Database statistics are not limited to tables, columns, and indexes. PL/SQL functions also have a number of associated statistics, namely costs (CPU, I/O, network), selectivity, and cardinality (for functions that return collections). These statistics have default values that only somewhat represent reality. However, these values are always used by Oracle's cost-based optimizer to build execution plans. This session uses real-life examples to illustrate how properly managed PL/SQL statistics can significantly improve executions plans. It also demonstrates that Oracle's extensible optimizer is flexible enough to support packaged functions.

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The Hidden Face
of the Cost Based Optimizer:
PL/SQL-Specific Statistics
[UGF2781]
Michael Rosenblum
Dulcian, Inc.
www.dulcian.com

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Who Am I? – “Misha”
Oracle ACE
Co-author of 3 books
 PL/SQL for Dummies
 Expert PL/SQL Practices
 Oracle PL/SQL Performance Tuning Tips & Techniques
(Rosenblum & Dorsey, Oracle Press, July 2014)
Won ODTUG 2009 Speaker of the Year
Known for:
 SQL and PL/SQL tuning
 Complex functionality
 Code generators
 Repository-based development

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Did you know that…?
User-defined functions have a number of statistics
associated with them.
These statistics impact decisions made by the Cost Based
Optimizer (CBO).
Default values of these statistics are …well…less than
adequate.
… but you can adjust them manually!

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Defaults
Hardware resources
CPU cost – 3000 [CPU instructions]
I/O cost – 0 [data blocks to be read/written]
Network cost – 0 [data blocks to be read/written]
Cardinality – 8168 [rows]
Selectivity – 1% [out of total set]

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Cost of Standalone Function

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Basic Case
Problem:
There are two functions in SQL statement.
You want to tell CBO that one of them is expensive.
Solution:
ASSOCIATE STATISTICS WITH FUNCTIONS f_light_tx
DEFAULT COST (0,0,0) /* CPU,IO,Network */; -- light
ASSOCIATE STATISTICS WITH FUNCTIONS f_heavy_tx
DEFAULT COST (99999,99999,99999); -- heavy

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Impact
SQL> set autotrace on explain
SQL> SELECT count(*) FROM emp
2 WHERE f_heavy_tx(deptno) = 'A' OR f_light_tx(empno) = 'B';
COUNT(*)
----------
0
Execution Plan
-----------------------------------------------------
| Id | Operation | Name | Rows | Bytes |
---------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 7 |
| 1 | SORT AGGREGATE | | 1 | 7 |
|* 2 | TABLE ACCESS FULL| EMP | 1 | 7 |
---------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("F_LIGHT_TX"("EMPNO")='B‘ OR "F_HEAVY_TX"("DEPTNO")='A')
Change of execution
order

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Increasing Complexity
Problem:
Hardcoding high values is a cheat
 … although, in some cases it may be enough.
Solution:
Get actual statistics
 … if you can simulate a real world environment.

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Measuring Statistics – Sample Function
CREATE FUNCTION f_getdeptinfo_tx (i_deptno NUMBER)
RETURN VARCHAR2 IS
v_out_tx VARCHAR2(256);
BEGIN
SELECT dname
INTO v_out_tx
FROM scott.dept@remoteDB
WHERE deptno = i_deptno;
SELECT v_out_tx||':'||count(*)
INTO v_out_tx
FROM scott.emp
WHERE deptno = i_deptno;
RETURN v_out_tx;
END;
Remote call

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Measuring Statistics - Snapshot Before
SQL> SELECT f_getdeptinfo_tx (10) FROM DUAL;
SQL> SELECT a.name, b.value
2 FROM v$statname a, v$mystat b
3 WHERE a.statistic# = b.statistic#
4 AND name IN ('db block gets', -- physical reads
5 'consistent gets', -- logical reads
6 'CPU used by this session', -- CPU
7 'bytes sent via SQL*Net to dblink', -- DB-link
8 'bytes received via SQL*Net from dblink' -- DB-link
9 );
NAME VALUE
-------------------------------------- ----------
CPU used by this session 9
db block gets 12
consistent gets 226
bytes sent via SQL*Net to dblink 3459
bytes received via SQL*Net from dblink 4070
Cause parse
and ignore

