Oracle Scratchpad

February 24, 2020

Fake Baselines – 2

Filed under: Bugs,dbms_xplan,Execution plans,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 3:38 pm GMT Feb 24,2020

Many years ago (2011) I wrote a note describing how you could attach the Outline Information from one query to the SQL_ID of another query using the official Oracle mechanism of calling dbms_spm.load_plans_from_cursor_cache(). Shortly after publishing that note I drafted a follow-up note with an example demonstrating that even when the alternative outline was technically relevant the optimizer might still fail to use the SQL Plan Baseline. Unfortunately I didn’t quite finish the draft – until today.

The example I started with nearly 10 years ago behaved correctly against 11.1.0.7, but failed to reproduce the plan when I tested it against 11.2.0.3, and it still fails against 19.3.0.0. Here’s the test data and the query we’re going to attempt to manipulate:


rem
rem     Script:         fake_sql_baseline_4.sql
rem     Author:         Jonathan Lewis
rem     Dated:          Oct 2010
rem

create table emp1 (
        dept_no         number /* not null */,
        sal             number,
        emp_no          number,
        padding         varchar2(200),
        constraint e1_pk primary key(emp_no)
)
;

create table emp2 (
        dept_no         number /* not null */,
        sal             number,
        emp_no          number,
        padding         varchar2(200),
        constraint e2_pk primary key(emp_no)
)
;

insert into emp1
select
        mod(rownum,6),
        rownum,
        rownum,
        rpad('x',200)
from
        all_objects
where
        rownum <= 20000 -- > comment to avoid wordpress format issue
;

insert into emp2
select
        mod(rownum,6),
        rownum,
        rownum,
        rpad('x',200)
from
        all_objects
where
        rownum <= 20000 -- > comment to avoid wordpress format issue
;

begin
        dbms_stats.gather_table_stats(
                ownname                 => user,
                tabname                 => 'EMP1',
                cascade                 => true,
                method_opt              =>'for all columns size 1'
        );

        dbms_stats.gather_table_stats(
                ownname                 => user,
                tabname                 => 'EMP2',
                cascade                 => true,
                method_opt              =>'for all columns size 1'
        );
end;
/

select
        /*+ target_query */
        count(*)
from
        emp1
where
        emp1.dept_no not in (
                select  dept_no
                from    emp2
        )
;

select * from table(dbms_xplan.display_cursor(null, null, 'outline'));

I haven’t included the code I run on my testbed to delete all existing SQL Plan Baselines before running this test, I’ll post that at the end of the article.

The query is very simple and will, of course, return no rows since emp1 and emp2 are identical and we’re looking for departments in emp1 that don’t appear in emp2. The “obvious” plan for the optimizer is to unnest the subquery into a distinct (i.e. aggregate) inline view then apply an anti-join. It’s possible that the optimizer will also decide to do complex view merging and postpone the aggregation. Here’s the execution plan from 19.3:


----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |       |       |   168 (100)|          |
|   1 |  SORT AGGREGATE     |      |     1 |     6 |            |          |
|*  2 |   HASH JOIN ANTI NA |      |  3333 | 19998 |   168   (5)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| EMP1 | 20000 | 60000 |    83   (4)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| EMP2 | 20000 | 60000 |    83   (4)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("EMP1"."DEPT_NO"="DEPT_NO")

Hint Report (identified by operation id / Query Block Name / Object Alias):

Total hints for statement: 1 (E - Syntax error (1))
---------------------------------------------------------------------------
   0 -  SEL$1
         E -  target_query

As expected the subquery unnested, we have the anti-join (in this case, since dept_no can be null, it’s a “Null-Aware” antijoin); and the optimizer has, indeed, decided to do the join before the aggregation.

Assume, now, that for reasons known only to me a merge (anti-)join would be more effective than a hash join. To get the optimizer to do this I’m going to capture the query and connect it to a plan that uses a merge join. There are several minor variations on how we could do this, but I’m going to follow the steps I took in 2011 – but cut out a couple of the steps where I loaded redundant baselines into the SMB (SQLPlan Management Base). As a starting point I’ll just record the sql_id and plan_hash_value for the query (and the child_number just in case I want to use dbms_xplan.display_cursor() to report the in-memory execution plan):

column  sql_id                  new_value       m_sql_id_1
column  plan_hash_value         new_value       m_plan_hash_value_1
column  child_number            new_value       m_child_number_1

select
        sql_id, plan_hash_value, child_number
from
        v$sql
where
        sql_text like '%target_query%'
and     sql_text not like '%v$sql%'
and     rownum = 1
;

Now I’ll hack the query to produce a plan that does the merge join. An easy first step is to look at the current outline and take advantage of the hints there. You’ll notice I included the ‘outline’ format in my call to dbms_xplan.display_cursor() above, even though I didn’t show you that part of the output – here it is now:

Outline Data
-------------
  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('19.1.0')
      DB_VERSION('19.1.0')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$5DA710D3")
      UNNEST(@"SEL$2")
      OUTLINE(@"SEL$1")
      OUTLINE(@"SEL$2")
      FULL(@"SEL$5DA710D3" "EMP1"@"SEL$1")
      FULL(@"SEL$5DA710D3" "EMP2"@"SEL$2")
      LEADING(@"SEL$5DA710D3" "EMP1"@"SEL$1" "EMP2"@"SEL$2")
      USE_HASH(@"SEL$5DA710D3" "EMP2"@"SEL$2")
      END_OUTLINE_DATA
  */

So I’m going to take the useful-looking hints, get rid of the use_hash() hint and, for good measure, turn it into a no_use_hash() hint. Here’s the resulting query, with its execution plan:

select
        /*+
                unnest(@sel$2)
                leading(@sel$5da710d3 emp1@sel$1 emp2@sel$2)
                no_use_hash(@sel$5da710d3 emp2@sel$2)
                full(@sel$5da710d3 emp2@sel$2)
                full(@sel$5da710d3 emp1@sel$1)
                alternate_query
        */
        count(*)
from
        emp1
where
        emp1.dept_no not in (
                select  dept_no
                from    emp2
        )
;

-----------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |       |       |   178 (100)|          |
|   1 |  SORT AGGREGATE      |      |     1 |     6 |            |          |
|   2 |   MERGE JOIN ANTI NA |      |  3333 | 19998 |   178  (11)| 00:00:01 |
|   3 |    SORT JOIN         |      | 20000 | 60000 |    89  (11)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| EMP1 | 20000 | 60000 |    83   (4)| 00:00:01 |
|*  5 |    SORT UNIQUE       |      | 20000 | 60000 |    89  (11)| 00:00:01 |
|   6 |     TABLE ACCESS FULL| EMP2 | 20000 | 60000 |    83   (4)| 00:00:01 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   5 - access("EMP1"."DEPT_NO"="DEPT_NO")
       filter("EMP1"."DEPT_NO"="DEPT_NO")

