Oracle 11g SQL Plan Management – SQL Plan Baselines
Note, this post applies to 11gR1 (specifically 11.1.0.7).
11g has a new method of dealing with plan instability (i.e. the tendency of a statement to flip flop between plans). The goal has always been to provide the best performance, but until 11g there has not been a mechanism that had as a design goal, the idea of eliminating backwards movement (“performance regressions” as the Oracle documentation calls them). That is, not allowing a statement to switch to a plan that is significantly slower than the one it has already been executing. This new mechanism depends on a construct called a Baseline. So for this post I am not going to focus on the process of evolving plans to prevent “performance regressions”, but rather on the Baselines themselves. (hopefully I’ll get to the plan management part soon)
Here’s a little excerpt from the 11gR1 documentation:
Each time a SQL statement is compiled, the optimizer first uses a cost-based search method to build a best-cost plan, then tries to find a matching plan in the SQL plan baseline. If a match is found, the optimizer will proceed using this plan. Otherwise, it evaluates the cost of each accepted plan in the SQL plan baseline and selects the plan with the lowest cost. The best-cost plan found by the optimizer that does not match any plans in the plan history for the SQL statement represents a new plan, and is added as a non-accepted plan to the plan history. The new plan is not used until it is verified to not cause a performance regression. However, if a change in the system (such as a dropped index) causes all accepted plans to become non-reproducible, the optimizer will select the best-cost plan. Thus, the presence of a SQL plan baseline causes the optimizer to use conservative plan selection strategy for the SQL statement.
Clear as mud?
Now my description:
Baselines are the latest evolution in Oracle’s efforts to allow a plan to be locked. The previous efforts were Outlines and SQL Profiles. Both of those efforts were based on the idea that hints could be applied to limit the optimizers choices down to one. That approach seems a little flawed. Why not just save the plan and be done with it? I believe that’s what Baselines are intended to do. Unfortunately, they don’t appear to do it yet. But they do have an advantage over Outlines and SQL Profiles in that they at least save the plan_hash_value, so they know if they are reproducing the correct plan or not. (see my previous post, Do Baselines Use Hints? – Take 2) And hopefully, they will evolve to the point where the actual plan is saved along with the hash value – we’ll see.
So anyway, here’s some basic housekeeping stuff on Baselines:
- Baselines will be used by default in 11gR1, if they exist. There is a parameter to control whether they are used or not (OPTIMIZER_USE_SQL_PLAN_BASELINE). It is set to TRUE by default.
- Baselines will not be created by default in 11gR1. So, much like with the older Outlines or SQL Profiles, you must do something to create them.
- There is a view called DBA_SQL_PLAN_BASELINES that exposes the Baselines that have been created.
- Just like Outlines and SQL Profiles, Baselines apply to all instances in a RAC environment (they are not localized to a specific instance).
Baselines can be created a couple of different ways. They can be created automatically by the database as it’s running by setting the OPTIMIZER_CAPTURE_SQL_PLAN_BASELINES parameter. They can also be created for statements in a SQL Tuning Set. But I’m not going to talk about either one of those approaches in this post, as I don’t think they really make much sense in the case where you want to use a Baseline to lock a plan for a specific statement. Instead, I am going to discuss creating a baseline for a statement that is in the shared pool via the DBMS_SPM.LOAD_PLANS_FROM_CURSOR_CACHE function.
All the function needs is a SQL_ID and a PLAN_HASH_VALUE. Optionally a parameter can be used to define the baseline as FIXED. If it’s FIXED then it gets priority over any other Baselines for that statement, except other FIXED Baselines. Confused? Well it’s not exactly the most straight forward setup. I’m kind of a simple guy, so at this point I’m thinking one FIXED Baseline is enough.
