beacon ora perf class 2

2007-12-251

Beacon Infotech Corporation

www.oracleact.com

Oracle Performance Tuning

Tamilselvan G

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2007-12-252


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Tuning Joins and Sub-Queries

1. Nested Loops Join

2. Sort Merge Join

3. Hash Join

4. Hash Join Outer

5. Cartesian Join

6. Outer Join

7. Hierarchical (Self) Join

8. Sub Query

9. Semi Join

10. Anti Join

2007-12-253


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Nested loops join is the workhorse join of any RDBMS engine.

How it works : Oracle performs a search of the inner table for each row found in the

outer table, and join the image, then output the result.

SQL> select a.empid, a.bonus old_bonus, b.bonus new_bonus

from emp a, new_bonus b

where a.empid = b.empid ;

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE

1 0 NESTED LOOPS

2 1 TABLE ACCESS (FULL) OF 'NEW_BONUS'

3 1 TABLE ACCESS (BY INDEX ROWID) OF 'EMP'

4 3 INDEX (UNIQUE SCAN) OF 'EMP_UQ' (UNIQUE)

Oracle can work with 2 tables at the most at any time in a

join operation, no matter how many tables are used in the

from clause.

Oracle Performance Tuning 1. Nested Loops Join

2007-12-254


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Pseudo code

For x in (select EMPID, BONUS from NEW_BONUS) loop

Look up the index EMP_UQ using x.EMPID,

if a matching KEY is found in EMP_UQ index, get the ROWID of EMP

Get the BONUS column from EMP for the ROWID

output the result

end if

End loop

Advantages of NL Join

1. Output the first row as fast as possible

2. No temporary space is needed in most of the cases, unless a aggregate function

is used

3. FIRST_ROW or RULE hint always favors NL Join

4. Uses Index scan if suitable index is available.

Disadvantages of NL Join

1. If the result set is very large, then performance is horrible.

2. Not suitable for DW system because of processing large data sets.

Oracle Performance Tuning 1.Nested Loops Join

2007-12-255


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Oracle Performance Tuning 1.Nested Loop Join

HINT Usage

SELECT /*+ LEADING(B) USE_NL(A) */ A.ID, A.NAME, B.DOB

FROM BIG_TABLE A, SMALL_TABLE B

WHERE A.ID = B.ID

Rows Row Source Operation

------- ---------------------------------------------------

20000 NESTED LOOPS (cr=44109 pr=3121 pw=0 time=1725511 us)

20000 TABLE ACCESS FULL SMALL_TABLE (cr=2774 pr=1540 pw=0 time=244921 us)

20000 TABLE ACCESS BY INDEX ROWID BIG_TABLE (cr=41335 pr=1581 pw=0 time=1405038 us)

20000 INDEX UNIQUE SCAN BIG_TABLE_IDX (cr=21335 pr=42 pw=0 time=637526 us)(object id 66371)

SELECT /*+ LEADING(A) USE_NL(B) */ A.NAME, A.ID, B.DOB

FROM BIG_TABLE A, SMALL_TABLE B

WHERE A.ID = B.ID


------- ---------------------------------------------------


20000 TABLE ACCESS FULL BIG_TABLE (cr=2775 pr=1541 pw=0 time=261823 us)

20000 TABLE ACCESS BY INDEX ROWID SMALL_TABLE (cr=41335 pr=1581 pw=0 time=1457905 us)

20000 INDEX UNIQUE SCAN SMALL_TABLE_IDX (cr=21335 pr=42 pw=0 time=661071 us)(object id 66373)

********************************************************************************

2007-12-256


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Oracle Performance Tuning 2.Sort Merge Join

select a.id, a.name, b.dob

from (select * from big_table

where rownum < 11

order by dbms_random.value) a,

(select * from small_table

where rownum < 11

order by dbms_random.value) b

where a.dob >= b.dob and

a.id b.id

call count cpu elapsed disk query current rows

------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.05 0.04 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 4 0.02 0.01 0 8 0 45

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 6 0.07 0.06 0 8 0 45

Sort-Merge Join is used:

1. When Oracle sees IN-

EQUIVALITY Operator in the

WHERE Clause.

2. When USE_MERGE hint is added

in the Query.