Outline Data
-------------
  /*+
      BEGIN_OUTLINE_DATA
      IGNORE_OPTIM_EMBEDDED_HINTS
      OPTIMIZER_FEATURES_ENABLE('19.1.0')
      DB_VERSION('19.1.0')
      ALL_ROWS
      OUTLINE_LEAF(@"SEL$5DA710D3")
      UNNEST(@"SEL$2")
      OUTLINE(@"SEL$1")
      OUTLINE(@"SEL$2")
      FULL(@"SEL$5DA710D3" "EMP1"@"SEL$1")
      FULL(@"SEL$5DA710D3" "EMP2"@"SEL$2")
      LEADING(@"SEL$5DA710D3" "EMP1"@"SEL$1" "EMP2"@"SEL$2")
      USE_MERGE(@"SEL$5DA710D3" "EMP2"@"SEL$2")
      END_OUTLINE_DATA
  */

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (E - Syntax error (1))
---------------------------------------------------------------------------
   0 -  SEL$1
         E -  alternate_query

Note that I’ve included the text “alternative_query” at the end of the hint list as something to use when I’m searaching v$sql. Note also, that the “no_use_hash()” hint has disappeared and been replaced by “use_merge()” hint.

The plan tells us that the optimizer is happy to use a “merge join anti NA”, so we can load this plan’s outline into the SMB by combining the sql_id and plan_hash_value for this query with (for older versions of Oracle, though you can now use the sql_id in recent versions) the text of the previous query so that we can store the old text with the new plan.


column  sql_id                  new_value       m_sql_id_2
column  plan_hash_value         new_value       m_plan_hash_value_2
column  child_number            new_value       m_child_number_2

select
        sql_id, plan_hash_value, child_number
from
        v$sql
where
        sql_text like '%alternate_query%'
and     sql_text not like '%v$sql%'
and     rownum = 1
;

declare
        m_clob  clob;
begin
        select
                sql_fulltext
        into
                m_clob
        from
                v$sql
        where
                sql_id = '&m_sql_id_1'
        and     child_number = &m_child_number_1
        ;

        dbms_output.put_line(m_clob);

        dbms_output.put_line(
                'Number of plans loaded: ' ||
                dbms_spm.load_plans_from_cursor_cache(
                        sql_id                  => '&m_sql_id_2',
                        plan_hash_value         => &m_plan_hash_value_2,
                        sql_text                => m_clob,
                        fixed                   => 'YES',
                        enabled                 => 'YES'
                )
        );

end;
/

At this point we have one SQL Plan Baseline in the SMB, and it says the old query should execute usng the new plan. So let’s give it a go:

set serveroutput off
alter system flush shared_pool;

alter session set events '10053 trace name context forever';

select
        /*+ target_query */
        count(*)
from
        emp1
where
        emp1.dept_no not in (
                select  dept_no
                from    emp2
        )
/

alter session set events '10053 trace name context off';

select * from table(dbms_xplan.display_cursor(null, null, 'alias outline'));

I’ve enabled the 10053 (optimizer) trace so that I can report a critical few lines from it later on. Here’s the execution plan, omitting the outline but including the alias information.

----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |       |       |   168 (100)|          |
|   1 |  SORT AGGREGATE     |      |     1 |     6 |            |          |
|*  2 |   HASH JOIN ANTI NA |      |  3333 | 19998 |   168   (5)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| EMP1 | 20000 | 60000 |    83   (4)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| EMP2 | 20000 | 60000 |    83   (4)| 00:00:01 |
----------------------------------------------------------------------------

Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$5DA710D3
   3 - SEL$5DA710D3 / EMP1@SEL$1
   4 - SEL$5DA710D3 / EMP2@SEL$2

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("EMP1"."DEPT_NO"="DEPT_NO")

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (E - Syntax error (1))
---------------------------------------------------------------------------
   0 -  SEL$1
         E -  target_query

Note
-----
   - Failed to use SQL plan baseline for this statement

We haven’t used the SQL Plan Baseline – and in 19.3 we even have a note that the optimizer knew there was at least one baseline available that it failed to use! So what went wrong?

I have two diagnostics – first is the content of the baseline itself (warning – the SQL below will report ALL currently saved SQL Plan Baselines); I’ve just made sure that I have only one to report:

set linesize 90

select
        pln.*
from
        (select sql_handle, plan_name
         from   dba_sql_plan_baselines spb
         order by
                sql_handle, plan_name
        ) spb,
        table(dbms_xplan.display_sql_plan_baseline(spb.sql_handle, spb.plan_name)) pln
;


PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------
SQL handle: SQL_ce3099e9e3bdaf2f
SQL text: select         /*+ target_query */         count(*) from         emp1
          where         emp1.dept_no not in (                 select  dept_no
                        from    emp2         )
--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
Plan name: SQL_PLAN_cwc4tx7jvvbtg02bb0c12         Plan id: 45812754
Enabled: YES     Fixed: YES     Accepted: YES     Origin: MANUAL-LOAD-FROM-CURSOR-CACHE
Plan rows: From dictionary
--------------------------------------------------------------------------------

Plan hash value: 1517539632

-----------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      |       |       |   178 (100)|          |
|   1 |  SORT AGGREGATE      |      |     1 |     6 |            |          |
|   2 |   MERGE JOIN ANTI NA |      |  3333 | 19998 |   178  (11)| 00:00:01 |
|   3 |    SORT JOIN         |      | 20000 | 60000 |    89  (11)| 00:00:01 |
|   4 |     TABLE ACCESS FULL| EMP1 | 20000 | 60000 |    83   (4)| 00:00:01 |
|*  5 |    SORT UNIQUE       |      | 20000 | 60000 |    89  (11)| 00:00:01 |
|   6 |     TABLE ACCESS FULL| EMP2 | 20000 | 60000 |    83   (4)| 00:00:01 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   5 - access("EMP1"."DEPT_NO"="DEPT_NO")
       filter("EMP1"."DEPT_NO"="DEPT_NO")

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (E - Syntax error (1))
---------------------------------------------------------------------------
   0 -  SEL$1
         E -  alternate_query

We have an SQL Plan baseline that is accepted, enabled, and fixed; and it’s supposed to produce a “merge join anti NA”, and it clearly “belongs” to our query. So it should have been used.