Before you get too bored, here’s a quick example:
SQL> select avg(pk_col) from kso.little_skew where col1 = 99320;
AVG(PK_COL)
-----------
628922
SQL> @find_sql
Enter value for sql_text: %little%
Enter value for sql_id:
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
4dd2qjxbjav6u 0 2709260180 1 .02 89 select avg(pk_col) from kso.little_skew where col1 = 99320
SQL> @dplan
Enter value for sql_id: 4dd2qjxbjav6u
Enter value for child_no: 0
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 4dd2qjxbjav6u, child number 0
-------------------------------------
select avg(pk_col) from kso.little_skew where col1 = 99320
Plan hash value: 2709260180
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 4 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| LITTLE_SKEW | 1 | 9 | 4 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | LITTLE_SKEW_COL1 | 1 | | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("COL1"=99320)
20 rows selected.
SQL> !cat create_baseline.sql
var ret number
exec :ret := dbms_spm.load_plans_from_cursor_cache(-
sql_id=>'&sql_id', -
plan_hashvalue=>&plan_hash_value,-
fixed=>'&fixed');
SQL> @create_baseline
Enter value for sql_id: 4dd2qjxbjav6u
Enter value for plan_hash_value: 2709260180
Enter value for fixed: NO
PL/SQL procedure successfully completed.
SQL> !cat baselines.sql
set lines 155
col sql_text for a50 trunc
col last_executed for a28
col enabled for a7
col plan_hash_value for a16
col last_executed for a16
select spb.sql_handle, spb.plan_name, spb.sql_text,
spb.enabled, spb.accepted, spb.fixed,
to_char(spb.last_executed,'dd-mon-yy HH24:MI') last_executed
from
dba_sql_plan_baselines spb
where spb.sql_text like nvl('%'||'&sql_text'||'%',spb.sql_text)
and spb.sql_handle like nvl('&name',spb.sql_handle)
and spb.plan_name like nvl('&plan_name',spb.plan_name)
/
SQL> @baselines
Enter value for sql_text: %little%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_b77256b604589dec SYS_SQL_PLAN_04589dec2fdfd157 select avg(pk_col) from kso.little_skew where col1 YES YES NO
SQL> select avg(pk_col) from kso.little_skew where col1 = 99320;
AVG(PK_COL)
-----------
628922
SQL> select avg(pk_col) from kso.little_skew where col1 = 99320;
AVG(PK_COL)
-----------
628922
SQL> @find_sql
Enter value for sql_text:
Enter value for sql_id: 4dd2qjxbjav6u
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
4dd2qjxbjav6u 0 2709260180 1 .02 89 select avg(pk_col) from kso.little_skew where col1 = 99320
4dd2qjxbjav6u 1 2709260180 1 .00 5 select avg(pk_col) from kso.little_skew where col1 = 99320
SQL> @dplan
Enter value for sql_id: 4dd2qjxbjav6u
Enter value for child_no: 1
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 4dd2qjxbjav6u, child number 1
-------------------------------------
select avg(pk_col) from kso.little_skew where col1 = 99320
Plan hash value: 2709260180
-------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 4 (100)| |
| 1 | SORT AGGREGATE | | 1 | 9 | | |
| 2 | TABLE ACCESS BY INDEX ROWID| LITTLE_SKEW | 1 | 9 | 4 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | LITTLE_SKEW_COL1 | 1 | | 3 (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("COL1"=99320)
Note
-----
- SQL plan baseline SYS_SQL_PLAN_04589dec2fdfd157 used for this statement
24 rows selected. |
So that’s a quick example of how easy it is to create a Baseline. Once a Baseline is created it behaves much like an Outline or SQL Profile, in that it will be applied to any SQL statement where the normalized text matches. It’s interesting to note that even though Outlines are way outdated in 11g, they still take precedence over Baselines. So if you have an Outline and a Baseline on the same statement, the Outline will be used. Here’s an example of that behavior:
SQL> @outlines
Enter value for category:
Enter value for name:
CATEGORY NAME USED ENABLED HINTS SQL_TEXT
--------------- ------------------------------ ------------ ------------ ------ ----------------------------------------------------------------------
DEFAULT KSOTEST1 USED ENABLED 6 select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
SQL> select /* test1 */ avg(pk_col) from kso.skew where col1 > 0;
AVG(PK_COL)
-----------
16093749.3
SQL> @find_sql
Enter value for sql_text: select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
Enter value for sql_id:
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
0r09xac72gvj1 0 568322376 3 9.97 162,297 select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
SQL> @dplan
Enter value for sql_id: 0r09xac72gvj1
Enter value for child_no: 0
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 0r09xac72gvj1, child number 0
-------------------------------------
select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
Plan hash value: 568322376
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 44497 (100)| |
| 1 | SORT AGGREGATE | | 1 | 11 | | |
|* 2 | TABLE ACCESS FULL| SKEW | 32M| 335M| 44497 (2)| 00:08:54 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("COL1">0)
Note
-----
- outline "KSOTEST1" used for this statement
23 rows selected.