2007-12-257


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Oracle Performance Tuning 2.Sort-Merge Join


------- ---------------------------------------------------

45 MERGE JOIN (cr=8 pr=0 pw=0 time=20991 us)

10 SORT JOIN (cr=4 pr=0 pw=0 time=19593 us)

10 VIEW (cr=4 pr=0 pw=0 time=18898 us) -- Materialized

10 SORT ORDER BY (cr=4 pr=0 pw=0 time=18871 us)

10 COUNT STOPKEY (cr=4 pr=0 pw=0 time=491 us)


45 FILTER (cr=4 pr=0 pw=0 time=1773 us)


10 VIEW (cr=4 pr=0 pw=0 time=1257 us) -- Materialized

10 SORT ORDER BY (cr=4 pr=0 pw=0 time=1224 us)

10 COUNT STOPKEY (cr=4 pr=0 pw=0 time=426 us)


********************************************************************************

1. No driving table concept in Sort-Merge Join

2. Each data set is sorted prior to merge.

3. The output rows are NOT sorted even though individual data

set is sorted.

4. You need to specify the ORDER BY clause if you want sorted

order output.

2007-12-258


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Oracle Performance Tuning 2.Sort-Merge Join

USE_MERGE HINT

SELECT /*+ USE_MERGE(A, B) */

COUNT(*), COUNT(A.ID), COUNT(B.DOB) , COUNT(A.NAME)

FROM BIG_TABLE A ,

SMALL_TABLE B

WHERE A.ID = B.ID


------- ---------------------------------------------------

1 SORT AGGREGATE (cr=3087 pr=3081 pw=0 time=819538 us)

20000 MERGE JOIN (cr=3087 pr=3081 pw=0 time=811605 us)





********************************************************************************

Note: The opposite of USE_MERGE is NO_USE_MERGE.

Event 10032 details about Sort, and event 10033 about Sort IO.

2007-12-259


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Oracle Performance Tuning 3. Hash Join

Before we study Hash Join, examine the following query :

select /*+ RULE */ count(*) ---- To simulate NL Join, I used RULE hint

from sales a, customer b

where a.cust_id = b.cust_id


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.01 0.01 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 2 31.33 30.60 0 1,004,659 0 1

------- ------ -------- ---------- ---------- ------------- ---------- ----------

total 4 31.34 30.61 0 1,004,659 0 1


------- ---------------------------------------------------



1000000 TABLE ACCESS FULL SALES (cr=4657 pr=0 pw=0 time=5000343 us)

1000000 INDEX UNIQUE SCAN CUSTOMER_IDX (cr=1000002 pr=0 pw=0 time=22641908 us)(object id 66385)

2007-12-2510


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Oracle Performance Tuning 3. Hash Join ...

select count(*)

from sales a, customer b

where a.cust_id = b.cust_id


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.04 0.04 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 2 5.64 5.51 4655 4701 0 1

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 4 5.68 5.55 4655 4701 0 1


------- ---------------------------------------------------


1000000 HASH JOIN (cr=4716 pr=4655 pw=0 time=7098567 us)

20000 INDEX FAST FULL SCAN CUSTOMER_IDX (cr=44 pr=0 pw=0 time=20331 us)(object id 66385)

1000000 TABLE ACCESS FULL SALES (cr=4657 pr=4655 pw=0 time=1004056 us)

2007-12-2511


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5.51 times faster

run time

5.5530.61

Run Time

in Seconds

Consumed 213

times less LIO

4,7011,004,659# of LIO

Hash Join

Performance

Improvement

Hash Join

Nested

Loops

Join

Comparison

How did it happen?

2007-12-2512


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Let us study how Hash Join works:

Under optimal conditions, Oracle builds a hash table for the smaller of the two data

sets. The hash table is not a Hash Cluster. The join key CUST_ID in this case was

hashed by an index in the hash table. The hash table is an array and the row that went

with the hash key was stored in that Index entry. We can also call the index key as

bucket one bucket may have one or many rows. CUSTOMER_IDX is hashed in our

case.

Hash Join does not require latches, because the hash table in the memory is private,

no one can access this hash table. Once hashed the smaller table into memory, Oracle

full scans the larger table ( SALES in our case, other named used is PROBE TABLE),

then for each row Oracle hashes the CUST_ID, finds the matching index key (bucket

number) in the HASH table. If a match is found, then Oracle scans the rows with in the

bucket. If one or more rows match, then Oracle returns the joined image.

When hash table is not fit into memory, then the hash table is broken into smaller

chunks, and stored in the TEMP Tablespace.

Note: Tom says Hash Table is not a Hash Cluster, but its similar to Hash Table used in

Java or C++, where as Jonathan writes Hash Table is equivalent to a single Table

Hash Cluster.

2007-12-2513


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Why faster execution ?

1. No latch is needed because the hash table is in private memory, hence cache

buffer chain latch is not acquired.

2. The hash table itself acts like an index

3. Full table scan on the larger data set if needed uses

DB_FILE_MULTIBLOCK_READ_COUNT parameter.

