Adaptive cursor sharing allows SQL statements with bind variables to potentially reuse execution plans from previous statements with the same bind variable values. When the optimizer detects that the execution plan for a statement with a bind variable could benefit from using an index, it will create a sharable child cursor. On subsequent executions of the statement with the same bind value, the child cursor can be reused, avoiding a hard parse and producing a more efficient execution plan making use of the index. This improves performance and reduces parsing overhead compared to statements using literal values.

1. Adaptive Cursor Sharing
Mohamed Houri
www.hourim.wordpress.com
Mohamed Houri
www.hourim.wordpress.com
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2. Agenda
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•Soft parse and hard parse
overview of the different steps a SQL statement undergoes before being executed
•Adaptive Cursor Sharing
- explain the concept
- explain the objective
- explain the appearance criteria
- explain the monitoring criteria
•Literal, bind variable and sharing cursors
overview of the bind variable role on sharing cursors and saving resource
compare this with the negative impact literal variables can have on memory (SGA)
and CPU
•Conclusion

3. Agenda
3
Syntactic check
Syntax,
keywords
Semantic check
Access, right,
exist
Softparse
Store parent cursor in
v$sql(SGA)
Parent
cursor
exist? Logical Optimization
Physical Optimization
Store child cursor in
v$sql(SGA)
Child
cursor
exist?
Hardparse
No
Yes
Yes
No
Execute SQL
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You should always try to avoid hard parsing
CBO
Estimator
Plan Generator
Statistics
Dictionary
Query Transformer
Optimizer
Execution plan • Child cursor
• C0 C1 C2
Execute SQL query
Soft parse and hard
parse

5. Literal, bind variable and sharing
cursors
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SQL with Literals
Almost always considered as never seen
No parent cursor to share
• Syntactic check
• Semantic check
• Parent cursor in the SGA
• Optimization of an execution
plan
• Put the child cursor into SGA
Execute the query

6. Literal, bind variable and sharing
cursors
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SQL with Literals
Almost always considered as never seen
No parent cursor to share
• Syntactic check
• Semantic check
• Parent cursor in the SGA
• Optimization of an execution
plan
• Put the child cursor into SGA
Execute the query
SQL with Binds
Almost always considered as already
seen.
There is a parent cursor to share
Execute the query
Share the existing child
cursor

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•Using bind variable
- avoids hard parsing
- makes the SGA attractive
- use less resource – CPU
- sharing plan is not always optimal
Literal, bind variable and sharing
cursors
•Using Literal variable
- hard parsing for each execution
- traumatize the SGA
- burn a lot CPU
- generate an optimal plan

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Literal, bind variable and sharing
cursors
•How to have both?
• Attractive SGA
• Less CPU consumption
• Optimal execution plan

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Adaptive Cursor
Sharing

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Adaptive Cursor
Sharing

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SQL> create table t1
(col1 number
,col2 varchar2(50)
,flag varchar2(2));
SQL> create index i1 on t1 (flag);
Adaptive Cursor
Sharing

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SQL> insert into t1
select
rownum
,dbms_random.string('s',20)
,case when rownum = 1
then 'Y1'
when rownum = 2
then 'Y2'
when mod(rownum,2) = 0
then 'N1'
else 'N2'
end
from dual
connect by rownum <= 100000;
Adaptive Cursor
Sharing

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SQL> select flag, count(1) from t1 group by flag;
FLAG COUNT(1)
---- ----------
N1 49999
N2 49999
Y1 1
Y2 1
We have an index on FLAG column
FLAG COUNT(1)
---- ----------
N1 49999  FULL TABLE SCAN
N2 49999  FULL TABLE SCAN
Y1 1  INDEX RANGE SCAN
Y2 1  INDEX RANGE SCAN
Adaptive Cursor
Sharing