SQL> @baselines
Enter value for sql_text: %test1%
Enter value for name:
Enter value for plan_name:
no rows selected
SQL> @create_baseline
Enter value for sql_id: 0r09xac72gvj1
Enter value for plan_hash_value: 568322376
Enter value for fixed: YES
PL/SQL procedure successfully completed.
SQL> @baselines
Enter value for sql_text: %test1%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_661b59bf2029dc56 SYS_SQL_PLAN_2029dc5655381d08 select /* test1 */ avg(pk_col) from kso.skew where YES YES YES
SQL> select /* test1 */ avg(pk_col) from kso.skew where col1 > 0;
AVG(PK_COL)
-----------
16093749.3
SQL> @baselines
Enter value for sql_text: %test1%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_661b59bf2029dc56 SYS_SQL_PLAN_2029dc5655381d08 select /* test1 */ avg(pk_col) from kso.skew where YES YES YES
SQL> -- note that the last_executed column is still blank, so it wasn't used
SQL>
SQL> @find_sql
Enter value for sql_text: %test1%
Enter value for sql_id:
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
0r09xac72gvj1 0 568322376 4 10.00 162,297 select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
SQL> - -and the existing cursor is still there with one more execution
SQL> -- just to verify let's check the plan
SQL>
SQL> @dplan
Enter value for sql_id: 0r09xac72gvj1
Enter value for child_no: 0
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 0r09xac72gvj1, child number 0
-------------------------------------
select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
Plan hash value: 568322376
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 44497 (100)| |
| 1 | SORT AGGREGATE | | 1 | 11 | | |
|* 2 | TABLE ACCESS FULL| SKEW | 32M| 335M| 44497 (2)| 00:08:54 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("COL1">0)
Note
-----
- outline "KSOTEST1" used for this statement
23 rows selected.
SQL> -- so Outlines do take precedence over Baselines
SQL> -- let's try disabling the Outline
SQL>
SQL> @outlines
Enter value for category:
Enter value for name:
CATEGORY NAME USED ENABLED HINTS SQL_TEXT
--------------- ------------------------------ ------------ ------------ ------ ----------------------------------------------------------------------
DEFAULT KSO1 USED ENABLED 6 select /*+ full(a) */ avg(pk_col) from kso.little_skew a where col1 =
DEFAULT KSOTEST1 USED ENABLED 6 select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
SQL> alter outline &outline_name disable;
Enter value for outline_name: KSOTEST1
Outline altered.
SQL> select /* test1 */ avg(pk_col) from kso.skew where col1 > 0;
AVG(PK_COL)
-----------
16093749.3
SQL> @find_sql
Enter value for sql_text: %test1%
Enter value for sql_id:
no rows selected
SQL> -- quirky behavior - no record of the first execution with the baseline ???
