will be desupported in a future release in favor of SQL plan management. In Oracle Database 12c Release 2, stored outlines continue to function as in past releases. However, Oracle strongly recommends that you use SQL plan management for new applications. SQL plan management creates SQL plan baselines, which offer superior SQL performance and stability compared with stored outlines. If you have existing stored outlines, consider migrating them to SQL plan baselines by using the LOAD_PLANS_FROM_CURSOR_CACHE or LOAD_PLANS_FROM_SQLSET procedure of the DBMS_SPM package. When the migration is complete, you should disable or remove the stored outlines.
SPM and Adaptive Cursor Sharing
Adaptive cursor sharing (ACS) may generate multiple cursors for a given bind sensitive SQL statement if it is determined that a single plan is not optimal under all conditions. Each cursor is generated by forcing a hard parse of the statement. The optimizer will normally select the plan with the best cost upon each hard parse. When you have a SQL plan baseline for a statement, the SPM aware optimizer will select the best accepted plan as the optimal plan. This also applies for the hard parse of a bind sensitive statement. There may be multiple accepted plans, each of which is optimal for different bind sets. With SPM and ACS enabled, the SPM aware optimizer will select the best plan for the current bind set. Thus, if a hard parse occurs, the normal SPM plan selection algorithm is used regardless of whether a statement is bind sensitive.
Enterprise Manager
You can view SQL plan baselines and configure and manage most SPM tasks through the Enterprise Manager. The screenshots below show two of these tasks.
Setting init.ora parameters for SPM
Loading SQL plan baselines from cursor cache
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