Then we have the 10053 trace file, in which we find the following:


SPM: planId in plan baseline = 45812754, planId of reproduced plan = 1410137244
------- START SPM Plan Dump -------
SPM: failed to reproduce the plan using the following info:
  parse_schema name        : TEST_USER
  plan_baseline signature  : 14857544400522555183
  plan_baseline plan_id    : 45812754
  plan_baseline hintset    :
    hint num  1 len 27 text: IGNORE_OPTIM_EMBEDDED_HINTS
    hint num  2 len 35 text: OPTIMIZER_FEATURES_ENABLE('19.1.0')
    hint num  3 len 20 text: DB_VERSION('19.1.0')
    hint num  4 len  8 text: ALL_ROWS
    hint num  5 len 29 text: OUTLINE_LEAF(@"SEL$5DA710D3")
    hint num  6 len 16 text: UNNEST(@"SEL$2")
    hint num  7 len 17 text: OUTLINE(@"SEL$1")
    hint num  8 len 17 text: OUTLINE(@"SEL$2")
    hint num  9 len 36 text: FULL(@"SEL$5DA710D3" "EMP1"@"SEL$1")
    hint num 10 len 36 text: FULL(@"SEL$5DA710D3" "EMP2"@"SEL$2")
    hint num 11 len 54 text: LEADING(@"SEL$5DA710D3" "EMP1"@"SEL$1" "EMP2"@"SEL$2")
    hint num 12 len 41 text: USE_MERGE(@"SEL$5DA710D3" "EMP2"@"SEL$2")

During optimization the optimizer has found that SQL Plan Baseline. We can see that the hints in the baseline are exactly the hints from the plan that we wanted – but the optimizer says it can’t reproduce the plan we wanted. In fact if you try adding exactly these hints to the query itself you’ll still find that the merge join won’t appear and Oracle will use a hash join.

Conclusion

This is just a simple example of how the optimizer may be able to produce a plan if hinted in one way, but the outline consists of a different set of hints that won’t reproduce the plan they describe. My no_use_hash() has turned into a use_merge() but that hint fails to reproduce the merge join in circumstances that makes me think there’s a bug in the optimizer.

If you happen to be unlucky you may find that the plan you really need to see can’t be forced through a SQL Plan Baseline. In this example it may be necessary to use the SQL Patch mechanism to include the no_use_hash() hint in a set of hints that I associate with the query.

 

June 3, 2019

Ignoring Hints

Filed under: 18c,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 8:36 am BST Jun 3,2019

One of the small changes (and, potentially big but temporary, threats) in 18.3 is the status of the “ignore hints” parameter. It ceases to be a hidden (underscore) parameter so you can now officially set parameter optimizer_ignore_hints to true in the parameter file, or at the system level, or at the session level. The threat, of course, it that some of your code may use the hidden version of the parameter (perhaps in an SQL_Patch as an opt_param() option rather than in its hint form) which no longer works after the upgrade.

But there’s more. The parameter (whether the old hidden version or the new revealed version) doesn’t make the optimizer ignore parallel() hints. But 18.3 now has a related parameter optimizer_ignore_parallel_hints to address this limitation. Here’s a quick demo – we start by creating a table and then running a query where the full tablescan is clearly the default strategy that the optimizer would take if we didn’t hint an indexed access path:

rem
rem     Script:         ignore_parallel_hints.sql
rem     Author:         Jonathan Lewis
rem     Dated:          May 2019
rem 

create table t1
as
with generator as (
        select 
                rownum id
        from dual 
        connect by 
                level <= 1e4 -- > comment to avoid WordPress format issue
)
select
        rownum                          id,
        mod(rownum,10)                  n1,
        lpad(rownum,10,'0')             v1,
        lpad('x',100,'x')               padding
from
        generator       v1,
        generator       v2
where
        rownum <= 1e4 -- > comment to avoid WordPress format issue
;

create index t1_i1 on t1(id);

set serveroutput off

prompt  =============
prompt  Baseline test
prompt  =============

select
        /*+ index(t1) */
        n1, sum(id)
from
        t1
where
        id > 0
group by
        n1
order by
        n1
;

select * from table(dbms_xplan.display_cursor);


SQL_ID  gudnnk7j7q5bz, child number 0
-------------------------------------
select  /*+ index(t1) */  n1, sum(id) from  t1 where  id > 0 group by
n1 order by  n1

Plan hash value: 356059923

----------------------------------------------------------------------------------------------
| Id  | Operation                            | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                     |       |       |       |   198 (100)|          |
|   1 |  SORT GROUP BY                       |       |    10 |    70 |   198   (2)| 00:00:01 |
|   2 |   TABLE ACCESS BY INDEX ROWID BATCHED| T1    | 10000 | 70000 |   196   (1)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN                  | T1_I1 | 10000 |       |    22   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   3 - access("ID">0)


Now we repeat the exercise with the version-specific “alter session” command below – and you should try each option with each version of Oracle if you want to do the complete test cycle – to see that the session will ignore hints and the plan will change (side note – using the underscore version  with 18.3 doesn’t raise an error, the statement is silently ignored):


alter session set "_optimizer_ignore_hints" = true;
alter session set "optimizer_ignore_hints" = true;

SQL_ID  gudnnk7j7q5bz, child number 1
-------------------------------------
select  /*+ index(t1) */  n1, sum(id) from  t1 where  id > 0 group by
n1 order by  n1

Plan hash value: 3946799371

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |       |       |    27 (100)|          |
|   1 |  SORT GROUP BY     |      |    10 |    70 |    27  (12)| 00:00:01 |
|*  2 |   TABLE ACCESS FULL| T1   | 10000 | 70000 |    25   (4)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("ID">0)

Then, of course, we have to test a query with a parallel() hint – or shared() hint, which is the internal equivalent you will notice occasionally in outlines or the “remote” statement for distributed execution plans – to show that we don’t yet ignore parallel queries – the plans following the code are from 18.3:


select
        /*+ parallel(t1 3) */
        n1, sum(id)
from
        t1
where
        id > 0
group by
        n1
order by
        n1
;

select * from table(dbms_xplan.display_cursor);

alter session set "optimizer_ignore_parallel_hints" = true;

select
        /*+ parallel(t1 3) */
        n1, sum(id)
from
        t1
where
        id > 0
group by
        n1
order by
        n1
;

select * from table(dbms_xplan.display_cursor);




SQL_ID  7jynurdtc48kv, child number 0
-------------------------------------
select  /*+ parallel(t1 3) */  n1, sum(id) from  t1 where  id > 0 group
by  n1 order by  n1

Plan hash value: 2919148568

------------------------------------------------------------------------------------------------------------------
| Id  | Operation                | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |          |       |       |    10 (100)|          |        |      |            |
|   1 |  PX COORDINATOR          |          |       |       |            |          |        |      |            |
|   2 |   PX SEND QC (ORDER)     | :TQ10001 |    10 |    70 |    10  (10)| 00:00:01 |  Q1,01 | P->S | QC (ORDER) |
|   3 |    SORT GROUP BY         |          |    10 |    70 |    10  (10)| 00:00:01 |  Q1,01 | PCWP |            |
|   4 |     PX RECEIVE           |          |    10 |    70 |    10  (10)| 00:00:01 |  Q1,01 | PCWP |            |
|   5 |      PX SEND RANGE       | :TQ10000 |    10 |    70 |    10  (10)| 00:00:01 |  Q1,00 | P->P | RANGE      |
|   6 |       HASH GROUP BY      |          |    10 |    70 |    10  (10)| 00:00:01 |  Q1,00 | PCWP |            |
|   7 |        PX BLOCK ITERATOR |          | 10000 | 70000 |     9   (0)| 00:00:01 |  Q1,00 | PCWC |            |
|*  8 |         TABLE ACCESS FULL| T1       | 10000 | 70000 |     9   (0)| 00:00:01 |  Q1,00 | PCWP |            |
------------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   8 - access(:Z>=:Z AND :Z<=:Z) -- > comment added to avoid wordpress format issue
       filter("ID">0)

Note
-----
   - Degree of Parallelism is 3 because of table property


Session altered.