SQL> -- let's try it again
SQL>
SQL> select /* test1 */ avg(pk_col) from kso.skew where col1 > 0;
AVG(PK_COL)
-----------
16093749.3
SQL> @find_sql
Enter value for sql_text: %test1%
Enter value for sql_id:
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
0r09xac72gvj1 0 568322376 1 9.57 162,297 select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
SQL> -- Now it's here but shows only one execution (not sure what happened on the first execution)
SQL> -- Let's check the plan
SQL>
SQL> @dplan
Enter value for sql_id: 0r09xac72gvj1
Enter value for child_no: 0
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 0r09xac72gvj1, child number 0
-------------------------------------
select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
Plan hash value: 568322376
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 44497 (100)| |
| 1 | SORT AGGREGATE | | 1 | 11 | | |
|* 2 | TABLE ACCESS FULL| SKEW | 32M| 335M| 44497 (2)| 00:08:54 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("COL1">0)
Note
-----
- SQL plan baseline SYS_SQL_PLAN_2029dc5655381d08 used for this statement
23 rows selected.
SQL> -- using the Baseline now
SQL> @baselines
Enter value for sql_text: %test1%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_661b59bf2029dc56 SYS_SQL_PLAN_2029dc5655381d08 select /* test1 */ avg(pk_col) from kso.skew where YES YES YES 13-apr-09 10:31
SQL> -- and the Baseline table now shows the last execution time as expected |
So being able to create a Baseline on an existing statement is useful, but being able to create a Baseline on one statement, and then attach it to a different statement is awesome. That gives us the ability to manipulate the text of the SQL statement in order to get the plan we want. Outlines can be tricked into doing something similar by swapping hints between statements. SQL Profiles also can do something similar by using a trick to load the hints from one statement into another (see my previous post for an example). But Baselines have the built-in ability to do this. No tricks, no sneaky stuff, just call the procedure directly and it’s done. Of course the optimizer will have to verify that the plan will work for the statement you attach it to. If it fails this validation step, then the optimizer will ignore it and go on about it’s normal business. Here’s an example:
SQL> @flush_pool
System altered.
SQL> select avg(pk_col) from kso.little_skew where col1 > 0;
AVG(PK_COL)
-----------
320093.5
SQL> @find_sql
Enter value for sql_text: %avg%little%
Enter value for sql_id:
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
a2h75xrkn1xh3 0 3498336203 1 .26 3,366 select avg(pk_col) from kso.little_skew where col1 > 0
SQL> @dplan
Enter value for sql_id: a2h75xrkn1xh3
Enter value for child_no: 0
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID a2h75xrkn1xh3, child number 0
-------------------------------------
select avg(pk_col) from kso.little_skew where col1 > 0
Plan hash value: 3498336203
----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | | 907 (100)| |
| 1 | SORT AGGREGATE | | 1 | 10 | | |
|* 2 | TABLE ACCESS FULL| LITTLE_SKEW | 640K| 6251K| 907 (2)| 00:00:11 |
----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("COL1">0)
19 rows selected.
SQL> select /*+ index (a LITTLE_SKEW_COL1) */ avg(pk_col) from kso.little_skew a where col1 > 0;
AVG(PK_COL)
-----------
320093.5
SQL> @find_sql
Enter value for sql_text: %avg%little%
Enter value for sql_id:
SQL_ID CHILD PLAN_HASH EXECS AVG_ETIME AVG_LIO SQL_TEXT
------------- ------ ---------- ---------- ------------- ------------ ------------------------------------------------------------
3trqfzj8yhu6j 0 2709260180 1 2.53 580,280 select /*+ index (a LITTLE_SKEW_COL1) */ avg(pk_col) from ks
o.little_skew a where col1 > 0
a2h75xrkn1xh3 0 3498336203 1 .26 3,366 select avg(pk_col) from kso.little_skew where col1 > 0
SQL> @create_baseline
Enter value for sql_id: a2h75xrkn1xh3
Enter value for plan_hash_value: 3498336203
Enter value for fixed: NO
PL/SQL procedure successfully completed.