Where to use Hash Join:

1 When joining 2 big data sets

2 When joining one big data sets with smaller data sets

Categories in Hash Join

There are 3 categories of Workarea executions Optimal, Onepass and Multipass

which you can see in V$SYSSTAT.

Hash Join Trace

By enabling 10104 events, you can get a lot of details.

2007-12-2514


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Monitoring

SQL> select * from v$sysstat where name like 'workarea exec%'

STATISTIC# NAME CLASS VALUE STAT_ID

---------- ------------------------------- ------ ------ ----------

294 workarea executions - optimal 64 7423 3211650785

295 workarea executions - onepass 64 5 798730793

296 workarea executions - multipass 64 0 3804491469

2007-12-2515


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Oracle Performance Tuning 3. Hash Join .

V$SQL_WORKAREA in 10g

Name Null? Type

-------------------------- -------- ------------------

ADDRESS RAW(8)

HASH_VALUE NUMBER

SQL_ID VARCHAR2(13)

CHILD_NUMBER NUMBER

WORKAREA_ADDRESS RAW(8)

OPERATION_TYPE VARCHAR2(20)

OPERATION_ID NUMBER

POLICY VARCHAR2(10)

ESTIMATED_OPTIMAL_SIZE NUMBER

ESTIMATED_ONEPASS_SIZE NUMBER

LAST_MEMORY_USED NUMBER

LAST_EXECUTION VARCHAR2(10)

LAST_DEGREE NUMBER

TOTAL_EXECUTIONS NUMBER

OPTIMAL_EXECUTIONS NUMBER

ONEPASS_EXECUTIONS NUMBER

MULTIPASSES_EXECUTIONS NUMBER

ACTIVE_TIME NUMBER

MAX_TEMPSEG_SIZE NUMBER

LAST_TEMPSEG_SIZE NUMBER

V$SQL_WORKAREA_ACTIVE

Name Null? Type

-------------------------- -------- ------------------

SQL_HASH_VALUE NUMBER

SQL_ID VARCHAR2(13)

WORKAREA_ADDRESS RAW(8)

OPERATION_TYPE VARCHAR2(20)

OPERATION_ID NUMBER

POLICY VARCHAR2(6)

SID NUMBER

QCINST_ID NUMBER

QCSID NUMBER

ACTIVE_TIME NUMBER

WORK_AREA_SIZE NUMBER

EXPECTED_SIZE NUMBER

ACTUAL_MEM_USED NUMBER

MAX_MEM_USED NUMBER

NUMBER_PASSES NUMBER

TEMPSEG_SIZE NUMBER

TABLESPACE VARCHAR2(31)

SEGRFNO# NUMBER

SEGBLK# NUMBER

2007-12-2516


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Oracle Performance Tuning 4. Hash Join Outer

SQL> select * from supp ;

SUPP_ NAME ST

----- ------------------------------ --

1001 ABC Inc CA

1002 Microsoft NC

1003 Oracle Corp CA

1004 IBM Corp MA

SQL> select * from state;

ST NAME

-- ------------------------------

GA Georgia

CA California

NY New York

AL Alabama

TX Texas

MA Massachusetts

SC South Carolina

NC North Carolina

AZ Arizona

9 rows selected.

2007-12-2517


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Oracle Performance Tuning 4. Hash Join Outer.

SQL> EXPLAIN PLAN FOR

SELECT B.NAME SUPPLIER, A.STATEID STATEID, A.NAME STATE

FROM STATE A , SUPP B

WHERE B.STATEID(+) = A.STATEID ;

Explained.

SQL> select * from table( dbms_xplan.display );

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost |

--------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 9 | 333 | 5 |

|* 1 | HASH JOIN OUTER | | 9 | 333 | 5 |

| 2 | TABLE ACCESS FULL | STATE | 9 | 144 | 2 |

| 3 | TABLE ACCESS FULL | SUPP | 4 | 84 | 2 |

-------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

PLAN_TABLE_OUTPUT

---------------------------------------------------

1 - access("B"."STATEID"(+)="A"."STATEID")

Note: cpu costing is off

2007-12-2518


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Oracle Performance Tuning 4. Hash Join Outer.

SQL> SELECT B.NAME SUPPLIER,

A.STATEID STATEID,

A.NAME STATE

FROM STATE A , SUPP B

WHERE B.STATEID(+) = A.STATEID ;

SUPPLIER ST STATE

------------------------- -- -----------------------------

ABC Inc CA California

Microsoft NC North Carolina

Oracle Corp CA California

IBM Corp MA Massachusetts

AZ Arizona

NY New York

GA Georgia

TX Texas

SC South Carolina

AL Alabama

10 rows selected.

How Hash Join Outer Works:

Oracle document says the outer table (whose

rows are preserved) is used to build the

hash table, and the inner table is used to

probe the hash table.