SQL_ID  7jynurdtc48kv, child number 1
-------------------------------------
select  /*+ parallel(t1 3) */  n1, sum(id) from  t1 where  id > 0 group
by  n1 order by  n1

Plan hash value: 3946799371

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |       |       |    27 (100)|          |
|   1 |  SORT GROUP BY     |      |    10 |    70 |    27  (12)| 00:00:01 |
|*  2 |   TABLE ACCESS FULL| T1   | 10000 | 70000 |    25   (4)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - filter("ID">0)


tl;dr

In 18.3 Oracle exposes the parameter optimizer_ignore_hints – any code using depending on the hidden version of this parameter will no longer behave as expected. 18.3 also introduces optimiser_ignore_parallel_hints to allow you to ignore parallel hints as well.

 

March 8, 2019

Append hint

Filed under: Hints,Ignoring Hints,Infrastructure,Oracle — Jonathan Lewis @ 1:13 pm GMT Mar 8,2019

One of the questions that came up on the CBO Panel Session at the UKOUG Tech2018 conference was about the /*+ append */ hint – specifically how to make sure it was ignored when it came from a 3rd party tool that was used to load data into the database. The presence of the hint resulted in increasing amounts of space in the table being “lost” as older data was deleted by the application which then didn’t reuse the space the inserts always went above the table’s highwater mark; and it wasn’t possible to change the application code.

The first suggestion aired was to create an SQL Patch to associate the hint /*+ ignore_optim_embedded_hints */ with the SQL in the hope that this would make Oracle ignore the append hint. This won’t work, of course, because the append hint is not an optimizer hint, it’s a “behaviour” hint.

There are, however, various things that will make the append hint invalid – sometimes to the great surprise of the people using it. The three things I can think of at present are:

  • row-level triggers
  • enabled foreign key constraints
  • non-unique indexes enforcing unique constraints

It’s been some time since I last ran a detailed check so I’m not going to guarantee that the following claims are still true – if it matters to you then it’s easy enough to create a little test of (say) 10,000 rows inserted into a non-empty, indexed table.

  • Row level triggers turn array processing into single row processing, so there’s a significant increase in redo generation.
  • Non-unique indexes enforcing unique constraints have (small but) potentially undesirable effects on the optimizer and on run-time and may turn array processing into single row processing.
  • Foreign key constraint require checking which may have some impact, but doesn’t turn array processing into single row processing.

Of the three options the foreign key constraint seemed to me to be the best strategy to disable the hint with minimal side effects, so my answer was:

“Create a new table with no data and a primary key constraint; add an invisible column to the target table, index it (online) and add a foreign key constraint from the column to the new table”.

My thinking on this was that the foreign key will disable the append hint. The column will always be null which means it will always satisfy the foreign key constraint without being checked and it won’t introduce any index maintenance overheads. Net effect: /*+ append */ is disabled with no extra overheads.

Footnote

The append hint is also ignored if the table is an Index Organized Table (IOT), but changing a heap table to an IOT is probably not a sensible choice if all you want to do is disable the hint – the potential for unexpected client side anomalies is too high, and if the table is heavily indexed the processing overhead for the secondary indexes could be fairly significant.

If I recall correctly the person asking the question said that the “do nothing” trigger option sounded like something they would be most comfortable with and they’d live with the overheads. I understand the feeling – an invisible column with an invisible index and extra foreign key constraint sounds too close to the boundary where mixing and matching simple features ends up hitting some unexpected behaviour (i.e. a bug).

Update 14th March 2019

A note on twitter has reminded me that distributed transactions introduce another limitation.  The append hint will work across a database link only if the insert is into a local table; the first insert below will accept the  hint, the second will not.


insert /*+ append */ into local_table select * from table@remote_location;

insert /*+ append */ into table@remote_location select * from local_table;

There are a couple more limitations given in the comments, viz: disable table locking, and adding a security policy (VPD / RLS / FGAC / OLS).

May 25, 2017

Parallelism

Filed under: 12c,CBO,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 3:48 pm BST May 25,2017

Headline – if you don’t want to read the note – the /*+ parallel(N) */ hint doesn’t mean a query will use parallel execution, even if there are enough parallel execution server processes to make it possible. The parallel(N) hint tells the optimizer to consider the cost of using parallel execution for each path that it examines, but ultimately the optimizer will still take the lowest cost path (bar the odd few special cases) and that path could turn out to be a serial path.

The likelihood of parallelism appearing for a given query changes across versions of Oracle so you can be fooled into thinking you’re seeing bugs as you test new versions but it’s (almost certainly) the same old rule being applied in different circumstances. Here’s an example – which I’ll start off on 11.2.0.4:


create table t1
segment creation immediate
nologging
as
with generator as (
        select
                rownum id
        from dual
        connect by
                level <= 1e4
)
select
        rownum                          id,
        lpad(rownum,10,'0')             v1,
        lpad('x',100,'x')               padding
from
        generator       v1,
        generator       v2
where
        rownum <= 1e6 ; create index t1_i1 on t1(id); begin dbms_stats.gather_table_stats( ownname => user,
                tabname          =>'T1',
                method_opt       => 'for all columns size 1'
        );
end;
/

set autotrace traceonly explain

select
        count(v1)
from    t1
where   id = 10
;

select
        /*+ parallel(4) */
        count(v1)
from    t1
where   id = 10
;

select
        /*+ parallel(4) full(t1) */
        count(v1)
from    t1
where   id = 10
;

set autotrace off

I haven’t declare the index to be unique, but it clearly could be; and it’s obvious that with 1M rows and about 120M of table a parallel full scan is probably a bad idea to acquire one row (even if you’re running Exadata!). So what do we get for the three plans – I’ll skip the predicate section – when we want to collect one row.


