1. Oracle12c Join Techniques
Carl Dudley
Tradba Ltd
Oracle ACE Director

2. Introduction Working with Oracle since 1986
Oracle DBA - OCP Oracle7, 8, 9, 10
Oracle DBA of the Year – 2002
Oracle ACE Director
Beta tester – Oracle8, 9, 10, 11, 12
Regular Presenter at Oracle Conferences
Consultant and Trainer
Technical Editor for a number of Oracle texts
UK Oracle User Group Official
Member of IOUC
Fellow of British Computer Society
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

3. The emp and dept Tables dept emp DEPTNO DNAME LOC
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
emp
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
7934 MILLER CLERK JAN
7782 CLARK MANAGER JUN
7839 KING PRESIDENT NOV
7369 SMITH CLERK DEC
7876 ADAMS CLERK JAN
7566 JONES MANAGER APR
7902 FORD ANALYST DEC
7788 SCOTT ANALYST DEC
7900 JAMES CLERK DEC
7521 WARD SALESMAN FEB
7654 MARTIN SALESMAN SEP
7844 TURNER SALESMAN SEP
7499 ALLEN SALESMAN FEB
7698 BLAKE MANAGER MAY

4. ANSI Joins ANSI defined join syntax in ANSI SQL2 Standard (1992)
Oracle finally fully implemented this syntax in Oracle9i
Types of joins specified by additional keywords
INNER JOIN
OUTER JOIN (LEFT RIGHT FULL)
CROSS JOIN
UNION JOIN (not supported by Oracle)

5. Inner Joins – ANSI and Oracle Syntax
Equivalent ANSI – based joins
SELECT ename,dname,dept.deptno
FROM emp JOIN dept
ON dept.deptno = emp.deptno;
SELECT ename,dname,deptno
FROM emp NATURAL JOIN dept;
SELECT ename,dname,deptno
FROM emp NATURAL JOIN dept
USING (deptno);
Note the absence of the table qualifier for deptno in some of the examples
Equivalent Oracle join
SELECT ename,dname,dept.deptno
FROM emp,dept
WHERE emp.deptno = dept.deptno;

6. The Join Condition Join condition (ON clause) must be included
Helps safeguard against inadvertant cartesian products
SELECT ename,dname,dept.deptno
FROM emp JOIN dept;
ORA-00905: missing keyword
But join condition does not have to make any sense
Could cause cartesian products
Oracle join syntax has no protection
SELECT ename,dname,dept.deptno
FROM emp JOIN dept
ON emp.empno = emp.empno;

7. Multi-table Joins If several tables are to be joined
Use a step-wise process by first joining two of the tables
Then add a further JOIN keyword to join to the third table
Repeat this for each subsequent table
Each join may have its own condition(s)
The joins may be of different types
SELECT emp.empno
,emp.ename
,dept.loc
,proj.pname
FROM emp
JOIN dept
ON emp.deptno = dept.deptno
JOIN proj
ON emp.proj_id = proj.proj_id;

8. LEFT and RIGHT Outer Joins
The driving table is specified with LEFT or RIGHT
This table will have all its rows included
Dummy NULL rows may be included from the other table
Refers to placement of the table name WITHIN the FROM ... JOIN clause
The following join clauses are all equivalent
FROM emp,dept
WHERE emp.deptno(+) = dept.deptno
FROM dept LEFT OUTER JOIN emp
ON emp.deptno = dept.deptno
FROM emp RIGHT OUTER JOIN dept

9. Modified Employee Data
empa
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
7782 CLARK MANAGER JUN
7788 SCOTT ANALYST APR
7844 TURNER SALESMAN SEP
7499 ALLEN SAlESMAN FEB
7902 FORD ANALYST DEC
1111 EXTRA CEO JAN
dept
DEPTNO DNAME LOC
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON

10. RIGHT Outer Joins SELECT ename,dname,dept.deptno
FROM empa RIGHT OUTER JOIN dept
ON empa.deptno = dept.deptno;
ENAME DNAME DEPTNO
CLARK ACCOUNTING
SCOTT RESEARCH
TURNER SALES
FORD RESEARCH
ALLEN SALES
OPERATIONS
Equivalent Oracle join statement
SELECT ename,dname,dept.deptno
FROM empa,dept WHERE dept.deptno = emp.deptno(+);

11. LEFT Outer Joins SELECT ename,dname,dept.deptno
FROM empa LEFT OUTER JOIN dept
ON empa.deptno = dept.deptno;
ENAME DNAME DEPTNO
CLARK ACCOUNTING
SCOTT RESEARCH
TURNER SALES
FORD RESEARCH
ALLEN SALES
EXTRA
Equivalent Oracle join statement
SELECT ename,dname,dept.deptno
FROM empa, dept WHERE dept.deptno(+) = empa.deptno;

12. FULL Outer Joins
SELECT ename,dname,dept.deptno d_deptno, empa.deptno e_deptno
FROM empa FULL OUTER JOIN dept
ON empa.deptno = dept.deptno;
ENAME DNAME D_DEPTNO E_DEPTNO
CLARK ACCOUNTING
SCOTT RESEARCH
TURNER SALES
FORD RESEARCH
ALLEN SALES
EXTRA
OPERATIONS
All rows included
from both tables
Equivalent (ILLEGAL) Oracle join statement
SELECT ename,dname,dept.deptno,empa.deptno
FROM empa, dept WHERE dept.deptno(+) = empa.deptno(+);

13. FULL OUTER JOIN – Oracle11g
Oracle now performs a 'NATIVE' full outer join
SELECT ename
,dname
FROM empa e FULL OUTER JOIN dept d
ON e.deptno = d.deptno;
| Id | Operation | Name |Rows |Bytes |Cost(%CPU
| 0| SELECT STATEMENT | | 15 | 240 | 7 (15
| 1| VIEW | VW_FOJ_0 | 15 | 240 | 7 (15
|* 2| HASH JOIN FULL OUTER| | 15 | 330 | 7 (15
| 3| TABLE ACCESS FULL | DEPT | 4 | 52 | 3 (0
| 4| TABLE ACCESS FULL | EMPA | 14 | 126 | 3 (0