In our example, STATE table is hashed, and the

SUPP table is used to probe the hash table.

Steps involved:

1. STATE table is hashed

2. Full Scan SUPP table for each row

3. Using HASH function on STATEID from

both tables, rows are output and mark the

matched row in STATE table

4. Finally, again full scan on STATE table and

output unmarked rows.

There is NO WAY a outer joined table

can be hashed.

2007-12-2519


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Oracle Performance Tuning 5. Cartesian Join

In a Cartesian join, the product of two data sets are output.

SQL> SELECT STUDENT_ID ID,

STUDENT_NAME NAME

FROM STUDENT ;

ID NAME

---------- ----------

101 Tamil

102 Ram

103 Tom

SQL> SELECT SUBJECT_ID,

SUBJECT_NAME

FROM SUBJECTS;

SUBJECT_ID SUBJECT_NAME

---------- --------------------

21 English

22 Math

23 History

24 Computer Science

SQL> SELECT STUDENT_ID,

STUDENT_NAME NAME,

SUBJECT_NAME SUB_NAME

FROM SUBJECTS, STUDENT ;

STUDENT_ID NAME SUB_NAME

---------- ---------- --------------------

101 Tamil English

101 Tamil Math

101 Tamil History

101 Tamil Computer Science

102 Ram English

102 Ram Math

102 Ram History

102 Ram Computer Science

103 Tom English

103 Tom Math

103 Tom History

103 Tom Computer Science

12 rows selected.

2007-12-2520


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Oracle Performance Tuning 5. Cartesian Join

SQL> SELECT * FROM EMP ;

EMPNO ENAME SALARY DEPTNO

---------- --------------- ---------- ----------

1001 Tamil 900 10

1002 Tandon 1900 10

1003 Muthu 3100 20

1004 Ravi 4100 20

1005 Veera 2700 30

SQL> SELECT A.EMPNO, A.ENAME, SALARY ,

ROUND((SALARY*100/TOT),2) PCT

FROM EMP A ,

(SELECT SUM(SALARY) TOT FROM EMP) ;

EMPNO ENAME SALARY PCT

---------- --------------- ---------- ----------

1001 Tamil 900 7.09

1002 Tandon 1900 14.96

1003 Muthu 3100 24.41

1004 Ravi 4100 32.28

1005 Veera 2700 21.26

We can change

the Cartesian

Product SQL into

Analytical

Function.

2007-12-2521


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Oracle Performance Tuning 6.Outer Join

What is a Outer Join?

A join condition using the outer join operator (+) with one or more columns of

one of the tables. Oracle returns all rows that meet the join condition. Oracle

also returns all rows from the table without the outer join operator for which

there are no matching rows in the table with the outer join operator.


EMPNO ENAME SALARY DEPTNO

---------- ------- ---------- ----------

1001 Tamil 1000 10

1002 Tandon 2000 10

1003 Muthu 3000 20

1004 Ravi 4000 20

SQL> SELECT * FROM DEPT;

DEPTNO DEPTNAME

---------- -----------------------

10 Sales

20 Marketing

30 Accounts

SQL> SELECT A.ENAME, B.DEPTNO,

B.DEPTNAME

FROM EMP A, DEPT B

WHERE B.DEPTNO = A.DEPTNO(+) ;

ENAME DEPTNO DEPTNAME

--------------- ---------- -------------------

Tamil 10 Sales

Tandon 10 Sales

Muthu 20 Marketing

Ravi 20 Marketing

30 Accounts -----

2007-12-2522


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Oracle Performance Tuning 6.Outer Join..

Types of Outer Joins: 1 Right Outer Join, 2 Left Outer Join, 3 Full Outer Join

SQL> SELECT A.ENAME,

NVL(A.DEPTNO,

B.DEPTNO) DEPTNO,

B.DEPTNAME

FROM EMP A, DEPT B

WHERE A.DEPTNO (+) = B.DEPTNO ;


--------------- --------- ------------

Tamil 10 Sales

Tandon 10 Sales

Muthu 20 Marketing

Ravi 20 Marketing

30 Accounts

ANSI SQL


NVL(A.DEPTNO, B.DEPTNO) DEPTNO,

B.DEPTNAME

FROM EMP A RIGHT OUTER JOIN DEPT B

ON (A.DEPTNO = B.DEPTNO) ;


--------------- ---------- ------------

Tamil 10 Sales

Tandon 10 Sales

Muthu 20 Marketing

Ravi 20 Marketing

30 Accounts

Right Outer Join

2007-12-2523


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2 NVL(A.DEPTNO, B.DEPTNO) DEPTNO,