Base plan - unhinted
--------------------------------------------------------------------------------------
| Id  | Operation                    | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |       |     1 |    16 |     4   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE              |       |     1 |    16 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1    |     1 |    16 |     4   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | T1_I1 |     1 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Hinted parallel(4)
--------------------------------------------------------------------------------------
| Id  | Operation                    | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |       |     1 |    16 |     4   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE              |       |     1 |    16 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| T1    |     1 |    16 |     4   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | T1_I1 |     1 |       |     3   (0)| 00:00:01 |
--------------------------------------------------------------------------------------

Hinted parallel(4) and full(t1)
----------------------------------------------------------------------------------------------------------------
| Id  | Operation              | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |          |     1 |    16 |   606   (2)| 00:00:02 |        |      |            |
|   1 |  SORT AGGREGATE        |          |     1 |    16 |            |          |        |      |            |
|   2 |   PX COORDINATOR       |          |       |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM) | :TQ10000 |     1 |    16 |            |          |  Q1,00 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE     |          |     1 |    16 |            |          |  Q1,00 | PCWP |            |
|   5 |      PX BLOCK ITERATOR |          |     1 |    16 |   606   (2)| 00:00:02 |  Q1,00 | PCWC |            |
|*  6 |       TABLE ACCESS FULL| T1       |     1 |    16 |   606   (2)| 00:00:02 |  Q1,00 | PCWP |            |
----------------------------------------------------------------------------------------------------------------

In 11.2.0.4 the optimizer did consider the parallel hint when it appeared on its own – but it has compared the parallel(4) cost of 606 with the serial index cost of 4 and chosen the indexed access path. This is not a case of ignoring the hint, it’s an example of being fooled if you don’t know how the hint is really supposed to work.

But here’s an interesting change that appeared in 12.2 – this time just the plan with the parallel(4) hint on its own:


---------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                               | Name     | Rows  | Bytes | Cost (%CPU)| Time     |    TQ  |IN-OUT| PQ Distrib |
---------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                        |          |     1 |    16 |     4   (0)| 00:00:01 |        |      |            |
|   1 |  SORT AGGREGATE                         |          |     1 |    16 |            |          |        |      |            |
|   2 |   PX COORDINATOR                        |          |       |       |            |          |        |      |            |
|   3 |    PX SEND QC (RANDOM)                  | :TQ10001 |     1 |    16 |            |          |  Q1,01 | P->S | QC (RAND)  |
|   4 |     SORT AGGREGATE                      |          |     1 |    16 |            |          |  Q1,01 | PCWP |            |
|   5 |      TABLE ACCESS BY INDEX ROWID BATCHED| T1       |     1 |    16 |     4   (0)| 00:00:01 |  Q1,01 | PCWP |            |
|   6 |       PX RECEIVE                        |          |     1 |       |     3   (0)| 00:00:01 |  Q1,01 | PCWP |            |
|   7 |        PX SEND HASH (BLOCK ADDRESS)     | :TQ10000 |     1 |       |     3   (0)| 00:00:01 |  Q1,00 | S->P | HASH (BLOCK|
|   8 |         PX SELECTOR                     |          |       |       |            |          |  Q1,00 | SCWC |            |
|*  9 |          INDEX RANGE SCAN               | T1_I1    |     1 |       |     3   (0)| 00:00:01 |  Q1,00 | SCWP |            |
---------------------------------------------------------------------------------------------------------------------------------

You get a parallel execution plan – although it starts with a serial index range scan which is operated for the new (12c) PX Selector operator that allocates a serial operation to one of the parallel execution slaves – which, approximately, is why the indexed access cost doesn’t change in this example – rather than running it through the query coordinator (QC). The serial range scan does a hash distribution (hashed by block address of the rowids it finds to avoid collisions between parallel execution slave as they do their table accesses.

This is just one cute little trick that makes it worth looking at the upgrade to 12c – this new path is likely to be of benefit to people who had to create global (as opposed to globally partitioned) indexes on partitioned tables.

This note was prompted by a recent twitter comment by Timur Akhmadeev followed in short order by an OTN posting that added further confusion to the problem by running Siebel – which is just one of several 3rd party products that love to configure optimizer parameters with non-standard values like: optimizer_index_cost_adj = 1, or optimizer_mode = first_rows_10. (At the last update I’ve seen on the thread, there seemed to be some other reason why parallelism was being blocked.)

Footnote

In a follow-up tweet, Timue directed me to the 11.2 SQL Language Reference manual – specifically a section on the Parallel Hint, asking if this was an example of a documentation bug.

The trouble with the manuals is that sometimes they are obviously wrong, sometimes they are wrong but it’s not obvious they are wrong, sometimes they omit important information, and sometimes they are badly written and, most specfically, the writing can be ambiguous.

Here’s an extract we could consider:

For PARALLEL, if you specify integer, then that degree of parallelism will be used for the statement.

But my example above shows a “parallel({integer})” hint where we didn’t use that degree of parallelism for the statement.

However the next two sentences read as follows:

If you omit integer, then the database computes the degree of parallelism. All the access paths that can use parallelism will use the specified or computed degree of parallelism.

So what if the optimizer uses the degree of parallelism while calculating the lowest cost plan and ends up with a serial plan ? How comfortable would you feel saying that Oracle has “used the degree of parallelism for the statement”. Or would you say that the first sentence means Oracle isn’t allowed to use a serial plan even if it finds one when doing the arithmetic with the appropriate degree of parallelism.

My call is that this is one of those ambiguous cases – the manual should say something more like:

For PARALLEL, if you specify integer, then that degree of parallelism will be used by the optimizer while calculating the best execution  plan for the statement.

Even then I’m not sure that that’s a complete statement of how the hint works because when you have a full set of system statistics, or have used the dbms_resource_manager.calibrate_io mechanism to tell Oracle about the I/O capacity of the system the optimizer may do some working that says something like: “the hint says degree 64, but the stats say the maximum effective degree will be 38 so I’ll calculate using 38” (This type of thing happens with the older usage of the parallel hint with manual parallelism – I haven’t examined what happens with an automatic policy and the newer option for the hint.)

 

March 17, 2016

Hinting

Filed under: Hints,Ignoring Hints,Oracle,Upgrades — Jonathan Lewis @ 1:10 pm GMT Mar 17,2016

A posting on the OTN database forum a few days ago demonstrated an important problem with hinting (though it didn’t come up in the thread)  especially in the face of upgrades. A simple query needed a couple of hints to produce the correct plan but a slight change to the query seemed to result in Oracle ignoring the hints. The optimizer doesn’t ignore hints, of course, but there are many reasons why it might look as if it has, so I created a little demonstration of the problem – starting with the following data set:

rem
rem     Script:  OTN_DAG.sql
rem     Author:  J.P.Lewis
rem     Dated:   March 2016
rem

create table t1
nologging
as
with generator as (
        select  --+ materialize
                rownum id
        from dual
        connect by
                level <= 1e4 -- > comment to avoid wordpress format issue
)
select
        mod(rownum,200)         n1,
        mod(rownum,200)         n2,
        rpad(rownum,180)        v1
from
        generator       g1,
        generator       g2
where
        rownum <= 24000 -- > comment to avoid wordpress format issue
;

create table t2
nologging
as
with generator as (
        select  --+ materialize
                rownum id
        from dual
        connect by
                level <= 1e4 -- > comment to avoid wordpress format issue
)
select
        trunc((rownum-1)/15)    n1,
        trunc((rownum-1)/15)    n2,
        rpad(rownum,180)        v1
from    generator
where
        rownum <= 3000 -- > comment to avoid wordpress format issue
;

begin
        dbms_stats.gather_table_stats(
                ownname          => user,
                tabname          =>'T1',
                method_opt       => 'for all columns size 1'
        );

        dbms_stats.gather_table_stats(
                ownname          => user,
                tabname          =>'T2',
                method_opt       => 'for all columns size 1'
        );
end;
/

(Ignore the silliness of the way I’ve created the data, it’s a consequence of using my standard template).