14. FULL OUTER JOIN – Oracle10g
ALTER SESSION SET optimizer_features_enable = ' ';
SELECT ename
,dname
FROM empa e FULL OUTER JOIN dept d
ON e.deptno = d.deptno;
| Id | Operation | Name | Rows | Bytes | Cost |
| 0 | SELECT STATEMENT | | | | 13|
| 1 | VIEW | | | | 13|
| 2 | UNION-ALL | | | | |
|* 3 | HASH JOIN OUTER | | | | 7|
| 4 | TABLE ACCESS FULL| EMPA | | | 3|
| 5 | TABLE ACCESS FULL| DEPT | | | 3|
|* 6 | HASH JOIN ANTI | | | | 7|
| 7 | TABLE ACCESS FULL| DEPT | | | 3|
| 8 | TABLE ACCESS FULL| EMPA | | | 3|

15. FULL OUTER JOIN – Oracle10g (continued)
Statement is transformed to a UNION construct
Generates same execution plan as for 10g FULL OUTER JOIN
Native full outer join available in and with a hint
/*+NATIVE_FULL_OUTER_JOIN */
Undocumented parameter
_optimizer_native_full_outer_join
SELECT ename
,dname
FROM empa e
,dept d
WHERE e.deptno = d.deptno(+)
UNION ALL
SELECT null
FROM dept d2 WHERE NOT EXISTS (SELECT 'x'
FROM empa e2
WHERE e2.deptno = d2.deptno);

16. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

17. Two-way Outer Joins empb dept proj
project table linked to a cut down modified version of emp, called empb
Project number 3 (DESIGN) has no employees
empb
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO PROJ_ID
7782 CLARK MANAGER JUN
7788 SCOTT ANALYST APR
7844 TURNER SALESMAN SEP
7499 ALLEN SAlESMAN FEB
proj
dept
DEPTNO DNAME LOC
10 ACCOUNTING NEW YORK
20 RESEARCH DALLAS
30 SALES CHICAGO
40 OPERATIONS BOSTON
PROJ_ID PNAME START_DATE
1 BPR JUL-2002
2 MIGRATION 12-OCT-2002
3 DESIGN NOV-2002

18. Two-way Outer Joins (+) (+)
Show details of employees with their projects and departments
Include projects with no employees and departments with no employees
project
empb
dept
(+)
(+)

19. ANSI Two-way Outer Joins – Expected result
Two-way outer joins are allowed but the result may not be as expected
Outer join of empb to both dept and project based on deptno and proj_id
values (as requested on the previous slide) should give the following result
ENAME HIREDATE DEPTNO DNAME PROJ_ID PNAME
ALLEN FEB SALES BPR
TURNER SEP SALES BPR
CLARK JUN ACCOUNTING BPR
SCOTT APR RESEARCH MIGRATION
40 OPERATIONS
3 DESIGN

20. ANSI Two-way Outer Joins – Missing Rows
Unexpected results?
SELECT ename, hiredate,d.deptno,dname,p.proj_id,pname
FROM dept d LEFT OUTER JOIN empb e
ON e.deptno = d.deptno
RIGHT OUTER JOIN proj p
ON e.proj_id = p.proj_id;
ENAME HIREDATE DEPTNO DNAME PROJ_ID PNAME
CLARK JUN ACCOUNTING BPR
SCOTT APR RESEARCH MIGRATION
ALLEN FEB SALES BPR
TURNER SEP SALES BPR
3 DESIGN

21. ANSI Two-way Outer Joins – Missing Rows (continued)
Reordering the joins still gives another different unexpected result?
Outer joins are not 'symmetrical'
SELECT ename, hiredate,d.deptno,dname,p.proj_id,pname
FROM proj p LEFT OUTER JOIN empb e
ON e.proj_id = p.proj_id
RIGHT OUTER JOIN dept d
ON e.deptno = d.deptno;
ENAME HIREDATE DEPTNO DNAME PROJ_ID PNAME
CLARK JUN ACCOUNTING BPR
SCOTT APR RESEARCH MIGRATION
TURNER SEP SALES BPR
ALLEN FEB SALES BPR
40 OPERATIONS

22. ANSI Two-way Outer Joins - Explanation
Explanation of unexpected behaviour
Examine the result of the first join in the first example
The fifth row does not have a value in the proj_id column
This row can not take part in the final join with the proj table
SELECT ename, hiredate,d.deptno,dname,proj_id
FROM dept d LEFT OUTER JOIN empb e
ON e.deptno = d.deptno;
ENAME HIREDATE DEPTNO DNAME PROJ_ID
CLARK JUN ACCOUNTING
SCOTT APR RESEARCH
TURNER SEP SALES
ALLEN FEB SALES
40 OPERATIONS

23. ANSI Two-way Outer Joins – use of FULL
Outer joining dept to empb gives a row with NULL in proj_id
This row can not take part in the second outer join
The second join needs to be a FULL OUTER JOIN
SELECT ename,hiredate,d.deptno,dname,p.proj_id,pname
FROM dept d LEFT OUTER JOIN empb e
ON e.deptno = d.deptno
FULL OUTER JOIN proj p
ON e.proj_id = p.proj_id;
ENAME HIREDATE DEPTNO DNAME PROJ_ID PNAME
ALLEN FEB SALES BPR
TURNER SEP SALES BPR
CLARK JUN ACCOUNTING BPR
SCOTT APR RESEARCH MIGRATION
40 OPERATIONS
3 DESIGN
SELECT STATEMENT |
VIEW | VW_FOJ_0
HASH JOIN FULL OUTER|
TABLE ACCESS FULL | PROJ
VIEW |
HASH JOIN OUTER |
TABLE ACCESS FULL| DEPT
TABLE ACCESS FULL| EMPB