3 B.DEPTNAME

4 FROM EMP A, DEPT B

5 WHERE A.DEPTNO = B.DEPTNO (+)

6 /


--------------- ---------- ------------

Tamil 10 Sales

Tandon 10 Sales

Muthu 20 Marketing

Ravi 20 Marketing

Veera

ANSI SQL


2 NVL(A.DEPTNO,B.DEPTNO) DEPTNO,

3 B.DEPTNAME

4 FROM EMP A

5 LEFT OUTER JOIN DEPT B

6 ON (A.DEPTNO = B.DEPTNO)

7 /


--------------- ---------- ------------

Tamil 10 Sales

Tandon 10 Sales

Muthu 20 Marketing

Ravi 20 Marketing

Veera

Left Outer Join

2007-12-2524


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SQL> select a.ename,

2 nvl(a.deptno,b.deptno) deptno,

3 b.deptname

4 from emp a , dept b

5 where a.deptno = b.deptno(+)

6 union

7 select a.ename,

8 b.deptno deptno ,

9 b.deptname

10 from emp a, dept b

11* where a.deptno(+) = b.deptno

SQL> /


--------------- ---------- ------------

Muthu 20 Marketing

Ravi 20 Marketing

Tamil 10 Sales

Tandon 10 Sales

Veera

30 Accounts

6 rows selected.

ANSI SQL

SQL> select a.ename,


3 b.deptname

4 from emp a

5 full outer join dept b

6 on (a.deptno = b.deptno)

7 /


--------------- --------- ------------

Tamil 10 Sales

Tandon 10 Sales

Muthu 20 Marketing

Ravi 20 Marketing

Veera

30 Accounts

6 rows selected.

FULL Outer Join

2007-12-2525


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FULL Outer Join

In the previous query I used

UNION for the FULL OUTER

JOIN.

Can any one rewrite the query

with UNION ALL?

2007-12-2526


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FULL Outer Join with ANSI Standard

SQL> explain plan for

2 select a.ename,


4 b.deptname

5 from emp a

6 full outer join dept b

7 on (a.deptno = b.deptno)

8 /

Explained.

SQL> select * from table( dbms_xplan.display );

PLAN_TABLE_OUTPUT

-----------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost |

----------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 6 | 312 | 12 |

| 1 | VIEW | | 6 | 312 | 12 |

| 2 | UNION-ALL | | | | |

| 3 | NESTED LOOPS OUTER | | 5 | 205 | 7 |

| 4 | TABLE ACCESS FULL | EMP | 5 | 110 | 2 |

|* 5 | TABLE ACCESS CLUSTER | DEPT | 1 | 19 | 1 |

| 6 | NESTED LOOPS ANTI | | 1 | 32 | 5 |

| 7 | TABLE ACCESS FULL | DEPT | 3 | 57 | 2 |

|* 8 | TABLE ACCESS CLUSTER| EMP | 5 | 65 | 1 |

---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):

---------------------------------------------------

5 - filter("A"."DEPTNO"="B"."DEPTNO"(+))

8 - filter("A"."DEPTNO"="B"."DEPTNO")

2007-12-2527


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Oracle Performance Tuning 7. Hierarchical Join

The Hierarchical query allows you to select rows in hierarchical order

Example:

SQL> SELECT * FROM EMP ORDER BY EMPID ;

EMPID LASTNAME DEPTID CUMSALARY MGRID

---------- ---------- ---------- ---------- ----------

1001 Scott 10 10600 ---------- CEO

1002 Tom 10 2500 1001 -------- Tom reports to Scott

1003 Veer 10 700 1002 -------- Veer reports to Tom

1004 Ram 10 500 1002

1005 Ragu 10 3100 1001 ---------- Ragu reports to Scott

1006 Bush 10 800 1005 ---------- Bush reports to Ragu

1007 Huss 10 600 1005

1008 Gopi 10 400 1005

8 rows selected. Note the CUMSALARY column. This column stores individual salary for those

employees who do not have subordinate, but stores cumulative salary for those employees who

have subordinate plus his salary.

Now our requirement is list all employees individual salary.

2007-12-2528


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Now our requirement is list all employees individual salary.

SQL> SELECT EMPID, MGRID, CUMSALARY,

2 ( CUMSALARY - (SELECT NVL(SUM(B.CUMSALARY),0) FROM EMP B

3 WHERE B.MGRID = A.EMPID

4 ) ) IND_SAL

5 FROM EMP A

6 START WITH MGRID IS NULL

7 CONNECT BY PRIOR EMPID = MGRID ; If the table is large, then create

separate indexes on EMPID

EMPID MGRID CUMSALARY IND_SAL and MGRID columns for

---------- ---------- ---------- ---------- performance improvement.

