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Adaptive Cursor Sharing in Oracle Database 11g Release 1

DBAs are always encouraging developers to use bind variables, but when bind variables are used against columns containing skewed data they sometimes lead to less than optimum execution plans. This is because the optimizer peeks at the bind variable value during the hard parse of the statement, so the value of a bind variable when the statement is first presented to the server can affect every execution of the statement, regardless of the bind variable values.

Oracle 11g uses Adaptive Cursor Sharing to solve this problem by allowing the server to compare the effectiveness of execution plans between executions with different bind variable values. If it notices suboptimal plans, it allows certain bind variable values, or ranges of values, to use alternate execution plans for the same statement. This functionality requires no additional configuration. The following code provides an example of adaptive cursor sharing.

Related articles.

Setup

First we create and populate a test table. We give every row with an even ID value a RECORD_TYPE of "2", so half of the rows in the table have a RECORD_TYPE of "2", and all the other rows have a unique RECORD_TYPE value. Notice we have an index on the RECORD_TYPE column.

drop table acs_test_tab;

create table acs_test_tab (
  id          number,
  record_type number,
  description varchar2(50),
  constraint acs_test_tab_pk primary key (id)
);

create index acs_test_tab_record_type_i on acs_test_tab(record_type);

declare
  type t_acs_test_tab is table of acs_test_tab%rowtype;
  l_tab t_acs_test_tab := t_acs_test_tab();
  
begin
  for i in 1 .. 100000 loop
    l_tab.extend;
    if mod(i,2)=0 then
      l_tab(l_tab.last).record_type := 2;
    else
      l_tab(l_tab.last).record_type := i;
    end if;
    
    l_tab(l_tab.last).id          := i;
    l_tab(l_tab.last).description := 'description for ' || i;
  end loop;

  forall i in l_tab.first .. l_tab.last
    insert into acs_test_tab values l_tab(i);

  commit;
end;
/

exec dbms_stats.gather_table_stats(user, 'acs_test_tab', estimate_percent =>100, method_opt=>'for all indexed columns size skewonly', cascade=>true);

The data in the RECORD_TYPE column is skewed, as shown by the presence of a histogram against the column.

-- Demonstrate data skew.
with data as (
  select (case record_type
            when 2 then '2'
            else 'unique'
          end) record_type
  from   acs_test_tab
)
select record_type, count(*) as amount
from   data
group by record_type
order by record_type;

RECORD_TYPE       AMOUNT
-------------- ---------
2                  50000
unique             50000

SQL>


-- Check for histograms and distinct values.
select column_name, histogram, num_distinct
from   user_tab_cols
where  table_name = 'ACS_TEST_TAB';

COLUMN_NAME    HISTOGRAM             NUM_DISTINCT
-------------- ------------------ ---------------
ID             NONE                        100000
RECORD_TYPE    HEIGHT BALANCED              50001
DESCRIPTION    NONE

SQL>

Test Adaptive Cursor Sharing

We query the table and limit the rows returned based on the RECORD_TYPE column with a literal value of "1".

set linesize 200

select max(id) from acs_test_tab where record_type = 1;
select * from table(dbms_xplan.display_cursor);

   MAX(ID)
----------
         1

1 row selected.

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
SQL_ID  cgt92vnmcytb0, child number 0
-------------------------------------
select max(id) from acs_test_tab where record_type = 1

Plan hash value: 3987223107

-----------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                       | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                            |       |       |     2 (100)|          |
|   1 |  SORT AGGREGATE              |                            |     1 |     9 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB               |     1 |     9 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | ACS_TEST_TAB_RECORD_TYPE_I |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------

This query has used the index as we would expect. Now we repeat the query, but this time use a bind variable.

variable l_record_type number;
exec :l_record_type := 1;

select max(id) from acs_test_tab where record_type = :l_record_type;
select * from table(dbms_xplan.display_cursor);

   MAX(ID)
----------
         1

1 row selected.

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
SQL_ID  9bmm6cmwa8saf, child number 0
-------------------------------------
select max(id) from acs_test_tab where record_type = :l_record_type

Plan hash value: 3987223107

-----------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                       | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                            |       |       |     2 (100)|          |
|   1 |  SORT AGGREGATE              |                            |     1 |     9 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB               |     1 |     9 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | ACS_TEST_TAB_RECORD_TYPE_I |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------

So we ran what amounted to the same query, and got the same result and execution plan. The optimizer picked an execution plan that it thought was optimum for query by peeking at the value of the bind variable. The only problem is, this would be totally the wrong plan if the bind variable value were "2". If we change the bind variable value to "2", we can see we get the same plan used again.

variable l_record_type number;
exec :l_record_type := 2;

select max(id) from acs_test_tab where record_type = :l_record_type;
select * from table(dbms_xplan.display_cursor);

   MAX(ID)
----------
    100000

1 row selected.

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
SQL_ID  9bmm6cmwa8saf, child number 0
-------------------------------------
select max(id) from acs_test_tab where record_type = :l_record_type

Plan hash value: 3987223107

-----------------------------------------------------------------------------------------------------------
| Id  | Operation                    | Name                       | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |                            |       |       |     2 (100)|          |
|   1 |  SORT AGGREGATE              |                            |     1 |     9 |            |          |
|   2 |   TABLE ACCESS BY INDEX ROWID| ACS_TEST_TAB               |     1 |     9 |     2   (0)| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | ACS_TEST_TAB_RECORD_TYPE_I |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------

So we have used an index to visit 50% of the rows in the table, evenly spread throughout the table blocks. This seems like a bad decision. A full table scan would have been more efficient.