24. Two-way Outer Joins – Oracle Style
In 11g, Oracle syntax cannot outer join a table to more than one other table
SELECT ename,hiredate,d.deptno,dname,p.proj_id,pname
FROM dept d, empb e, proj p
WHERE e.deptno(+) = d.deptno
AND e.proj_id(+) = p.proj_id;
ORA-01417: a table may be outer joined to at most one other table

25. Two-way Outer Joins – Oracle Style (continued)
But in Oracle12c, this would be the output
Can be imitated using ANSI syntax
ENAME HIREDATE DEPTNO DNAME PROJ_ID PNAME
SCOTT APR RESEARCH MIGRATION
ALLEN FEB SALES BPR
TURNER SEP SALES BPR
CLARK JUN ACCOUNTING BPR
40 OPERATIONS BPR
40 OPERATIONS MIGRATION
40 OPERATIONS DESIGN
30 SALES MIGRATION
30 SALES DESIGN
20 RESEARCH BPR
20 RESEARCH DESIGN
10 ACCOUNTING MIGRATION
10 ACCOUNTING DESIGN
SELECT ename,hiredate,d.deptno,dname,p.proj_id,pname
FROM dept d CROSS JOIN proj p
LEFT OUTER JOIN empb e
ON e.deptno = d.deptno AND e.proj_id = p.proj_id
SELECT STATEMENT |
HASH JOIN OUTER |
MERGE JOIN CARTESIAN|
TABLE ACCESS FULL | PROJ
BUFFER SORT |
TABLE ACCESS FULL | DEPT
TABLE ACCESS FULL | EMPB

26. Oracle Outer Joins with Subqueries
Show each department with its longest serving employee
Subqueries are not allowed in combination with an outer join condition
SELECT ename, hiredate,d.deptno,dname
FROM dept d LEFT OUTER JOIN emp e
ON e.deptno = d.deptno
AND e.hiredate = (SELECT MIN(e.hiredate)
FROM emp e
WHERE e.deptno = d.deptno);
SELECT ename, hiredate,d.deptno,dname
FROM dept d, empa e
WHERE e.deptno(+) = d.deptno
AND e.hiredate(+) = (SELECT MIN(e.hiredate)
FROM empa e
WHERE e.deptno = d.deptno);
ENAME HIREDATE DEPTNO DNAME
CLARK JUN ACCOUNTING
SMITH DEC RESEARCH
ALLEN FEB SALES
40 OPERATIONS
ORA-01799: a column may not be outer-joined to a subquery
What about ANSI joins?

27. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

28. Join and Filter Conditions
Oracle uses the ON clause for join conditions
Conditions specified in a WHERE clause are treated as filter conditions
Filter conditions can also be specified in the ON (or USING) clause
Join operations and conditions are effectively processed before the WHERE clause conditions

29. Oracle Join and Filter Conditions
SELECT dept.deptno,dname,ename,job
FROM dept, empa
WHERE empa.deptno(+) = dept.deptno
AND empa.job = 'SALESMAN';
DEPTNO DNAME ENAME JOB
30 SALES TURNER SALESMAN
30 SALES ALLEN SALESMAN
Filter applied after the join
SELECT dept.deptno,dname,ename,job
FROM dept, empa
WHERE empa.deptno(+) = dept.deptno
AND empa.job(+) = 'SALESMAN';
DEPTNO DNAME ENAME JOB
10 ACCOUNTING
20 RESEARCH
30 SALES TURNER SALESMAN
30 SALES ALLEN SALESMAN
40 OPERATIONS
Filter applied during the join
Somewhat awkward syntax

30. ANSI Join and Filter Conditions
SELECT dept.deptno,dname,ename,job
FROM dept LEFT OUTER JOIN empa
ON empa.deptno = dept.deptno
WHERE empa.job = 'SALESMAN‘;
DEPTNO DNAME ENAME JOB
30 SALES ALLEN SALESMAN
30 SALES TURNER SALESMAN
Filter applied after the join
Easier to understand due to
separation of join conditions
from filter conditions
SELECT dept.deptno,dname,ename,job
FROM dept LEFT OUTER JOIN empa
ON empa.deptno = dept.deptno
AND empa.job = 'SALESMAN';
DEPTNO DNAME ENAME JOB
30 SALES TURNER SALESMAN
30 SALES ALLEN SALESMAN
40 OPERATIONS
20 RESEARCH
10 ACCOUNTING
Filter applied during the join
Note the three outer joined rows
- one row for each department
having no salesmen

31. Oracle Outer Joins and OR
Oracle syntax cannot cope
SELECT dname
,ename
,job
,loc
,sal
FROM emp e
,dept d
WHERE e.deptno(+) = d.deptno
AND (sal(+) > 2999 OR loc(+) = ‘NEW YORK')
ORDER BY dname;
AND (sal(+) > 2999 OR loc(+) = ‘NEW YORK')
ERROR :
ORA-01719:outer join operator (+) not allowed in operand of OR or IN

32. ANSI Outer Joins and OR
ANSI join can process OR when combined with outer join
SELECT dname
,ename
,job
,loc
,sal
FROM emp e RIGHT OUTER JOIN dept d
ON e.deptno = d.deptno
AND (sal > 2999 OR loc = 'NEW YORK')
ORDER BY dname;
DNAME ENAME JOB LOC SAL
ACCOUNTING CLARK MANAGER NEW YORK
ACCOUNTING KING PRESIDENT NEW YORK
ACCOUNTING MILLER CLERK NEW YORK
OPERATIONS BOSTON
RESEARCH FORD ANALYST DALLAS
RESEARCH SCOTT ANALYST DALLAS
SALES CHICAGO

33. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

34. 'Chaining on' the Outer Join
course
attendance
student
All courses have attendances, all attendances have offerings
Hence outer join is never necessary when joining offering and attendance
Some students do not have attendances
Query :
Show ALL students along with their courses

35. Data in the Sample Tables
student
course
STUDENT_ID STUDENT_LNAME
1111 BROWN
2299 ADAMS
4568 COX
5556 TYLER
6789 ROSE
COURSE_ID START_DATE
DEC-11
JUL-12
NOV-12
attendance
COURSE_ID STUDENT_ID
Two students do not have any attendances
All courses and attendances are related
Outer join between these tables should NOT be needed