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Measuring Statistics - Snapshot After
SQL> SELECT f_getdeptinfo_tx (10) FROM DUAL;
SQL> SELECT a.name, b.value
2 FROM v$statname a, v$mystat b
3 WHERE a.statistic# = b.statistic#
4 AND name IN ('db block gets', -- physical reads
5 'consistent gets', -- logical reads
6 'CPU used by this session', -- CPU
7 'bytes sent via SQL*Net to dblink', -- DB-link
8 'bytes received via SQL*Net from dblink' -- DB-link
9 );
NAME VALUE
-------------------------------------- ----------
CPU used by this session 11 [was 9]
db block gets 12 [was 12]
consistent gets 232 [was 226]
bytes sent via SQL*Net to dblink 4113 [was 3459]
bytes received via SQL*Net from dblink 4603 [was 4070]
Real call

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Real Numbers
Difference:
CPU time = 2 hs
I/O = 6 blocks
 Physical Reads = 0 blocks
 Logical Reads = 6 blocks
Network = 2 blocks
 Sent via DBLink = 654 bytes ~ 1 block
 Received via DBLink = 533 bytes ~ 1 block
Needs translation

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CPU Time Format Conversion
Convert CPU time into CPU instructions:
SQL> DECLARE
2 v_units_nr NUMBER;
3 v_time_nr NUMBER:=0.02; -- time in seconds
4 BEGIN
5 v_units_nr:=
6 DBMS_ODCI.ESTIMATE_CPU_UNITS (v_time_nr)* 1000;
7 DBMS_OUTPUT.PUT_LINE
8 ('Instructions:'||round(v_units_nr));
9 END;
10 /
Instructions:18783086
Function output is in
thousands of instructions

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Final Step
Associate real statistics:
ASSOCIATE STATISTICS WITH FUNCTIONS f_getDeptInfo_tx
DEFAULT COST (
18783086, -- CPU instructions
6, -- local IO
2); -- network

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Cost of Packaged Functions

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Problem
Task:
Multiple functions in the same package.
Need to associate different statistics with each of them.
Problem:
No syntax to hard code statistics using the
PACKAGE.FUNCTION format.
 … but you can hardcode statistics to the whole package (i.e. all
functions will share the same numbers)
 … via: ASSOCIATE STATISTICS WITH PACKAGES <name>
Solution:
ODCI object type interface!

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Sample Package
CREATE PACKAGE perf_pkg IS
FUNCTION f_heavy_tx (i_deptno NUMBER) RETURN VARCHAR2;
FUNCTION f_light_tx (i_empno NUMBER) RETURN VARCHAR2;
FUNCTION f_medium_tx (i_name VARCHAR2) RETURN VARCHAR2;
END;
CREATE OR REPLACE PACKAGE BODY perf_pkg is
FUNCTION f_heavy_tx (i_deptno NUMBER) RETURN VARCHAR2 IS
BEGIN RETURN 'heavy:'||i_deptno; END;
FUNCTION f_light_tx (i_empno NUMBER) RETURN VARCHAR2 IS
BEGIN RETURN 'light:'||i_empno; END;
FUNCTION f_medium_tx (i_name VARCHAR2) RETURN VARCHAR2 IS
BEGIN RETURN initcap(i_name); END;
END;

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Key Discovery
ODCI interface:
Does not care about names of function parameters, but does
care about datatypes:
You need to record all possible combinations of inputs.
In this case:
 2 functions with NUMBER inputs
 1 function with VARCHAR2 input

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Object Type (1)
CREATE OR REPLACE TYPE function_stat_oty AS OBJECT (
dummy_attribute NUMBER,
STATIC FUNCTION ODCIGetInterfaces (p_interfaces OUT sys.odciobjectlist)
RETURN NUMBER,
STATIC FUNCTION ODCIStatsFunctionCost
(p_func_info IN sys.odcifuncinfo,
p_cost OUT sys.odcicost,
p_args IN sys.odciargdesclist,
i_single_nr IN NUMBER,
p_env IN sys.odcienv) RETURN NUMBER,
STATIC FUNCTION ODCIStatsFunctionCost
(p_func_info IN sys.odcifuncinfo,
p_cost OUT sys.odcicost,
p_args IN sys.odciargdesclist,
i_single_tx IN varchar2,
p_env IN sys.odcienv) RETURN NUMBER
)
One function for each
datatype permutation

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Object Type (2)
CREATE OR REPLACE TYPE BODY function_stat_oty as
STATIC FUNCTION ODCIGetInterfaces
(p_interfaces OUT sys.odciobjectlist)
RETURN NUMBER IS
BEGIN
p_interfaces := sys.odciobjectlist(
sys.odciobject ('sys', 'odcistats2')
);
RETURN odciconst.success;
END odcigetinterfaces;
Required by ODCI