For every row in t2 there are 8 rows in t1, so when I join t1 to t2 on n2, selecting the same length of row from bothe tables, it would obviously be sensible for the resulting hash join to use the t2 (smaller byte count) rowsource as the build table and the t1 (larger byte count) rowsource as the probe table; but I’m going to tell the optimizer to use t1 as the build table and t2 as the probe table. Here’s a query and execution plan from 11.2.0.4:

explain plan for
select
        /*+ leading(t1) use_hash(t2) no_swap_join_inputs(t2) */
        count(t1.n2)
from
        t1, t2
where
        t2.n2 = t1.n2
and     t1.n1 = 15
and     t2.n1 = 15
;

select * from table(dbms_xplan.display(null,null,'outline alias'));

----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |     1 |    16 |    97   (3)| 00:00:01 |
|   1 |  SORT AGGREGATE     |      |     1 |    16 |            |          |
|*  2 |   HASH JOIN         |      |    20 |   320 |    97   (3)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL| T1   |   120 |   960 |    85   (3)| 00:00:01 |
|*  4 |    TABLE ACCESS FULL| T2   |    15 |   120 |    12   (0)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("T2"."N2"="T1"."N2")
   3 - filter("T1"."N1"=15)
   4 - filter("T2"."N1"=15)

As you can see, the optimizer has obeyed my hinting – the join order is t1 -> t2, I’ve used a hash join to join t2, and Oracle hasn’t swapped the join inputs despite the fact that the t1 rowsource is larger than the t2 rowsource (960 bytes vs. 120 bytes) which should have persuaded it to do the  swap. (Technically, the leading() hint seems to block the option for swapping the first two tables anyway – see the “Special Case” section at this URL – but I’ve included the no_swap_join_inputs() hint anyway to make the point explicit.)

So now, instead of just counting n2, we’ll modify the query to count the number of distinct values for n2:


explain plan for
select
        /*+ leading(t1) use_hash(t2) no_swap_join_inputs(t2) */
        count(distinct t1.n2) 
from
        t1, t2
where
        t2.n2 = t1.n2
and     t1.n1 = 15
and     t2.n1 = 15
;

select * from table(dbms_xplan.display(null,null,'outline alias'));

----------------------------------------------------------------------------------
| Id  | Operation             | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |          |     1 |    13 |    98   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE       |          |     1 |    13 |            |          |
|   2 |   VIEW                | VW_DAG_0 |    20 |   260 |    98   (4)| 00:00:01 |
|   3 |    HASH GROUP BY      |          |    20 |   320 |    98   (4)| 00:00:01 |
|*  4 |     HASH JOIN         |          |    20 |   320 |    97   (3)| 00:00:01 |
|*  5 |      TABLE ACCESS FULL| T2       |    15 |   120 |    12   (0)| 00:00:01 |
|*  6 |      TABLE ACCESS FULL| T1       |   120 |   960 |    85   (3)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   4 - access("T2"."N2"="T1"."N2")
   5 - filter("T2"."N1"=15)
   6 - filter("T1"."N1"=15)

Check operations 5 and 6 – Oracle has swapped the join inputs: t2 (the obvious choice) is now the build table. Has Oracle ignored the hint ? (Answer: No).

If you look at operation 2 you can see that Oracle has generated an internal view called VW_DAG_0 – this is an example of the “Distinct Aggregate” transformation taking place. It seems to be a pointless exercise in this case and the 10053 trace file suggests that in 11.2.0.4 it’s a heuristic transformation rather than a cost-based transformation (i.e. the optimizer does it because it can, not because it’s going to be lower cost). Oracle has transformed the SQL to the following (to which I have applied a little cosmetic tidying):


SELECT  /*+ LEADING (T1) */
        COUNT(VW_DAG_0.ITEM_1) "COUNT(DISTINCTT1.N2)"
FROM    (
        SELECT  T1.N2 ITEM_1
        FROM    TEST_USER.T2 T2,TEST_USER.T1 T1
        WHERE   T2.N2=T1.N2
        AND     T1.N1=15
        AND     T2.N1=15
        GROUP BY
                T1.N2
        ) VW_DAG_0

Notice how the use_hash() and no_swap_join_input() hints have disappeared. I am slightly surprised that the leading() hint is still visible, I would have expected all three to stay or all three to disappear; regardless of that, though, the single remaining hint references an object that does not exist in the query block where the hint has been placed. The original hint has not been “ignored” it has become irrelevant. (I’ll be coming back to an odd little detail about this transformed query a little later on but for the moment I’m going to pursue the problem of making the optimizer do what we want.)

UPDATE (June 2020): By 12.2.0.1 the vw_dag_0 view of the unparsed query included all three of the original hints, so the execution plan reverted to using t1 as the build table and t2 as the probe table.

We have three strategies we could pursue at this point. We could tell the optimizer that we don’t want it to do the transformation; we could work out the query block name of the query block that holds t1 and t2 after the transformation and direct our hints into that query block; or we could tell Oracle to pretend it was using an older version of the optimizer because the Distinct Aggregate transformation didn’t appear until 11.2.0.1.

You’ll notice that I used the ‘alias’ formatting option in my call to dbms_xplan.display(). Here’s the Query Block Name / Object Alias section of the resulting output:


Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$C33C846D
   2 - SEL$5771D262 / VW_DAG_0@SEL$C33C846D
   3 - SEL$5771D262
   5 - SEL$5771D262 / T1@SEL$1
   6 - SEL$5771D262 / T2@SEL$1

Based on this information we can say:

  • Strategy A says: try adding the hint: /*+ no_transform_distinct_agg(@sel$1) */
  • Strategy B says: try using the hints: /*+ leading(@sel$5771d262 t1@sel$1 t2@sel$1) use_hash(@sel$5771d262 t2@sel$1 no_swap_join_inputs(@sel$5771d262 t2@sel$1) */
  • Strategy C says: try adding the hint: /*+ optimizer_features_enable(‘11.1.0.7’) */

Strategies A and C (stopping the transformation) produce the following plan:


----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |     1 |    16 |    98   (4)| 00:00:01 |
|   1 |  SORT GROUP BY      |      |     1 |    16 |            |          |
|*  2 |   HASH JOIN         |      |    20 |   320 |    98   (4)| 00:00:01 |
|*  3 |    TABLE ACCESS FULL| T1   |   120 |   960 |    85   (3)| 00:00:01 |
|*  4 |    TABLE ACCESS FULL| T2   |    15 |   120 |    12   (0)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("T2"."N2"="T1"."N2")
   3 - filter("T1"."N1"=15)
   4 - filter("T2"."N1"=15)

Strategy B (allowing the transformation but addressing the hints to the generated query block) produces this plan:


----------------------------------------------------------------------------------
| Id  | Operation             | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |          |     1 |    13 |    98   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE       |          |     1 |    13 |            |          |
|   2 |   VIEW                | VW_DAG_0 |    20 |   260 |    98   (4)| 00:00:01 |
|   3 |    HASH GROUP BY      |          |    20 |   320 |    98   (4)| 00:00:01 |
|*  4 |     HASH JOIN         |          |    20 |   320 |    97   (3)| 00:00:01 |
|*  5 |      TABLE ACCESS FULL| T1       |   120 |   960 |    85   (3)| 00:00:01 |
|*  6 |      TABLE ACCESS FULL| T2       |    15 |   120 |    12   (0)| 00:00:01 |
----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   4 - access("T2"."N2"="T1"."N2")
   5 - filter("T1"."N1"=15)
   6 - filter("T2"."N1"=15)

All three Strategies have produced plans that use t1, the larger rowsource, as the build table.