36. Joining course and attendance
SELECT c.course_id c_course_id
,a.course_id a_course_id
,a.student_id
FROM course c
INNER JOIN attendance a
ON c.course_id = a.course_id;
SELECT o.course_id c_course_id
,a.course_id a_course_id
,a.student_id a_student_id
FROM attendance a
LEFT OUTER JOIN course c
ON a.course_id = c.course_id;
Exact same result
C_COURSE_ID A_COURSE_ID A_STUDENT_ID
Outer join is completely unnecessary as all rows match

37. Joining attendance and student
Outer join is necessary to include students without attendances
SELECT s.student_id s_student_id
,s.student_lname s_student_lname
,a.student_id a_student_id
,a.course_id a_course_id
FROM student s
LEFT OUTER JOIN attendance a
ON s.student_id = a.student_id;
S_STUDENT_ID S_LNAME A_STUDENT_ID A_COURSE_ID
1111 BROWN
2299 ADAMS
2299 ADAMS
6789 ROSE
1111 BROWN
4568 COX
5556 TYLER

38. Join the Three Tables - inner join 'loses' students
SELECT s.student_id s_student_id,s.student_lname s_lname
,a.student_id a_student_id,a.course_id a_course_id
,c.course_id c_course_id,c.start_date
FROM student s LEFT OUTER JOIN attendance a
ON s.student_id = a.student_id INNER JOIN course c
ON c.course_id = a.course_id;
S_STUDENT_ID S_LNAME A_STUDENT_ID A_COURSE_ID C_COURSE_ID START_DATE
1111 BROWN DEC-11
2299 ADAMS JUL-12
2299 ADAMS NOV-12
6789 ROSE JUL-12
1111 BROWN NOV-12
| Id | Operation | Name |Rows |
| 0| SELECT STATEMENT | | 5|
|* 1| HASH JOIN | | 5|
|* 2| HASH JOIN | | 5|
| 3| TABLE ACCESS FULL| COURSE | 3|
| 4| TABLE ACCESS FULL| ATTENDANCE | 5|
| 5| TABLE ACCESS FULL | STUDENT | 5|
Rows are lost by the final inner join
Oracle is smart enough not to perform the outer join

39. Chaining on the OUTER JOIN
SELECT s.student_id s_student_id,s.student_lname s_lname
,a.student_id a_student_id,a.course_id a_course_id
,c.course_id c_course_id,c.start_date
FROM student s LEFT OUTER JOIN attendance a
ON s.student_id = a.student_id LEFT OUTER JOIN course c
ON c.course_id = a.course_id;
S_STUDENT_ID S_LNAME A_STUDENT_ID A_COURSE_ID C_COURSE_ID START_DATE
1111 BROWN DEC-11
2299 ADAMS JUL-12
2299 ADAMS NOV-12
6789 ROSE JUL-12
1111 BROWN NOV-12
4568 COX
5556 TYLER
| Id | Operation | Name |Rows | | 0| SELECT STATEMENT | | 5| |* 1| HASH JOIN OUTER | | 5| |* 2| HASH JOIN OUTER | | 5| | 3| TABLE ACCESS FULL| STUDENT | 5| | 4| TABLE ACCESS FULL| ATTENDANCE | 5| | 5| TABLE ACCESS FULL | COURSE | 3|
Outer join must be performed between attendance and course when not strictly necessary

40. Avoiding the second OUTER JOIN
SELECT s.student_id s_student_id,s.student_lname s_lname
,a.student_id a_student_id,a.course_id a_course_id
,c.course_id c_course_id,c.start_date
FROM course c JOIN attendance a
ON c.course_id = a.course_id RIGHT OUTER JOIN student s
ON s.student_id = a.student_id;
S_STUDENT_ID S_LNAME A_STUDENT_ID A_COURSE_ID C_COURSE_ID START_DATE
1111 BROWN DEC-11
2299 ADAMS JUL-12
2299 ADAMS NOV-12
6789 ROSE JUL-12
1111 BROWN NOV-12
4568 COX
5556 TYLER
| Id | Operation | Name |Rows |
| 0| SELECT STATEMENT | | 5|
|* 1| HASH JOIN OUTER | | 5|
| 2| TABLE ACCESS FULL | STUDENT | 5|
| 3| VIEW | | 5|
|* 4| HASH JOIN | | 5|
| 5| TABLE ACCESS FULL| COURSE | 3|
| 6| TABLE ACCESS FULL| ATTENDANCE | 5|
Inner join performed first
Outer join preserves all student rows

41. Ordering the joins 'Right'
SELECT s.student_id s_student_id,s.student_lname s_lname
,a.student_id a_student_id,a.course_id a_course_id
,c.course_id c_course_id,c.start_date
FROM attendance a RIGHT OUTER JOIN student s
ON s.student_id = a.student_id JOIN course c
ON c.course_id = a.course_id;
S_STUDENT_ID S_LNAME A_STUDENT_ID A_COURSE_ID C_COURSE_ID START_DATE
1111 BROWN DEC-11
2299 ADAMS JUL-12
2299 ADAMS NOV-12
6789 ROSE JUL-12
1111 BROWN NOV-12
Right join would be performed first?
Inner join would then lose the outer joined student rows
No outer joins performed
| Id | Operation | Name |Rows |
| 0| SELECT STATEMENT | | 5|
|* 1 HASH JOIN | | 5|
|* 2| HASH JOIN | | 5|
| 3| TABLE ACCESS FULL| COURSE | 3|
| 4| TABLE ACCESS FULL| ATTENDANCE | 5|
| 5| TABLE ACCESS FULL | STUDENT | 5|

42. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

43. Processing a Hash Join Build Table (customers) Hash table
NAME
CITY
...
SCOTT
BATH
PAYNE
YORK
FOX
LEEDS
SMITH
ELY
WOOD
ADAMS
LONDON
Hash table
ADAMS
LONDON
...
FOX
LEEDS
PAYNE
YORK
SCOTT
BATH
WOOD
SMITH
ELY
...
ORD_ID
NAME
... 1
SMITH 2
PAYNE 3
FOX 4
SCOTT 5
WOOD 6
ADAMS
Probe Table (orders)

44. Join Trees for T1 T2 T3 T4 Left deep tree T2 T1 T3 T4 Right deep tree
Hash table
built on T3
probed by result
of T1 join T2
built on T2
probed by T1
built on T4
of T1 join T2 join T3
Output of T1 join T2 join T3
probed by T4 T2 T1 T3 T4
Output of T1 join T2
probed by T3
Hash table
built on T1
probed by T2

45. Summary of Left-deep and Right-deep Hash Join Trees
Result sets are formed before build of next hash table
Workarea discarded after each subsequent join
Only one workarea used
Right-deep
Hash tables on T3 and T4 can be built 'in parallel' with hash table on T1
Three workareas can be used
Could be useful when the results of joins are larger than the two sets of rows being joined

46. Join Trees for T1 T2 T3 T4 (continued)
Zig-zag tree
Bushy tree T1 T2 T3 T4

47. Tree Combinations
Optimizer normally considers only left-deep trees (until latest release?)
Left deep joins cover most requirements
Number of left-deep trees = n!
Total number of join trees = (2n-2)!/(n-1)!
Tables
Left-Deep Trees
Bushy Trees
(inc. Left and Right) 1 2 3 6 12 4 24
120 5
1,680
720
30,240
Can Oracle be forced to perform a bushy tree?

48. Four Table Join Scenariosa
ID NAME WA_ID
1 x
2 y
3 z
: : : wa
WA_ID WA_NAME
1 name1
2 name2
3 name3
: : p
P_ID P_NAME WA_ID PT_ID
1 pname
2 pname
3 pname
: : : : pt
PT_ID PT_NAME
1 ptname1
2 ptname2
3 ptname3
: :

49. Right Deep Join Tree
Oracle uses hints internally to force a right deep join in this case
SELECT /*+GATHER_PLAN_STATISTICS */ name
FROM sa
JOIN wa ON wa.wa_id = sa.wa_id
JOIN p ON wa.wa_id = p.wa_id
JOIN pt ON pt.pt_id = p.pt_id;
Internally
generated hints
| SELECT STATEMENT | |
| HASH JOIN | |
| TABLE ACCESS FULL | SA |
| HASH JOIN | |
| TABLE ACCESS FULL | WA |
| HASH JOIN | |
| TABLE ACCESS FULL| PT |
| TABLE ACCESS FULL| P |

50. Left Deep Join Tree Left deep tree can be forced using hints
SELECT /*+GATHER_PLAN_STATISTICS
no_swap_join_inputs(wa)
no_swap_join_inputs(p)
no_swap_join_inputs(pt)
no_swap_join_inputs(sa)*/ name
FROM sa
JOIN wa ON wa.wa_id = sa.wa_id
JOIN p ON wa.wa_id = p.wa_id
JOIN pt ON pt.pt_id = p.pt_id;
| SELECT STATEMENT | |
| HASH JOIN | |
| HASH JOIN | |
| HASH JOIN | |
| TABLE ACCESS FULL| PT |
| TABLE ACCESS FULL| P |
| TABLE ACCESS FULL | WA |
| TABLE ACCESS FULL | SA |

51. Forcing a Bushy Tree
In complex decision support scenarios a bushy tree can be a good plan
Cannot be forced using '(', ')' or ORDERED, LEADING or other 'conventional' hints
Must use inline views and a selection of less well known hints to stop Oracle merging the views internally
SELECT /*+no_merge(v1) no_merge(v2)*/ name
FROM (SELECT sa.name
,wa.wa_id
FROM sa JOIN wa
ON wa.wa_id = sa.wa_id) v1
JOIN
(SELECT p.wa_id
FROM p JOIN pt
ON pt.pT_id = p.pT_id) v2
ON v1.wa_id = v2.wa_id;
| SELECT STATEMENT | |
| HASH JOIN | |
| VIEW | |
| HASH JOIN | |
| TABLE ACCESS FULL| PT |
| TABLE ACCESS FULL| P |
| TABLE ACCESS FULL| WA |
| TABLE ACCESS FULL| SA |

52. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

53. Hash Join Plans Semi-joins Often occur with EXISTS and IN subqueries
Used when existence tests are required
Find departments which have employees
Semi-join returns a department only once no matter how many employees work for it
SELECT *
FROM dept
WHERE EXISTS (SELECT 'x'
FROM emp
WHERE deptno = dept.deptno);

54. Right Semi Join
From Oracle 10g, the driving table can be automatically switched
Can depend on size of input rowsets
SWAP_JOIN_INPUTS hint is used internally
Often used to avoid high memory usage (monitor OMEM with dbms_xplan)
dept : 8192 rows
emp : 7168 rows
dept : 4 rows
emp : 7168 rows
|Operation |Name |Rows |
|SELECT STATEMENT | |8192 |
| HASH JOIN RIGHT SEMI| |8192 |
| VIEW |VW_SQ_1|7168 |
| TABLE ACCESS FULL |EMP |7168 |
| TABLE ACCESS FULL |DEPT |8192 |
|Operation |Name |Rows|
|SELECT STATEMENT | | 4|
| HASH JOIN SEMI | | 4|
| TABLE ACCESS FULL |DEPT | 4|
| VIEW |VW_SQ_1|7168|
| TABLE ACCESS FULL|EMP |7168|

55. Departments with no Employees
SELECT *
FROM dept
WHERE deptno NOT IN (SELECT deptno
FROM emp);
SELECT *
FROM dept x
WHERE NOT EXISTS (SELECT 1
FROM emp y
WHERE y.deptno = x.deptno);
DEPTNO DNAME LOC
40 OPERATIONS BOSTON