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Object Type (3)
STATIC FUNCTION ODCIStatsFunctionCost
(p_func_info IN sys.odcifuncinfo,
p_cost OUT sys.odcicost,
p_args IN sys.odciargdesclist,
i_single_nr IN NUMBER,
p_env IN sys.odcienv
) RETURN NUMBER IS
BEGIN
IF LOWER(p_func_info.methodname) LIKE '%heavy%' THEN
p_cost := sys.odcicost
(cpucost=>NULL,
iocost=>1000,
networkcost=>NULL,
indexcostinfo=>NULL);
END IF;
RETURN odciconst.success;
END;
Record type containing:
- ObjectSchema
- ObjectName – name of package
or standalone function
- MethodName – name of
packaged function
- Flags

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Object Type (4)
STATIC FUNCTION ODCIStatsFunctionCost
(p_func_info IN sys.odcifuncinfo,
p_cost OUT sys.odcicost,
p_args IN sys.odciargdesclist,
i_single_tx IN VARCHAR2,
p_env IN sys.odcienv
) RETURN NUMBER IS
BEGIN
IF LOWER(p_func_info.methodname) LIKE '%medium%' THEN
p_cost := sys.odcicost(NULL, 10, NULL, NULL);
END IF;
RETURN odciconst.success;
END;
END;
Second permutation

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Test Case
SQL> ASSOCIATE STATISTICS WITH PACKAGES perf_pkg
2 USING function_stat_oty;
SQL> SET AUTOTRACE ON EXPLAIN
SQL> SELECT count(*) FROM emp
2 WHERE perf_pkg.f_heavy_tx(empno)='A'
3 OR perf_pkg.f_light_tx(deptno)='B'
4 OR perf_pkg.f_medium_tx(job)='C';
COUNT(*)
----------
0
Starting from HEAVY

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Impact
Execution Plan
----------------------------------------------------------
Plan hash value: 2083865914
----------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
----------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 15 | 13863 (0)|
| 1 | SORT AGGREGATE | | 1 | 15 | |
|* 2 | TABLE ACCESS FULL| EMP | 1 | 15 | 13863 (0)|
----------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("PERF_PKG"."F_LIGHT_TX"("DEPTNO")='B' OR
"PERF_PKG"."F_MEDIUM_TX"("JOB")='C' OR
"PERF_PKG"."F_HEAVY_TX"("EMPNO")='A')
… but CBO started
from LIGHT!

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Function Cardinality

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Issue
Task:
Use Object Collections as a part of a SQL statement with the
TABLE clause.
Problem:
Oracle’s default cardinality of the collection causes the CBO
to make bad decisions.

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Test Case
-- create table
CREATE TABLE inlist_tab AS
SELECT object_id, created, object_type
FROM all_objects
WHERE object_id IS NOT NULL;
ALTER TABLE inlist_tab
ADD CONSTRAINT inlist_tab_pk PRIMARY KEY (object_id) USING INDEX;
BEGIN
dbms_stats.gather_table_stats(user,'INLIST_TAB');
END;
-- create object collection
CREATE TYPE id_tt IS TABLE OF NUMBER;

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Problem Illustration
SELECT /*+ gather_plan_statistics */ MAX(created)
FROM inlist_tab
WHERE object_id IN (SELECT t.column_value
FROM TABLE(id_tt(100,101)) t)
-- run DBMS_XPLAN.DISPLAY_CURSOR
-----------------------------------------------------------------------
|Id | Operation |Name |E-Rows|A-Rows
-----------------------------------------------------------------------
| 0 | SELECT STATEMENT | | | 1
| 1 | SORT AGGREGATE | | 1| 1
|*2 | HASH JOIN | | 8168| 2
| 3 | COLLECTION ITERATOR CONSTRUCTOR FETCH| | 8168| 2
| 4 | TABLE ACCESS FULL |INLIST_TAB| 29885| 89761
-----------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID"=VALUE(KOKBF$))
Only 2 objects…
… cause full-table scan!