It’s hard to resist asking if it’s possible to claim that one of the three strategies is the best strategy; it’s hard to say, but I think I’d favour using the no_transform_distinct_agg() hint because it’s precisely targetted – so avoids the brute force thuggish nature of reverting back to an old version and also avoids the (possible) fragility of needing to know a very precise query block name which (possibly) might change for some reason if the query were to be modified very slightly. This argument, of course, comes from the perspective of a friendly consultant who visits for a couple of days, gets a bit clever with your SQL, then walks away leaving you to worry about whether you understand why your SQL now works the way it does.

Upgrades

My opening comment was about the difficulty of hinting across upgrades. Imagine you had been running this count(distinct) query in Oracle 10.2.0.5 and, after some experimentation, had found that you got the path you needed by adding the hints: /*+ leading(t1 t2)  full(t1) use_hash(t2) no_swap_join_inputs(t2) full(t2) */. This is a careful and thorough piece of hinting (and it does work, of course, in 10.2.0.5).

When the big day for upgrading to 11.2 arrives you find that this query changes its execution plan. This scenario is NOT a rare occurrence. I’ve said it before, and I’ll keep saying it: hinting – especially with “micro-management” hints – is undesirable in a production system. You probably haven’t done it right, and even if the hints are (broadly speaking) perfect in the current version they may be pushed out of context by a new feature in the next version.  If you’ve hinted your code you have to check every single hinted statement to make sure the hints still have the same effect on the upgrade.

This is why I came up with the sound-bite: “if you can hint it, baseline it”.  If you had generated an SQL Baseline in 10g from a query with these hints Oracle would have included the hint /*+ optimizer_features_enable(‘10.2.0.5’) */ with the rest of the hints and the upgrade wouldn’t have produced a different plan.

Technically, of course, you might have remembered to add the optimizer_features_enable() hint to your production code – but in many cases Oracle introduces far more hints in an SQL Baseline than you might want to put into your code; and by using the SQL Baseline approach you’ve given yourself the option to get rid of the “hidden hinting” in a future version of Oracle by dropping the baseline rather than editing the code and (perhaps) recompiling the application.

Inevitably there are cases where setting the optimizer_features_enable() backwards doesn’t rescue you from a new plan – there are probably a few cases where the internal code forgets to check the value and bypass some subroutines; more significantly there are cases where one version of Oracle will give you an efficient plan because of an optimizer bug and setting the version backwards won’t re-introduce that bug.

Footnote

I said I’d come back to the “unparsed” query that the optimizer generated from the original count(distinct) statement and the way it left the leading(t1) hint in place but lost the use_hash(t2) and no_swap_join_inputs(t2). I got curious about how Oracle would optimize the unparsed query if I supplied it from SQL*Plus – and this is the plan I got:


explain plan for
SELECT  /*+ LEADING (T1) */
        COUNT(VW_DAG_0.ITEM_1) "COUNT(DISTINCTT1.N2)"
FROM    (
        SELECT  T1.N2 ITEM_1
        FROM    TEST_USER.T2 T2,TEST_USER.T1 T1
        WHERE   T2.N2=T1.N2
        AND     T1.N1=15
        AND     T2.N1=15
        GROUP BY
                T1.N2
        ) VW_DAG_0
;

-----------------------------------------------------------------------------------
| Id  | Operation             | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |           |     1 |    13 |    98   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE       |           |     1 |    13 |            |          |
|   2 |   VIEW                | VM_NWVW_0 |    20 |   260 |    98   (4)| 00:00:01 |
|   3 |    HASH GROUP BY      |           |    20 |   320 |    98   (4)| 00:00:01 |
|*  4 |     HASH JOIN         |           |    20 |   320 |    97   (3)| 00:00:01 |
|*  5 |      TABLE ACCESS FULL| T1        |   120 |   960 |    85   (3)| 00:00:01 |
|*  6 |      TABLE ACCESS FULL| T2        |    15 |   120 |    12   (0)| 00:00:01 |
-----------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------
   4 - access("T2"."N2"="T1"."N2")
   5 - filter("T1"."N1"=15)
   6 - filter("T2"."N1"=15)

Oracle has managed to do a transformation to this statement that it didn’t do when it first generated the statement – too much recursion, perhaps – and that floating leading(t1) hint has been squeezed back into action by a view-merging step in the optimization that got the hint back into a query block that actually contained t1 and t2!  At this point I feel like quoting cod-philosophy from the great Dune trilogy by Frank Herbert: “Just when you think you understand …”

 

May 19, 2014

Ignoring Hints

Filed under: Bugs,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 6:21 pm BST May 19,2014

Does Oracle ignore hints – not if you use them correctly, and sometimes it doesn’t ignore them even when you use them incorrectly!

Here’s an example that I’ve run on 11.2.0.4 and 12.1.0.1


create table t1
as
with generator as (
	select	--+ materialize
		rownum id
	from dual
	connect by
		level <= 1e4  -- > comment to avoid WordPress format issue.
)
select
	rownum			id,
	rownum			n1,
	rpad('x',100)		padding
from
	generator	v1
;

begin
	dbms_stats.gather_table_stats(
		ownname		 => user,
		tabname		 =>'T1',
		method_opt	 => 'for all columns size 1'
	);
end;
/

create index t1_i1 on t1(id);
alter index t1_i1 unusable;

select n1 from t1 where id = 15;
select /*+ index(t1 (id)) */ n1 from t1 where id = 15;

Any guesses about the output from the last 4 statements ?

Index created.

Index altered.

        N1
----------
        15

1 row selected.

select /*+ index(t1 (id)) */ n1 from t1 where id = 15
*
ERROR at line 1:
ORA-01502: index 'TEST_USER.T1_I1' or partition of such index is in unusable state

That’s a pretty convincing display of Oracle not ignoring hints. Unhinted the optimizer finds a path which can produce a result; hinted to use an unusable index the optimizer reports a failure rather than silently ignoring the hint.