56. No Index and no NOT NULL Constraint on emp.deptno
Oracle
Version
NOT IN plan
NOT EXISTS plan 8i
SELECT STATEMENT
FILTER
TABLE ACCESS FULL DEPT
TABLE ACCESS FULL EMP 9i
HASH JOIN ANTI
10g
11g
HASH JOIN ANTI NA

57. NOT NULL constraint on emp.deptno
Oracle
Version
NOT IN plan
NOT EXISTS plan 8i
SELECT STATEMENT
FILTER
TABLE ACCESS FULL DEPT
TABLE ACCESS FULL EMP 9i
HASH JOIN ANTI
10g
11g
HASH JOIN ANTI SNA

58. Index on emp.deptno, no NOT NULL constraint
Oracle
Version
NOT IN plan
NOT EXISTS plan 8i
SELECT STATEMENT
FILTER
TABLE ACCESS FULL DEPT
TABLE ACCESS FULL EMP
INDEX RANGE SCAN EMP$DEPTNO 9i
10g
NESTED LOOPS ANTI
11g
HASH JOIN ANTI NA
HASH JOIN ANTI
INDEX FAST FULL SCAN EMP$DEPTNO
SELECT STATEMENT
NESTED LOOPS ANTI
TABLE ACCESS FULL DEPT
INDEX RANGE SCAN EMP$DEPTNO
12c

59. Index and NOT NULL Constraint on emp.deptno
Version
NOT IN plan
NOT EXISTS plan 8i
SELECT STATEMENT
FILTER
TABLE ACCESS FULL DEPT
INDEX RANGE SCAN EMP$DEPTNO 9i
INDEX FULL SCAN EMP$DEPTNO
10g
NESTED LOOPS ANTI
11g
HASH JOIN ANTI SNA
If the column on the right hand side of the condition (emp.deptno) has no NOT NULL constraint, then NA is used
If it is declared NOT NULL, SNA is used, along with an available index

60. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

61. Lateral Joins
Inline views cannot reference columns outside of the view
Considered to be out of scope
SELECT dname
,ename
FROM dept d
,(SELECT ename
FROM emp e
WHERE e.deptno = d.deptno);
WHERE e.deptno = d.deptno) *
ERROR : ORA-00904: "D"."DEPTNO": invalid identifier
Lateral views allow such references
Used internally
Similar to correlated subqueries

62. Lateral Joins in Oracle12c
SELECT dname
,ename
FROM dept d
,LATERAL(SELECT ename
FROM emp e
WHERE e.deptno = d.deptno);
DNAME ENAME
RESEARCH SMITH
SALES ALLEN
SALES WARD
: :
Identical execution plan to equivalent equi-join
Need to set an event on Oracle11g
LATERAL keyword can then be used to allow correlation
ALTER SESSION SET EVENTS '22829 TRACE NAME CONTEXT FOREVER';

63. Lateral Outer Join Syntax – Oracle12c
SELECT dname
,ename
,job
,loc
,sal
FROM dept
,LATERAL(SELECT ename,job,sal
FROM emp
WHERE emp.deptno = dept.deptno
AND (sal < 1000 OR job = 'ANALYST'))(+);
DNAME ENAME JOB LOC SAL
ACCOUNTING NEW YORK
RESEARCH SMITH CLERK DALLAS
RESEARCH SCOTT ANALYST DALLAS
RESEARCH FORD ANALYST DALLAS
SALES JAMES CLERK CHICAGO
OPERATIONS BOSTON

64. CROSS APPLY and OUTER APPLY
Attempts to solve compatibility issues with SQL Server migration tools
SELECT dname,ename,job,loc,sal
FROM dept
CROSS APPLY (SELECT ename,job,sal
FROM emp
WHERE emp.deptno = dept.deptno
AND (sal < 1000 OR job = 'ANALYST'))
SELECT dname,ename,job,loc,sal
FROM dept
OUTER APPLY (SELECT ename,job,sal
FROM emp
WHERE emp.deptno = dept.deptno
AND (sal < 1000 OR job = 'ANALYST'))

65. Equivalent Outer Join Plans
SELECT dname,ename,job,loc,sal
FROM dept
OUTER APPLY (SELECT ename,job,sal
FROM emp
WHERE emp.deptno = dept.deptno
AND (sal < 1000 OR job = 'ANALYST'));
SELECT dname,ename,job,loc,sal
FROM dept
,LATERAL(SELECT ename,job,sal
FROM emp
WHERE emp.deptno = dept.deptno
AND (sal < 1000
OR job = 'ANALYST'))(+);
SELECT dname,ename,job,loc,sal
FROM dept, emp
WHERE emp.deptno(+) = dept.deptno
AND (sal(+) < 1000
OR job(+) = 'ANALYST');
| Id | Operation | Name | Rows |
| 0 | SELECT STATEMENT | | |
|* 1 | HASH JOIN OUTER | | |
| 2 | TABLE ACCESS FULL| DEPT | |
|* 3 | TABLE ACCESS FULL| EMP | |

66. Use of CROSS/OUTER APPLY
When join sets are not known in advance
Example purely for illustration :
Number of emp rows to join is governed by highest salary in that department
Note use of subquery in FETCH clause
Can use any appropriate variable or expression
SELECT deptno
,empno
,ename
,sal
FROM dept OUTER APPLY
(SELECT empno
FROM emp
WHERE emp.deptno = dept.deptno ORDER BY sal DESC
FETCH FIRST (SELECT MAX(sal)/1500
WHERE emp.deptno = dept.deptno) ROWS ONLY)
ORDER BY dept.deptno;

67. CROSS APPLY and OUTER APPLY with Table Level Functions
CREATE TYPE emp_row_type as OBJECT (
empno VARCHAR2(8),
ename VARCHAR2(20),
deptno VARCHAR2(20));
CREATE TYPE emp_table_type as TABLE OF emp_row_type;
CREATE OR REPLACE FUNCTION get_all_emps(pi_deptno in number)
RETURN emp_table_type PIPELINED AS
BEGIN
FOR cur IN (SELECT * FROM emp where deptno = pi_deptno) LOOP
PIPE ROW(emp_row_type(cur.empno,cur.ename,cur.deptno));
END LOOP;
RETURN;
END;