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SQL> BEGIN
dbms_stats.gather_table_stats
(user
,'T1'
,method_opt => 'FOR ALL COLUMNS FLAG SIZE
AUTO'
,cascade => true
,no_invalidate => FALSE);
END;
/
PL/SQL procedure successfully completed.
Adaptive Cursor
Sharing

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Adaptive Cursor
Sharing
SQL> select column_name, histogram from
user_tab_col_statistics
where table_name = 'T1';
COLUM HISTOGRAM
----- ---------------
FLAG FREQUENCY
COL2 NONE
COL1 NONE
SQL> var n varchar2(2)
SQL> exec :n := 'N1’

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Adaptive Cursor
Sharing
SQL> select count(*), max(col2) from t1 where flag = :n;
COUNT(*) MAX(COL2)
---------- --------------------
49999 ZZZAKHHQDDEERMBJIVYA
---------------------------------------------------
| Id | Operation | Name | Rows | Bytes |
---------------------------------------------------
| 0 | SELECT STATEMENT | | | |
| 1 | SORT AGGREGATE | | 1 | 24 |
|* 2 | TABLE ACCESS FULL| T1 | 49999 | 1171K|
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("FLAG"=:N)

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Adaptive Cursor
Sharing
SQL> select sql_id
2 , child_number
3 , is_bind_aware
4 , is_bind_sensitive
5 , is_shareable
6 , executions
7 from v$sql
8 where sql_id ='731b98a8u0knf'
9 ;
SQL_ID CHILD_NUMBER B_AWARE B_SENS SHAR EXECUTIONS
------------- ------------ -------- ------ ---- ---------
731b98a8u0knf 0 N Y Y 1

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Adaptive Cursor
SharingSQL> exec :n :='Y1‘
SQL> select count(*), max(col2) from t1 where flag = :n;
COUNT(*) MAX(COL2)
---------- ----------------------
1 FAYJDTZLLXQGUSYGHFDC
---------------------------------------------------
| Id | Operation | Name | Rows | Bytes |
---------------------------------------------------
| 0 | SELECT STATEMENT | | | |
| 1 | SORT AGGREGATE | | 1 | 24 |
|* 2 | TABLE ACCESS FULL| T1 | 49999 | 1171K|
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("FLAG"=:N)

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Adaptive Cursor
SharingSQL> exec :n :='Y1‘
SQL> select count(*), max(col2) from t1 where flag = :n;
COUNT(*) MAX(COL2)
---------- ----------------------
1 FAYJDTZLLXQGUSYGHFDC
--------------------------------------------------------------
| Id | Operation | Name | Rows |
--------------------------------------------------------------
| 0 | SELECT STATEMENT | | |
| 1 | SORT AGGREGATE | | 1 |
| 2 | TABLE ACCESS BY INDEX ROWID BATCHED| T1 | 1 |
|* 3 | INDEX RANGE SCAN | I1 | 1 |
--------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("FLAG"=:N)

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Adaptive Cursor
SharingSQL> select sql_id
2 , child_number
3 , is_bind_aware
4 , is_bind_sensitive
5 , is_shareable
6 , executions
7 from v$sql
8 where sql_id ='731b98a8u0knf'
9 ;
SQL_ID CHILD_NUMBER IS_B IS_B IS_S EXECUTIONS
------------- ------------ ---- ---- ---- ----------
731b98a8u0knf 0 N Y N 2
731b98a8u0knf 1 Y Y Y 1

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Adaptive Cursor
SharingSQL> exec :n := 'N2‘
SQL> select count(*), max(col2) from t1 where flag = :n;
COUNT(*) MAX(COL2)
---------- -----------------------
49999 ZZZYIRCRHXNEFONRFHPE
---------------------------------------------------
| Id | Operation | Name | Rows | Bytes |
---------------------------------------------------
| 0 | SELECT STATEMENT | | | |
| 1 | SORT AGGREGATE | | 1 | 24 |
|* 2 | TABLE ACCESS FULL| T1 | 49999 | 1171K|
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("FLAG"=:N)