1001 10600 5000

1002 1001 2500 1300

1003 1002 700 700

1004 1002 500 500

1005 1001 3100 1300

1006 1005 800 800

1007 1005 600 600

1008 1005 400 400

8 rows selected.

2007-12-2529


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Pseudo columns

1 CONNECT_BY_ISCYCLE

2 CONNECT_BY_ISLEAF

3 LEVEL

SQL> SELECT NAME, CONNECT_BY_ISLEAF "ISLEAF",

2 LEVEL,

3 SYS_CONNECT_BY_PATH(NAME,'/') "PATH"

4 FROM EMP

5 START WITH MGRID IS NULL

6* CONNECT BY PRIOR EMPID = MGRID

SQL> /

NAME IsLeaf LEVEL Path

---------- ---------- ---------- --------------------

Scott 0 1 /Scott

Tom 0 2 /Scott/Tom

Kumar 1 3 /Scott/Tom/Kumar

Veera 0 2 /Scott/Veera

Selvan 1 3 /Scott/Veera/Selvan


EMPID NAME MGRID

---------- ---------- ----------

1001 Scott

1002 Tom 1001

1003 Kumar 1002

1004 Veera 1001

1005 Selvan 1004

These pseudo columns are unique

features of Oracle only, You will not

find them in any other RDBMS.

2007-12-2530


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Oracle Performance Tuning 8. Sub Query

Simple Sub Query - Uncorrelated Sub Query


EMPNO ENAME SALARY DEPTNO BONUS

---------- --------------- ---------- ---------- ----------

1001 Tamil 900 10

1002 Tandon 1900 10

1003 Muthu 3100 20

1004 Ravi 4100 20

1005 Veera 2700

SQL> SELECT * FROM EMP A

WHERE A.SALARY = ( SELECT MAX(SALARY) -------- Sub Query is executed one time

FROM EMP_TEMP B ) ;

EMPNO ENAME SALARY DEPTNO BONUS

---------- --------------- ---------- ---------- ----------

1004 Ravi 4100 20


------- ---------------------------------------------------

1 TABLE ACCESS FULL EMP

1 SORT AGGREGATE

5 TABLE ACCESS FULL EMP_TEMP

2007-12-2531


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Correlated Sub Query

Sub query is executed for every row

from the outer query.

SELECT * FROM EMP A

WHERE A.SALARY = ( SELECT SALARY

FROM EMP_TEMP B

WHERE A.EMPNO = B.EMPNO )


------- ---------------------------------------------------

5 FILTER



Correlated Sub Query Using EXISTS

SELECT * FROM EMP A

WHERE EXISTS

( SELECT NULL FROM EMP_TEMP B

WHERE A.EMPNO = B.EMPNO )


------- ---------------------------------------------------

5 HASH JOIN SEMI



2007-12-2532


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IN Sub Query

SELECT * FROM EMP A

WHERE EMPNO

IN

( SELECT EMPNO FROM EMP_TEMP B)


------- ---------------------------------------------------

5 HASH JOIN SEMI



IN Sub Query is executed

only once, while EXISTS

sub query is executed once

per row of the parent

Query.

Where ever possible, allow the sub query to be executed

by a direct index lookup without a table access.

2007-12-2533


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Oracle Performance Tuning 9. Semi Join

MERGE SEMI JOIN

SELECT NAME, DOJ FROM CUSTOMER C

WHERE EXISTS ( SELECT /*+ MERGE_SJ */ NULL --- To simulate Merge Join

FROM SUPPLIER S

WHERE S.NAME = C.NAME AND S.DOJ = C.DOJ) ;

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |

----------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 41 | | 414 (8)| 00:00:04 |

| 1 | MERGE JOIN SEMI | | 1 | 41 | | 414 (8)| 00:00:04 |

| 2 | SORT JOIN | | 20000 | 507K| 1432K| 357 (7)| 00:00:03 |

| 3 | TABLE ACCESS FULL| CUSTOMER | 20000 | 507K| | 212 (6)| 00:00:02 |

|* 4 | SORT UNIQUE | | 5000 | 75000 | | 57 (11)| 00:00:01 |

| 5 | TABLE ACCESS FULL| SUPPLIER | 5000 | 75000 | | 54 (6)| 00:00:01 |

------------------------------------------------------------------------------------------------------------------------------------

RUN TIME = 0.85 Seconds LIO = 1949

When NO suitable index is available on the table used in the Sub Query, then

Oracle may use SEMI JOIN to optimize the query.