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Possible Options
Hints:
CARDINALITY hint – manual cardinality override
 Pro: Simple
 Con: Hardcoded
DYNAMIC_SAMPLING – let Oracle check the data
 Pro: Avoid hard coding
 Con: Involves extra SQL overhead, while PL/SQL already knows
how many objects are in the collection

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Impact of Hints
SELECT /*+ gather_plan_statistics */ MAX(created)
FROM inlist_tab
WHERE object_id IN (
SELECT /*+ dynamic_sampling (t 2) */ t.column_value
-- SELECT /*+ cardinality (t 2) */ t.column_value
FROM TABLE(id_tt(227011,227415)) t)
--------------------------------------------------------------------------
|Id|Operation |Name |E-Rows |A-Rows
--------------------------------------------------------------------------
| 0|SELECT STATEMENT | | | 1
| 1| SORT AGGREGATE | | 1 | 1
| 2| NESTED LOOPS | | | 2
| 3| NESTED LOOPS | | 2 | 2
| 4| COLLECTION ITERATOR CONSTRUCTOR FETCH| | 2 | 2
|*5| INDEX UNIQUE SCAN |INLIST_TAB_PK| 1 | 2
| 6| TABLE ACCESS BY INDEX ROWID |INLIST_TAB | 1 | 2
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("OBJECT_ID"=VALUE(KOKBF$))
Using index!

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+ Extra Option - ODCI
ODCI interface:
ODCIStatsTableFunction method
 It can work only with functions
 … so if you need to use a default constructor, you must create a
“transmitter” function that would have associated statistics:
CREATE OR REPLACE FUNCTION MyCard(i_tt id_tt)
RETURN id_tt IS
BEGIN
RETURN i_tt;
END;

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Object Type (1)
CREATE TYPE MyCard_OT AS OBJECT (
dummy_attribute NUMBER,
STATIC FUNCTION ODCIGetInterfaces
(p_interfaces OUT SYS.ODCIObjectList)
RETURN NUMBER,
STATIC FUNCTION ODCIStatsTableFunction (
p_function IN SYS.ODCIFuncInfo,
p_stats OUT SYS.ODCITabFuncStats,
p_args IN SYS.ODCIArgDescList,
i_tt IN id_tt)
RETURN NUMBER
); Object collection as input

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Object Type (2)
CREATE TYPE BODY MyCard_OT AS
STATIC FUNCTION ODCIGetInterfaces ...
STATIC FUNCTION ODCIStatsTableFunction
(p_function IN SYS.ODCIFuncInfo,
p_stats OUT SYS.ODCITabFuncStats,
p_args IN SYS.ODCIArgDescList,
i_tt IN id_tt) RETURN NUMBER IS
BEGIN
p_stats := SYS.ODCITabFuncStats(i_tt.COUNT);
RETURN ODCIConst.success;
END ODCIStatsTableFunction;
END;
Set
statistics

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Impact of Statistics
ASSOCIATE STATISTICS WITH FUNCTIONS MyCard USING mycard_ot;
SELECT /*+ gather_plan_statistics*/ MAX(created)
FROM inlist_tab
WHERE object_id IN (SELECT t.column_value
FROM table(MyCard(id_tt(100,101))) t)
---------------------------------------------------------------------
|Id|Operation |Name |E-Rows|A-Rows
---------------------------------------------------------------------
| 0|SELECT STATEMENT | | | 1
| 1| SORT AGGREGATE | | 1| 1
| 2| NESTED LOOPS | | | 2
| 3| NESTED LOOPS | | 2| 2
| 4| COLLECTION ITERATOR PICKLER FETCH|MYCARD | 2| 2
|*5| INDEX UNIQUE SCAN |INLIST_TAB_PK| 1| 2
| 6| TABLE ACCESS BY INDEX ROWID |INLIST_TAB | 1| 2
---------------------------------------------------------------------
Predicate Information (identified by operation id):
--------------------------------------------------
5 - access("OBJECT_ID"=VALUE(KOKBF$))
Valid stats
Constructor is wrapped

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Function Selectivity

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Issue
Default:
 1% ~ if you compare function to a literal, only every 1/100th row
satisfies the condition.
Problem:
 If your function is heavily skewed  CBO predicate analysis will
generate bad execution plans.
Solutions:
 ODCI object method can adjust selectivity setting.
 You can also hard code selectivity (if data is static):
ASSOCIATE STATISTICS WITH FUNCTIONS f_isSenior_yn
DEFAULT selectivity 50;

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Test Case
CREATE OR REPLACE FUNCTION f_isSenior_yn (i_job_id VARCHAR2)
RETURN VARCHAR2 IS
BEGIN
IF i_job_id IN ('AD_PRES','AD_VP') THEN
RETURN 'Y';
ELSE
RETURN 'N';
END IF;
END;
SQL> select f_isSenior_yn(job_id) isSenior_yn, count(*)
2 from hr.employees
3 group by f_isSenior_yn(job_id);
ISSENIOR_YN COUNT(*)
----------- ----------
Y 3
N 104