68. Table Function Joins SELECT * FROM dept,
TABLE(get_all_emps(dept.deptno));
SELECT * FROM dept CROSS APPLY
TABLE(get_all_emps(dept.deptno));
SELECT * FROM dept JOIN
TABLE(get_all_emps(dept.deptno))
ON 1 = 1;
DEPTNO DNAME LOC EMPNO ENAME DEPTNO
10 ACCOUNTING NEW YORK CLARK
: : : : : :
20 RESEARCH DALLAS SMITH
30 SALES CHICAGO JAMES
SELECT * FROM dept, TABLE(get_all_emps(dept.deptno))(+);
SELECT * FROM dept OUTER APPLY
TABLE(get_all_emps(dept.deptno));
SELECT * FROM dept LEFT OUTER JOIN TABLE(get_all_emps(dept.deptno))
ON NULL IS NULL;
DEPTNO DNAME LOC EMPNO ENAME DEPTNO
10 ACCOUNTING NEW YORK CLARK
: : : : : :
20 RESEARCH DALLAS SMITH
30 SALES CHICAGO JAMES
40 OPERATIONS BOSTON

69. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

70. Oracle11g Nested Loops Joins
SELECT /*+ LEADING(e) USE_NL(d)*/ *
FROM emp e
,dept d
WHERE e.deptno = d.deptno
AND e.sal = 3000;
Batch style operation using an index
ROWIDs collected from an index
Used in a second nested loops style operation against the driven table
Can reduce physical I/O and improve performance in certain cases
Could reduce readability of execution plans
| SELECT STATEMENT | |
| NESTED LOOPS | |
| NESTED LOOPS | |
TABLE ACCESS FULL | EMP |
| INDEX UNIQUE SCAN | PK_DEPT |
| TABLE ACCESS BY INDEX ROWID| DEPT |

71. Reverting to Oracle10g Nested Loops Joins
Use a hint or set a parameter
SELECT /*+ LEADING(e) USE_NL(d) NO_NLJ_BATCHING(d)*/ *
FROM emp e
,dept d
WHERE e.deptno = d.deptno
AND e.sal = 3000;
ALTER SESSION SET
OPTIMIZER_FEATURES_ENABLE = ' ';
| SELECT STATEMENT | |
| NESTED LOOPS | |
| TABLE ACCESS FULL | EMP |
| TABLE ACCESS BY INDEX ROWID| DEPT |
| INDEX UNIQUE SCAN | PK_DEPT |
Other relevant hints
NLJ_BATCHING()
NLJ_PREFETCH()
NO_NLJ_PREFETCH()
New undocumented parameter _nlj_batching_enabled (0 or 1)

72. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

73. Join Elimination – the Ins and Outs
Some unnecessary joins are eliminated
SELECT e.ename
,e.deptno
FROM emp e
,dept d
WHERE e.deptno = d.deptno
SELECT e.ename
,e.deptno
FROM emp e
,dept d
WHERE e.deptno = d.deptno(+)
| Id | Operation | Name | Rows |
| 0 | SELECT STATEMENT | | |
|* 1 | TABLE ACCESS FULL| EMP | |
Predicate Information (identified by operation id):
1 - filter("E"."DEPTNO" IS NOT NULL)
No access to dept table (or any dept table index) in both cases on 11g
Filter is unnecessary if deptno has a NOT NULL constraint

74. Conditions for Join Elimination
Oracle 10g
Oracle11g
Oracle12c
Inner Join
Outer join
Outer Join
No constraints 
Primary key on dept
(or UK)
Foreign key on deptno in emp 

75. Join Elimination - Antijoins
Anti joins can also be eliminated
SELECT e.empno, e.ename
FROM emp e
WHERE NOT EXISTS (SELECT 1
FROM dept d
WHERE d.deptno = e.deptno);
SELECT STATEMENT
NESTED LOOPS ANTI
TABLE ACCESS FULL EMP
INDEX UNIQUE SCAN PK_DEPT
filter("E"."DEPTNO"="D”.”DEPTNO”
No foreign key
SELECT STATEMENT
TABLE ACCESS FULL EMP
filter("E"."DEPTNO" IS NOT NULL)
Foreign key present
SELECT STATEMENT
FILTER
TABLE ACCESS FULL EMP
filter(NULL IS NOT NULL)
Foreign key and NOT NULL
Dummy filter showing NO access to ANY table

76. Join Elimination - Semijoins
SELECT e.empno, e.ename
FROM emp e
WHERE EXISTS (SELECT 1
FROM dept d
WHERE d.deptno = e.deptno);
SELECT STATEMENT
NESTED LOOPS SEMI
TABLE ACCESS FULL EMP
INDEX UNIQUE SCAN PK_DEPT
filter("E"."DEPTNO"="D”.”DEPTNO”
No foreign key
SELECT STATEMENT
TABLE ACCESS FULL EMP
filter("E"."DEPTNO" IS NOT NULL)
Foreign key present
SELECT STATEMENT
FILTER
TABLE ACCESS FULL EMP
Foreign key and NOT NULL
No filter is necessary

77. Outer Join Elimination - Implications
A view used on the system
Part of an application
No access made to dept table even though join is embedded in view
Access is only to the emp table
Does not eliminate access to emp when referencing columns in dept
Control with hints
ELIMINATE_JOIN NO_ELIMINATE_JOIN
Control with hidden parameter
_optimizer_join_elimination_enabled
CREATE VIEW empdept AS
SELECT e.ename
,e.job
,e.deptno
,d.dname
,d.loc
FROM emp e
,dept d
WHERE e.deptno = d.deptno(+);
SELECT ename,job FROM empdept;

78. Oracle12c Join Techniques
ANSI and Oracle Joins Two-way Outer Joins Join and Filter Conditions Chaining on the Outer Join Hash Join Trees Anti Joins and Semi Joins Lateral Joins Nested Loops Joins Join Elimination Adaptive Joins