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Adaptive Cursor
SharingSQL> select sql_id
2 , child_number
3 , is_bind_aware
4 , is_bind_sensitive
5 , is_shareable
6 , executions
7 from v$sql
8 where sql_id ='731b98a8u0knf'
9 ;
SQL_ID CHILD_NUMBER IS_B IS_B IS_S EXECUTIONS
------------- ------------ ---- ---- ---- ----------
731b98a8u0knf 0 N Y N 2
731b98a8u0knf 1 Y Y Y 1
731b98a8u0knf 2 Y Y Y 1

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Adaptive Cursor
SharingSQL> exec :n := 'Y2‘
SQL> select count(*), max(col2) from t1 where flag = :n;
COUNT(*) MAX(COL2)
---------- ----------------------
1 HSQMBZOWMBAUOGGCOGFC
-------------------------------------------------------------
| Id | Operation | Name | Rows |
-------------------------------------------------------------
| 0 | SELECT STATEMENT | | |
| 1 | SORT AGGREGATE | | 1 |
| 2 | TABLE ACCESS BY INDEX ROWID BATCHED| T1 | 1 |
|* 3 | INDEX RANGE SCAN | I1 | 1 |
-------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("FLAG"=:N)

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Adaptive Cursor
SharingSQL> select sql_id
2 , child_number
3 , is_bind_aware
4 , is_bind_sensitive
5 , is_shareable
6 , executions
7 from v$sql
8 where sql_id ='731b98a8u0knf'
9 ;
SQL_ID CHILD_NUMBER IS_B IS_B IS_S EXECUTIONS
------------- ------------ ---- ---- ---- ----------
731b98a8u0knf 0 N Y N 2
731b98a8u0knf 1 Y Y Y 2
731b98a8u0knf 2 Y Y Y 1
Child cursor n°1 for index scan plan
Child cursor n°2 for full table scan

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Adaptive Cursor
SharingThere are 3 dynamic views for ACS
monitoring :
SQL> desc V$SQL_CS_STATISTICS
Name Null? Type
------------------------------- -------- ----------------
1 ADDRESS RAW(8)
2 HASH_VALUE NUMBER
3 SQL_ID VARCHAR2(13)
4 CHILD_NUMBER NUMBER
5 BIND_SET_HASH_VALUE NUMBER
6 PEEKED VARCHAR2(1)
7 EXECUTIONS NUMBER
8 ROWS_PROCESSED NUMBER
9 BUFFER_GETS NUMBER
10 CPU_TIME NUMBER
11 CON_ID NUMBER

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Adaptive Cursor
SharingSQL> desc v$sql_cs_histogram
Name Null? Type
------------------------------- -------- --------------------
1 ADDRESS RAW(8)
2 HASH_VALUE NUMBER
3 SQL_ID VARCHAR2(13)
4 CHILD_NUMBER NUMBER
5 BUCKET_ID NUMBER
6 COUNT NUMBER
7 CON_ID NUMBER

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Adaptive Cursor
SharingSQL> desc v$sql_cs_selectivity
Name Null? Type
------------------------------- -------- ----------------
1 ADDRESS RAW(8)
2 HASH_VALUE NUMBER
3 SQL_ID VARCHAR2(13)
4 CHILD_NUMBER NUMBER
5 PREDICATE VARCHAR2(40)
6 RANGE_ID NUMBER
7 LOW VARCHAR2(10)
8 HIGH VARCHAR2(10)
9 CON_ID NUMBER

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Adaptive Cursor
Sharing
•Conclusion
• Literal variables are good for query performance
• very bad for resource and memory
• and they produce a non scalable application
• Bind variables are not always good for query performance
• very good for resource and memory
• and they produce a scalable application
• Adaptive cursor sharing allow query good performance
• even when using bind variable
• But be aware of the extra work it might introduce