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Problem Illustration
SELECT /*+ gather_plan_statistics*/
e.*,
d.department_name
FROM hr.employees e,
hr.departments d
WHERE e.department_id = d.department_id
AND f_isSenior_yn(e.job_id)='N'
--------------------------------------------------------------------
|Id|Operation |Name |E-Rows|A-Rows|Buffers|Used-Mem |
--------------------------------------------------------------------
| 0|SELECT STATEMENT | | | 103| 14| |
|*1| HASH JOIN | | 1| 103| 14| 901K(0)|
|*2| TABLE ACCESS FULL|EMPLOYEES | 1| 104| 7| |
| 3| TABLE ACCESS FULL|DEPARTMENTS| 1| 27| 7| |
--------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("EMPLOYEES"."DEPARTMENT_ID"="DEPARTMENTS"."DEPARTMENT_ID")
2 - filter("F_ISSENIOR_YN"("EMPLOYEES"."JOB_ID")='N')
1% of 107 is way off!
Heavy memory usage

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Object Type (1)
CREATE TYPE MySelect_OT AS OBJECT (
dummy_attribute NUMBER,
STATIC FUNCTION ODCIGetInterfaces
(p_interfaces OUT SYS.ODCIObjectList) RETURN NUMBER,
STATIC FUNCTION ODCIStatsSelectivity (
p_pred_info IN SYS.ODCIPredInfo,
p_selectivity OUT NUMBER,
p_args IN SYS.ODCIArgDescList,
p_start IN VARCHAR2,
p_stop IN VARCHAR2,
i_job IN VARCHAR2,
p_env IN SYS.ODCIEnv)
RETURN NUMBER
);

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Object Type (2)
CREATE or replace TYPE BODY MySelect_OT AS
STATIC FUNCTION ODCIGetInterfaces …;
STATIC FUNCTION ODCIStatsSelectivity (
p_pred_info IN SYS.ODCIPredInfo,
p_selectivity OUT NUMBER,
p_args IN SYS.ODCIArgDescList,
p_start IN VARCHAR2,
p_stop IN VARCHAR2,
i_job IN VARCHAR2,
p_env IN SYS.ODCIEnv
) RETURN NUMBER IS
BEGIN
if p_start='Y' then
p_selectivity:=3;
else
p_selectivity:=97;
end if;
RETURN ODCIConst.success;
END ODCIStatsSelectivity;
END;
START is used
for ‘=‘ comparison
START and STOP are used
for ‘BETWEEN‘ comparison

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Impact of Statistics
ASSOCIATE STATISTICS WITH FUNCTIONS f_isSenior_yn USING MySelect_OT;
SELECT /*+ gather_plan_statistics*/ e.*,
d.department_name
FROM hr.employees e,
hr.departments d
WHERE e.department_id = d.department_id
AND f_isSenior_yn(e.job_id)='N'
-----------------------------------------------------------------------------
|Id|Operation |Name |E-Rows|A-Rows|Buffers|Used-Mem|
-----------------------------------------------------------------------------
| 0|SELECT STATEMENT | | | 103| 9| |
| 1| MERGE JOIN | | 103| 103| 9| |
| 2| TABLE ACCESS BY INDEX ROWID|DEPARTMENTS| 27| 27| 2| |
| 3| INDEX FULL SCAN |DEPT_ID_PK | 27| 27| 1| |
|*4| SORT JOIN | | 104| 103| 7|14336(0)|
|*5| TABLE ACCESS FULL |EMPLOYEES | 104| 104| 7| |
-----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("EMPLOYEES"."DEPARTMENT_ID"="DEPARTMENTS"."DEPARTMENT_ID")
filter("EMPLOYEES"."DEPARTMENT_ID"="DEPARTMENTS"."DEPARTMENT_ID")
5 - filter("F_ISSENIOR_YN"("EMPLOYEES"."JOB_ID")='N')
Different execution plan!
Much less memory

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Summary
Keeping Oracle Statistics up to date is important
… otherwise the CBO can get confused.
Manual management of PL/SQL function statistics is non-
trivial.
… so it should be used only when needed.
Using PL/SQL functions within SQL should be tightly
controlled
… because usually statistics are the least of your problems!

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Contact Information
 Michael Rosenblum – mrosenblum@dulcian.com
 Dulcian, Inc. website - www.dulcian.com
 Blog: wonderingmisha.blogspot.com
Available NOW:
Oracle PL/SQL Performance Tuning Tips & Techniques

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