February 16, 2014

Recursive subquery factoring

Filed under: Hints,Ignoring Hints,Oracle,Subquery Factoring,Tuning — Jonathan Lewis @ 6:11 pm GMT Feb 16,2014

This is possibly my longest title to date – I try to keep them short enough to fit the right hand column of the blog without wrapping – but I couldn’t think of a good way to shorten it (Personally I prefer to use the expression CTE – common table expression – over “factored subquery” or “subquery factoring” or “with subquery”, and that would have achieved my goal, but might not have meant anything to most people.)

If you haven’t come across them before recursive CTEs appeared in 11.2, are in the ANSI standard, and are (probably) viewed by Oracle as the strategic replacement for “connect by” queries. Here, to get things started, is a simple (and silly) example:

(more…)

January 3, 2014

Index Hash

Filed under: Bugs,CBO,Hints,Ignoring Hints,Index Joins,Indexing,Oracle — Jonathan Lewis @ 6:56 pm GMT Jan 3,2014

I’m afraid this is one of my bad puns again – an example of the optimizer  making a real hash of the index hash join. I’m going to create a table with several indexes (some of them rather similar to each other) and execute a query that should do an index join between the obvious two indexes. To show how obvious the join should be I’m going to start with a couple of queries that show the cost of simple index fast full scans.

Here’s the data generating code:

(more…)

October 9, 2013

Hinting

Filed under: 12c,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 5:33 pm BST Oct 9,2013

I’ve spent so many years trying to explain that a “hint” to the Oracle optimizer is an order – if you know how to do it properly – that I finally decided to list the manual references that have made this point over the last 15 or so years. Here’s the list, which ends with a surprising change of flavour. (Emphasis in the body of the text is mine).

(more…)

June 14, 2013

Hints again

Filed under: CBO,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 6:17 pm BST Jun 14,2013

A recent posting on OTN came up with a potentially interesting problem – it started roughly like this:

I have two queries like this:

select * from emp where dept_id=10 and emp_id=15;
select * from emp where dept_id=10 and emp_id=16;

When I run them separately I get the execution plan I want, but when I run a union of the two the plans change.

This, of course, is extremely unlikely – even if we assume that the two queries are more complex than the text shown. On the other hand you might, after a little thought, come up with the idea that perhaps the optimizer had done something really clever like join factorization (moving a join that’s common to the two parts of the UNION from inside to outside the UNION), or maybe there’s some really new trick the optimizer had played because a UNION ultimately requires a SORT UNIQUE, and the optimizer had chosen a different path that returned the data from each part of the UNION in sorted order to decrease the cost of that final sort.

In fact it turned out to be a lot simpler than that. The query looked more like this:

(more…)

May 23, 2012

Logical tuning

Filed under: Hints,Ignoring Hints,Oracle,Performance,Tuning — Jonathan Lewis @ 6:22 pm BST May 23,2012

Here’s a model of a problem I solved quite recently at a client site. The client’s query was much more complex and the volume of data much larger, but this tiny, two table, example is sufficient to demonstrate the key principle. (Originally I thought I’d have to use three tables to model the problem, which is why you may find my choice of table names a little odd). I ran this example on 11.2.0.2 – which was the client version:
(more…)

November 18, 2011

Hinting

Filed under: Execution plans,Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 12:54 pm GMT Nov 18,2011

As I’ve often pointed out, this blog isn’t AskTom, or the OTN forum, so I don’t expect to have people asking me to solve their problems; neither do I answer email questions about specific problems. Occasionally, though, questions do appear that are worth a little public airing, and one of these came in by email a couple of weeks ago. The question is longer than the answer I sent, my contribution to the exchange doesn’t start until the heading: “My Reply”.
(more…)

January 16, 2011

Ignoring hints

Filed under: Hints,Ignoring Hints,Oracle — Jonathan Lewis @ 10:32 pm GMT Jan 16,2011

Yes, finally, really ignoring hints – but it’s a sort of bug, of course.

Thanks to Timur Akhmadeev for telling us about bug 8432870 (you’ll need an account on MOS for the link) in his reply to Charles Hooper’s recent post.

In the upgrade from 9i to 10g there was a change in the “hint parser”. If you put a valid SQL keyword inside the hint delimiters (the note says /*+  */ but doesn’t mention the –+ alternative for specifying a hint, thought it’s probably still true there) when the keyword is not a valid hint – for example the word NOLOGGING which I have seen people use as if it were a hint – then Oracle will ignore all the hints.

Earlier versions of Oracle simply noticed that you had embedded something that wasn’t a valid hint, but that didn’t stop the parser from reading the rest of the hints correctly.

If the invalid hint is not a valid SQL keyword then there are no nasty side effects.

This might explain why I ran into an odd problem a little while ago when I added a comment to my  hint list and found that the hints stopped working. I can’t remember the exact details any more but I think my comment was something along the lines of: “Do not … because …”, and this broke the hints until I changed it to “Don’t … because …”.

[Further reading on “ignoring hints”]

 

December 3, 2010

ANSI – argh

Filed under: ANSI Standard,CBO,Execution plans,Hints,Ignoring Hints — Jonathan Lewis @ 7:30 pm GMT Dec 3,2010

I’m not keen on ANSI standard SQL – even though it is, technically, the strategic option and even though you have to use it for full outer joins and partitioned outer joins.

One reason for disliking it is that it “separates join predicates from filter predicates” – a reason often given in praise of the syntax which, to my mind, claims a spurious distinction and introduces a mechanism that makes it harder to keep mental track of what’s going to happen as you walk  through the join order. (I have to admit that I was temporarily fooled into thinking it was quite a nice idea – in an abstract sort of way.)
(more…)

May 19, 2010

Ignoring Hints

Filed under: distributed,Hints,Ignoring Hints,Troubleshooting,Tuning — Jonathan Lewis @ 9:04 pm BST May 19,2010

I’ve previously published a couple of notes (hereand here) about the use of the driving_site() hint with distributed queries. The first note pointed out that the hint was deliberately ignored if you write a local CTAS or INSERT that did a distributed query. I’ve just found another case where the hint is ignored – this time in a simple SELECT statement.

Try running an ordinary distributed query from the SYS account, and then try using the driving_site() hint to make it run at the remote site. When I tried this a few days ago I ended up wasting half an hour translating some SQL from ANSI to Oracle dialect because I thought that the ANSI was making Oracle transform the query in a way that lost the hint – then I discovered that both versions of the code worked correctly if I logged in as a different user.

I was running my queries between two databases using 11.1.0.7 – I won’t guarantee you get the same results on other versions, but it looks like SYS doesn’t honour the driving_site() hint. I can’t think of a robust argument why this should be the case, but if I were forced to do some vague hand-waving I’d probably mumble something about potential security loopholes.

Footnote: I should, of course, have mentioned that there are all sorts of things that behave in unexpected ways if you are logged on as SYS, and that you shouldn’t be logged on as SYS – especially in a production system.

[Further reading on “ignoring hints”]

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