SQL> @baselines
Enter value for sql_text: select avg(pk_col) from kso.little_skew where col1 > 0
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_6560e8852671e3e3 SYS_SQL_PLAN_2671e3e38b1420d2 select avg(pk_col) from kso.little_skew where col1 YES YES NO
SQL> !cat create_baseline2.sql
var ret number
exec :ret := dbms_spm.load_plans_from_cursor_cache(-
sql_id=>'&sql_id', -
plan_hashvalue=>&plan_hash_value,-
sql_handle=>'&sql_handle',-
fixed=>'&fixed');
SQL> @create_baseline2
Enter value for sql_id: 3trqfzj8yhu6j
Enter value for plan_hash_value: 2709260180
Enter value for sql_handle: SYS_SQL_6560e8852671e3e3
Enter value for fixed: YES
PL/SQL procedure successfully completed.
SQL> @baselines
Enter value for sql_text: select avg(pk_col) from kso.little%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_6560e8852671e3e3 SYS_SQL_PLAN_2671e3e32fdfd157 select avg(pk_col) from kso.little_skew where col1 YES YES YES
SYS_SQL_6560e8852671e3e3 SYS_SQL_PLAN_2671e3e38b1420d2 select avg(pk_col) from kso.little_skew where col1 YES YES NO
SQL> -- since we made the one we added a fixed plan, it will take precedence
SQL>
SQL> |
Baselines Do Have a Few Quirks:
- The first execution of a statement using a Baseline appears to be immediately discarded from the shared pool (see the examples above). Not that big of deal since it’s only the 1st execution after the Baseline is created, but it’s a little weird.
- If a FIXED Baseline cannot be validated, NON-FIXED Baselines will NOT be used. That is to say, if a FIXED Baseline exists, and it cannot be validated, any NON-FIXED Baselines will be ignored. Instead, the normal optimizer costing calculations will kick in to create a new plan. (By the way, validation includes checking to make sure that the plan_hash_value arrived at by applying the hints associated with the Baseline matches the original plan_hash_value. Therefore, a different plan than the original cannot be used – unlike with Outlines and SQL Profiles).
- The LAST_EXECUTED field in DBA_SQL_PLAN_BASLINES appears to be updated somewhat inconsistently, so I wouldn’t rely on it.
- DBA_SQL_PLAN_BASELINES has a PLAN_ID field, but is not the same as the one in V$SQL. It would be handy to be able to relate the 2 plan hash values, but I haven’t figured out a way to do that yet (any ideas?).
- You can rename a Baseline. But then it won’t work! ???? Here’s an example:
SQL> @baselines
Enter value for sql_text: %test1%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_661b59bf2029dc56 SYS_SQL_PLAN_2029dc5655381d08 select /* test1 */ avg(pk_col) from kso.skew where YES YES YES
SQL> !cat alter_baseline.sql
-- Used to set attributes
--
-- FIXED - YES/NO
-- ENABLED - YES/NO
-- PLAN_NAME - doesn't work in 11.1.0.7
var ret number
exec :ret := dbms_spm.alter_sql_plan_baseline(-
sql_handle=>'&sql_handle',-
plan_name=>'&plan_name',-
attribute_name=>'&attribute_name',-
attributevalue=>'&attribute_value');
SQL> @alter_baseline
Enter value for sql_handle: SYS_SQL_661b59bf2029dc56
Enter value for plan_name: SYS_SQL_PLAN_2029dc5655381d08
Enter value for attribute_name: PLAN_NAME
Enter value for attribute_value: TEST1
PL/SQL procedure successfully completed.