If we look at the V$SQL view entry for this query, we can see the IS_BIND_SENSITIVE column is marked as 'Y', so Oracle is aware this query may require differing execution plans depending on the bind variable values, but currently the IS_BIND_AWARE column is marked as 'N', so Oracle has not acted on this yet.

select sql_id, child_number, is_bind_sensitive, is_bind_aware
from   v$sql
where  sql_text = 'select max(id) from acs_test_tab where record_type = :l_record_type';

SQL_ID        CHILD_NUMBER IS_BIND_SENSITIVE IS_BIND_AWARE
------------- ------------ ----------------- -------------
9bmm6cmwa8saf            0 Y                 N

1 row selected.

SQL>

If we run the statement using the bind variable value of "2", we can see Oracle has decided to use an alternate, more efficient plan for this statement.

variable l_record_type number;
exec :l_record_type := 2;

select max(id) from acs_test_tab where record_type = :l_record_type;
select * from table(dbms_xplan.display_cursor);

   MAX(ID)
----------
    100000

1 row selected.

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------
SQL_ID  9bmm6cmwa8saf, child number 1
-------------------------------------
select max(id) from acs_test_tab where record_type = :l_record_type

Plan hash value: 509473618

-----------------------------------------------------------------------------------
| Id  | Operation          | Name         | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |              |       |       |   138 (100)|          |
|   1 |  SORT AGGREGATE    |              |     1 |     9 |            |          |
|*  2 |   TABLE ACCESS FULL| ACS_TEST_TAB | 48031 |   422K|   138   (2)| 00:00:02 |
-----------------------------------------------------------------------------------

This change in behaviour is also reflected in the V$SQL view, which now has a new child cursor with the IS_BIND_AWARE column marked as "Y".

select sql_id, child_number, is_bind_sensitive, is_bind_aware
from   v$sql
where  sql_text = 'select max(id) from acs_test_tab where record_type = :l_record_type';

SQL_ID        CHILD_NUMBER IS_BIND_SENSITIVE IS_BIND_AWARE
------------- ------------ ----------------- -------------
9bmm6cmwa8saf            0 Y                 N
9bmm6cmwa8saf            1 Y                 Y

2 rows selected.

SQL>

Information about the cursor sharing histograms, statistics and selectivity is displayed using the V$SQL_CS_HISTOGRAM, V$SQL_CS_STATISTICS and V$SQL_CS_SELECTIVITY views respectively.

select * from v$sql_cs_histogram where sql_id = '9bmm6cmwa8saf';

ADDRESS  HASH_VALUE SQL_ID        CHILD_NUMBER  BUCKET_ID      COUNT
-------- ---------- ------------- ------------ ---------- ----------
319A4A1C 4171522382 9bmm6cmwa8saf            1          0          0
319A4A1C 4171522382 9bmm6cmwa8saf            1          1          1
319A4A1C 4171522382 9bmm6cmwa8saf            1          2          0
319A4A1C 4171522382 9bmm6cmwa8saf            0          0          1
319A4A1C 4171522382 9bmm6cmwa8saf            0          1          1
319A4A1C 4171522382 9bmm6cmwa8saf            0          2          0

SQL>


select * from v$sql_cs_statistics where sql_id = '9bmm6cmwa8saf';

ADDRESS  HASH_VALUE SQL_ID        CHILD_NUMBER BIND_SET_HASH_VALUE P EXECUTIONS ROWS_PROCESSED BUFFER_GETS   CPU_TIME
-------- ---------- ------------- ------------ ------------------- - ---------- -------------- ----------- ----------
319A4A1C 4171522382 9bmm6cmwa8saf            1          2064090006 Y          1          50001         499          0
319A4A1C 4171522382 9bmm6cmwa8saf            0          2342552567 Y          1              3           3          0

2 rows selected.

SQL>


select * from v$sql_cs_selectivity where sql_id = '9bmm6cmwa8saf';

ADDRESS  HASH_VALUE SQL_ID        CHILD_NUMBER PREDICATE                                  RANGE_ID LOW        HIGH
-------- ---------- ------------- ------------ ---------------------------------------- ---------- ---------- ----------
319A4A1C 4171522382 9bmm6cmwa8saf            1 =L_RECORD_T                                       0 0.432283   0.528346

1 row selected.

SQL>

BIND_AWARE and NO_BIND_AWARE Hints

From 11.1.0.7 onward it is possible to skip the monitoring that is required to detect bind-sensitive queries by using the BIND_AWARE hint. In the following example, the presence of the hint tells the optimizer that we believe the query is bind-sensitive, so it should use bind-aware cursor sharing from the first execution.

select /*+ bind_aware */ max(id) from acs_test_tab where record_type = :l_record_type;

The hint will only work if the query uses bind variables in WHERE clause predicates referencing columns with histograms.

There is also a NO_BIND_AWARE hint that tells the optimizer to ignore bind-sensitive queries, effectively hiding the query from the adaptive cursor sharing functionality.

Bind-aware cursor sharing has a small overhead associated with it, which is why Oracle use the "adaptive" approach to identifying queries that would benefit from bind-aware cursor sharing. Adding the hint to queries that will not benefit from it is a waste.

For more information see:

Hope this helps. Regards Tim...

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