2007-12-2534


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Oracle Performance Tuning 9. Semi Join

HASH JOIN RIGHT SEMI in 10g


WHERE EXISTS ( SELECT NULL

FROM SUPPLIER S

WHERE S.NAME = C.NAME AND S.DOJ = C.DOJ) ;

PLAN_TABLE_OUTPUT

----------------------------------------------------------------------------------------------------

Plan hash value: 1924774493

---------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

---------------------------------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 1 | 41 | 268 (6)| 00:00:03 |

|* 1 | HASH JOIN RIGHT SEMI | | 1 | 41 | 268 (6)| 00:00:03 |

| 2 | TABLE ACCESS FULL | SUPPLIER | 5000 | 75000 | 54 (6)| 00:00:01 |

| 3 | TABLE ACCESS FULL | CUSTOMER | 20000 | 507K| 212 (6)| 00:00:02 |

------------------------------------------------------------------------------------------------------------------------------------

Run Time 0.45 Seconds LIO = 2257

2007-12-2535


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Oracle Performance Tuning 9. Anti Join

Anti Join - An anti join is a query that returns rows in one table that

do not match some set of rows from another table.

NOT IN Sub Query


WHERE (NAME, DOJ) NOT IN

( SELECT NAME, DOJ FROM SUPPLIER S) ;

PLAN_TABLE_OUTPUT in 10g

------------------------------------------------------------------------------------------------------------------------

Plan hash value: 3490453085

------------------------------------------------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

------------------------------------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 50 | 1300 | 27791 (8)| 00:03:29 |

|* 1 | FILTER | | | | | |

| 2 | TABLE ACCESS FULL| CUSTOMER | 1000 | 26000 | 213 (6) | 00:00:02 |

|* 3 | TABLE ACCESS FULL| SUPPLIER | 13 | 195 | 55 (8) | 00:00:01 |

-------------------------------------------------------------------------------------------------------------------------

2007-12-2536


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Anti Join - NOT IN Sub Query

SELECT NAME, DOJ

FROM CUSTOMER C

WHERE (NAME, DOJ) NOT IN

( SELECT NAME, DOJ

FROM SUPPLIER S)


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.04 0.06 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 76 0.48 0.45 1945 2018 0 15000

------- ------ ------- ---------- ---------- ---------- ---------- ----------

total 78 0.52 0.52 1945 2018 0 15000

Rows Row Source Operation in 10g

------- ---------------------------------------------------

15000 HASH JOIN RIGHT ANTI (cr=2036 pr=0 pw=0 time=229979 us)

5000 TABLE ACCESS FULL SUPPLIER (cr=388 pr=0 pw=0 time=15230 us)

20000 TABLE ACCESS FULL CUSTOMER (cr=1630 pr=0 pw=0 time=100280 us)

10g optimizer has an improved execution plan for NOT IN Sub Query.

********************************************************************************

2007-12-2537


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NOT EXISTS Query

SELECT NAME, DOJ

FROM CUSTOMER C

WHERE NOT EXISTS

( SELECT 'X' FROM SUPPLIER S

WHERE S.NAME = C.NAME

AND S.DOJ = C.DOJ )


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.05 0.06 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 76 0.48 0.45 1945 2018 0 15000

------- ------ -------- ---------- ---------- --------- ---------- ----------

total 78 0.53 0.51 1945 2018 0 15000

Rows Row Source Operation in 10g

------- ---------------------------------------------------

15000 HASH JOIN RIGHT ANTI (cr=2036 pr=1945 pw=0 time=296890 us)

5000 TABLE ACCESS FULL SUPPLIER (cr=388 pr=386 pw=0 time=68124 us)

20000 TABLE ACCESS FULL CUSTOMER (cr=1630 pr=1559 pw=0 time=1522620 us)

********************************************************************************

2007-12-2538


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SQL> select count(*) from t1

where not exists

(select 'x' from t2 where t2.id = t1.id ) ;

COUNT(*)

----------

3

SQL> select count(*) from t1

2 where id not in (select id from t2) ;

COUNT(*)

----------

0

Because there is a NULL value in a row in

T2 table.

SQL> select * from t2;

ID NAME

---------- ----------

5 ABC

6 ABC

Tom

SQL> select * from t1;

ID

----------

1

5

7

9

NOT IN and NOT EXISTS are not the same.

2007-12-2539


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ANTI JOIN Using OUTER JOIN

select H1.object_name, H1.object_id

from H1, H2

where H1.object_id = H2.object_id(+)

and H1.object_name = H2.OBJECT_NAME (+)

and H2.OBJECT_NAME IS NULL


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.00 0.00 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 22 0.31 0.28 0 617 0 4010

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 24 0.31 0.28 0 617 0 4010


------- ---------------------------------------------------

4010 FILTER

24010 MERGE JOIN OUTER

24010 SORT JOIN

24010 TABLE ACCESS FULL H1

20000 SORT JOIN


There is NO index on H2

table, but still the

elapsed time is 0.28

seconds. ANTI JOIN

using Outer Join is one

of the fastest way to get

the result set.