SQL> @baselines
Enter value for sql_text: %test1%
Enter value for name:
Enter value for plan_name:
SQL_HANDLE PLAN_NAME SQL_TEXT ENABLED ACC FIX LAST_EXECUTED
------------------------------ ------------------------------ -------------------------------------------------- ------- --- --- ----------------
SYS_SQL_661b59bf2029dc56 TEST1 select /* test1 */ avg(pk_col) from kso.skew where YES YES YES
SQL> select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
2 /
select /* test1 */ avg(pk_col) from kso.skew where col1 > 0
*
ERROR at line 1:
ORA-38141: SQL plan baseline SYS_SQL_PLAN_2029dc5655381d08 does not exist
SQL> -- error caused by renaming a baseline
SQL> -- so it lets you rename them, but then the statements fail |
So Baselines seem to be a definite step in the right direction. Although I must say that at this point they are not very clearly documented. It does appear from looking at 10053 trace data that the complete optimization path is carried out, even though the optimizer recognizes that a Baseline exists for the statement. It seems like a Fixed Baseline would short circuit this process and just say, “I have a plan, let’s use it”. But it appears that the optimizer wants to evaluate other plans, probably in an attempt to add other opportunities to evolve a better plan.
I haven’t found a lot of information on Baselines on the web, but there is a pretty good post on them on the OptimizerMagic blog.
awesome write-ups.
I much enjoyed them because it was on my list of tasks to look at using the old outline swap to use hints on a statement for profiles and baselines. Baselines seem like a solid solution for most statements when hints are beneficial because of the evolve option.
Curious if there is any reason you see to use profiles over outlines.
Kyle,
Thanks for the kind words. Interesting question. I had an online dialog with a guy name Randolf Geist a year ago or so where he convinced me to look closer at SQL Profiles. Up until that point I had only seen SQL Profiles that were created by the SQL Tuning Advisor and I was not very impressed. They tended to go sour after a little time because of the OPT_ESTIMATE hint, but I digress. The answer is yes, I think SQL Profiles have a couple of advantages over Outlines at this point.
1. They have the ability to be used on multiple statements (ones that are the same except for literals) using the force matching signature bit.
2. They are newer. I’m starting to worry about Outlines getting buggy due to lack of attention. Documentation says they are deprecated and although they continue to work in 11g, I am a bit leery of continuing to use them at this point. (the whole hint based mechanism is tricky enough as it is).
11gR2 includes a procedure to migrate Outlines to Baselines by the way (DBMS_SPM.MIGRATE_STORED_OUTLINE). At any rate, I am not implementing Outlines at this point.
Kerry
I checked all Oracle documentation for the following error:
ORA-38141: SQL plan baseline SYS_SQL_PLAN_2029dc5655381d08 does not exist
I have verified that this baseline plan does not exist.
Could you please let me know how to resolve this error?
That error occurred when I altered the baseline (changing it’s name) and then tried to execute a statement which was trying to use the baseline. So, if I understand your question, why might you see this in the wild, I would guess that someone altered the baseline changing it’s name. To find which baseline has been changed you might look for baselines with unusual names, or ones that have been modified recently (see last_modified column in dba_sql_plan_baselines).
Kerry
Hello Osborne, I am using Oracle Database 11.1.0.7 on AIX
We have a congnos application installed and this morning they got the following error in their application log. This is turning out to be a show stopper.
Error Messages from Cognos:
Unable to execute the statement.
The SQL was:
SELECT object_name FROM sys.all_objects WHERE (object_type = ‘TABLE’) AND (owner = ‘MYSCHEMA’)
ORA-38141: SQL plan baseline SYS_SQL_PLAN_95dde1aac49074c9 does not exist
When I cehck the DBA_SQL_PLAN_BASELINE, there is no SQL Handel like SYS_SQL_PLAN_95dde1aac49074c9 and this confirms the message is right.
But when I run the same query -
SELECT object_name FROM sys.all_objects WHERE (object_type = ‘TABLE’) AND (owner = ‘MYSCHEMA’);
it works fine but when it runs through cognos, it throws the above mentioned error message. Not sure why it is looking for this baseline when it’s not there in the table itself.
I don’t think anybody changed the PLAN_NAME as all the plan name looks like as if it was generated by Oracle (SYS_SQL_PLAN_xxxxxxxxxxxxxxxx).