2007-12-2540


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ANTI JOIN Using MINUS

SELECT OBJECT_NAME, OBJECT_ID FROM H1

MINUS

SELECT OBJECT_NAME, OBJECT_ID FROM H2


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.00 0.00 0 0 0 0

Execute 1 0.01 0.00 0 0 0 0

Fetch 269 0.26 0.22 0 617 0 4010

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 271 0.27 0.22 0 617 0 4010


------- ---------------------------------------------------

4010 MINUS

24010 SORT UNIQUE


20000 SORT UNIQUE


If the parent query does

not contain a column

from the inner query,

then consider using

MINUS operator.

Also, the data types

should match in both

tables.

2007-12-2541


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ANTI JOIN Using HASH_AJ Hint

ALTER SESSION SET ALWAYS_ANTI_JOIN = HASH ;

SELECT OBJECT_NAME, OBJECT_ID

FROM H1

WHERE (OBJECT_NAME, OBJECT_ID) NOT IN

(SELECT /*+ HASH_AJ */

OBJECT_NAME, OBJECT_ID

FROM H2)


------- ------ -------- ---------- ---------- ---------- ---------- ----------

Parse 1 0.00 0.00 0 0 0 0

Execute 1 0.00 0.00 0 0 0 0

Fetch 269 0.10 0.09 0 617 0 4010

------- ------ -------- ---------- ---------- ---------- ---------- ----------

total 271 0.10 0.10 0 617 0 4010


------- ---------------------------------------------------

4010 HASH JOIN ANTI



Among the various tests, this is the BEST RUN TIME in 9i..

To take advantage of anti

join optimization, the

following must be true:

1. CBO must be enabled

2. Anti Join columns must

NOT be NULL. Or IS

NOT NULL clause

appears in the WHERE

clause.

3. Sub Query is not

correlated

4. Parent Query does not

contain OR clause.

5. ALWAYS_ANTI_JOIN

parameter must be set

to HASH or MERGE.

2007-12-2542


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Sub Query Categorization

NOT IN sub query is not same as NOT EXISTS, but can be transformed

into ANTI-joins.

NOT IN / NOT

EXISTS

6

IN sub query may be written as EXISTS sub query. These can be

transformed into semi joins.

IN/EXISTS5

Returns a single row. It can become a driving table. Single-row4

If a simple sub query contains some aggregation, then there are some

restrictions on how the optimizer may be able to transform them.

Aggregate3

Simple sub query contain a single table. Complex sub queries contain

many tables.

Simple/Complex2

A correlated sub query references columns from an outer query block.

Correlated sub queries can often be transformed into joins; non-

correlated sub queries have some chance of becoming driving sub

queries.

Correlated /

Noncorrelated

1

CharacteristicsCategory#

2007-12-2543


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Parameters in various Oracle releases affecting sub query

Always use this for semi join

(EXISTS)

ChooseChoo

se

Stand

ard

_ALWAYS_SEMI_JOIN6

Always use this method for anti-

join (not exists)

Choose Choo

se

NL_ALWAYS_ANTI_JOIN5

Enables costing of equiality semi

joins

TrueTruen/a_COST_EQUALITY_SEMI_JOIN4

Enables ordered semi-join

(exists) sub query

TrueTrueTrue_ORDERED_SEMI_JOIN3

Enables unnesting of correlated

sub query

trueTruefalse_UNNEST_SUBQUERY2

Unnests not exists sub query

with one or more tables if

possible

n/aSingl

e

n/a_UNNEST_NOT_EXISTS_SQ1

Description10g9i8iName#

2007-12-2544


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Parameters in various Oracle releases affecting sub query

Enables removal of subsumed

aggregate sub queries.

Truen/an/a_REMOVE_AGGR_SUBQUERY11

Enables right outer hash joinTruen/an/a_RIGHT_OUTER_HASH_ENABLE10

Enables unnesting of IN Sub query

into SELECT DISTINCT view

Falsen/an/a_DISTINCT_VIEW_UNNESTING9

Enables unnesting of sub queries in

a bottom up manner

Truen/an/a_OPTIMIZER_SQU_BOTTOMUP8

Forces correct computation of sub

query selectivity

Truen/an/a_OPTIMIZER_CORRECT_SQ_SEL

ECTIVITY

7

Description10g9i8iName#

2007-12-2545


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I value your questions, comments and suggestions.

Please send your comment to [email protected].

Beacon Infotech Corporation.

Web: www.oracleact.com

beacon ora perf class 2

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