Thanks in advance for giving your valuable time to this mystery.
Sorry – I don’t know the answer to that one – better open an SR
Kerry
I have the same issue as RK except with Toad and I upgraded the database itself from 11.1.0.7 to 11.2.0.2. ORA-38141 seemed to pop-up after that.
Also I noticed all plan names began with “SYS_SQL_PLAN” but a few of them started with “SQL_PLAN_”. When and why would the names be different? I deleted the few plans that started with “SQL_PLAN_” and the ORA-38141 errors disappeared.
[...] Osborne 1. Do SQL Plan Baselines use hints (pt.1) 2. Do SQL Plan Baselines use hints (pt. 2) 3. SQL Plan Management – SQL Plan Baselines 4. Why isn’t Oracle using my outline/ SQL profile / SQL Plan Baseline 5. Dynamic Sampling and [...]
Thanks for what I’m sure will be a useful post. Your other posts re sql profiles and about unstable plans have been very useful to me.
DBA_ADVISOR_SQLSTATS has both plan_id and plan_hash_value columns. I haven’t tried it, so this is a wild guess, but maybe if you created a plan baseline for your query that would give it a plan_id, and then if you ran sql tuning advisor on your query you’d end up seeing both columns populated in dba_advisor_sqlstats. Just maybe.
Thanks Paul,
I’ll give that a look when I have a few minutes.
Kerry
Kerry,
Above you say “I’m starting to worry about Outlines getting buggy due to lack of attention.”. It seems though that SPM is using outlines to (re)generate the plans. The data in sys.sqlobj$data.comp_data is basically an outline. It has the same info you would get if you run “select * from table(dbms_xplan.display_cursor(‘&sqlid’,&child_number,’ADVANCED’))” and look at the outline data.
Thought that was interesting… Maybe you don’t need to worry :-)
Jan
Jan,
Yeah, it’s very closely related. The Outline object consists of a set of hints that is applied to a statement and is similar to a SQL Profile and a Baseline. But they are stored in different ways and presumably use different code paths (although some of the code may be the same). At any rate, Outlines have been documented as deprecated since 11.1 (from the 11gR2 Database Upgrade Guide – “The use of stored outlines is deprecated in Oracle Database 11g Release 1 (11.1).”) I’d definitely prefer to use SQL Profiles or Baselines at this point.
The ADVANCED option on dbms_xplan.display_cursor spits out the content of the OTHER_XML field in V$SQL_PLAN. It is the set of hints that Oracle thinks are necessary to recreate the plan. I’ve written about that somewhere on this blog.
Kerry
[...] to Kerry Osborne again, and his article about SQL Plan Baselines. He (and others) write in detail about how and what SQL Plan Baselines are, but in essence it lets [...]
[...] the stored SPB http://kerryosborne.oracle-guy.com/2009/04/do-sql-plan-baselines-use-hints-take-2/ http://kerryosborne.oracle-guy.com/2009/04/oracle-11g-sql-plan-management-sql-plan-baselines/ GA_googleAddAttr("AdOpt", "1"); GA_googleAddAttr("Origin", "other"); GA_googleAddAttr("theme_bg", [...]
Hi Kerry,
here’s the connex between dba_sql_plan_baselines and v$sql:
select s.*, b.*
from dba_sql_plan_baselines b,
v$sql s
where s.exact_matching_signature = b.signature
and s.sql_plan_baseline = b.plan_name;
Thx 4 your great Blog.
Hi Kerry, does a stored outline still take precedence over baselines in Oracle 11gR2? Thanks.
Yes,
Outlines override Baselines even in 11gR2 (at least as of 11.2.0.2).
Thanks Kerry. I did a similar test in 11.2.0.2. It is true that outlines still overrides baselines as of this version.
[...] Oracle 11g SQL Plan Management – SQL Plan Baselines [...]