79. Adaptive Joins - Oracle12c
Oracle can switch execution plans during actual execution
Can occur in joins (e.g. emp with dept)
Generates subplans and uses a statistics collector to buffer rows
Detects if number of rows coming out of a table are more than a threshold value
If so, it switches to an alternative subplan
The new plan becomes the final plan
Subsequent executions will ALWAYS use the final plan
Statistics collector is bypassed
AUTOTRACE/EXPLAIN will show the initial default plan
dbms_xplan.display_cursor will show the final plan
Designed to optimize when an unexpected number of rows are received
Maybe because of inaccurate statistics

80. Adaptive Joins - Oracle12c
Common switch is from Nested Loops to hash join
Does not work with Sort-Merge
Nested Loops
Hash Join
Statistics Collector
Table Scan
dept
Index Scan
emp_dept_fki
emp
Final plan : Hash Join
threshold exceeded
Nested Loops
Hash Join
Statistics Collector
Table Scan
dept
Index Scan
emp_dept_fki
emp
Default plan : Nested Loops

81. Adaptive Joins - Tips Must have
optimizer_adaptive_features = (or higher)
Optimizer_adaptive_reporting_only = FALSE (default)
If set to TRUE, the optimizer can only report adaptive plans and not use them
New column is_resolved_adaptive_plan in v$sql with values of
‘Y’ – plan is the final adaptive plan
’N’ – plan is adaptive, but final plan has yet to be decided
NULL – plan is not adaptive
dbms_xplan.display_cursor has new arguments, e.g. format => ‘adaptive’
Shows full plan of query including unused steps
Unused steps marked with ‘-’

82. Undocumented Parameters
KSPPINM KSPPSTVL
_lm_node_join_opt FALSE
_abort_recovery_on_join FALSE
_hash_join_enabled TRUE
_multi_join_key_table_lookup TRUE
_force_hash_join_spill FALSE
_always_anti_join CHOOSE
_optimizer_null_aware_antijoin TRUE
_convert_set_to_join FALSE
_push_join_predicate TRUE
_push_join_union_view TRUE
_push_join_union_view TRUE
_optimizer_join_sel_sanity_check TRUE
_ordered_semijoin TRUE
_always_semi_join CHOOSE
_full_pwise_join_enabled TRUE
_partial_pwise_join_enabled TRUE
_index_join_enabled TRUE
_improved_outerjoin_card TRUE
_cost_equality_semi_join TRUE
_new_initial_join_orders TRUE
_oneside_colstat_for_equijoins TRUE
_gs_anti_semi_join_allowed TRUE
_optim_new_default_join_sel TRUE
_optimizer_new_join_card_computation TRUE
_optimizer_sortmerge_join_enabled TRUE
_optimizer_dim_subq_join_sel TRUE
_optimizer_join_order_control
_optimizer_join_elimination_enabled TRUE
_optimizer_sortmerge_join_inequality TRUE
_selfjoin_mv_duplicates TRUE
_optimizer_aw_join_push_enabled TRUE
_optimizer_native_full_outer_join FORCE
_optimizer_eliminate_filtering_join TRUE
_optimizer_join_factorization TRUE
_optimizer_outer_join_to_inner TRUE
_optimizer_full_outer_join_to_outer TRUE
_xsolapi_sql_all_multi_join_non_base_hints
_xsolapi_sql_enable_aw_join TRUE
SELECT ksppinm
,ksppstvl
FROM x$ksppi I
,x$ksppcv v
WHERE v.indx =i.indx
AND ksppinm LIKE '%join%';
Around 2750 undocumented parameters on 11g
Around 3968 undocumented parameters on 12c

83. Hints from v$sql_hint 273 hints on 11g 332 hints on 12c
NAME SQL_FEATURE CLASS
APPEND QKSFM_CBO APPEND
NO_MONITORING QKSFM_ALL NO_MONITORING
NO_SQL_TUNE QKSFM_ALL NO_SQL_TUNE
IGNORE_WHERE_CLAUSE QKSFM_ALL IGNORE_WHERE_CLAUSE
NO_QUERY_TRANSFORMATION QKSFM_TRANSFORMATION NO_QUERY_TRANSFORMATION
OPTIMIZER_FEATURES_ENABLE QKSFM_ALL OPTIMIZER_FEATURES_ENABL
GATHER_PLAN_STATISTICS QKSFM_GATHER_PLAN_STATISTICS GATHER_PLAN_STATISTICS
IGNORE_OPTIM_EMBEDDED_HINTS QKSFM_ALL IGNORE_OPTIM_EMBEDDED_HI
TABLE_STATS QKSFM_STATS TABLE_STATS
RULE QKSFM_RBO MODE
CHOOSE QKSFM_CHOOSE MODE
FIRST_ROWS QKSFM_FIRST_ROWS MODE
ORDERED QKSFM_CBO ORDERED
ORDERED_PREDICATES QKSFM_CBO ORDERED_PREDICATES
PUSH_PRED QKSFM_FILTER_PUSH_PRED PUSH_PRED
ELIMINATE_JOIN QKSFM_TABLE_ELIM ELIMINATE_JOIN
NO_ELIMINATE_JOIN QKSFM_TABLE_ELIM ELIMINATE_JOIN
OUTER_JOIN_TO_INNER QKSFM_OUTER_JOIN_TO_INNER OUTER_JOIN_TO_INNER
NO_OUTER_JOIN_TO_INNER QKSFM_OUTER_JOIN_TO_INNER OUTER_JOIN_TO_INNER
HASH_AJ QKSFM_CBO ANTIJOIN
NL_AJ QKSFM_CBO ANTIJOIN
SEMIJOIN QKSFM_TRANSFORMATION SEMIJOIN
HASH_SJ QKSFM_CBO SEMIJOIN
NL_SJ QKSFM_CBO SEMIJOIN
STAR QKSFM_STAR_TRANS STAR
SET_TO_JOIN QKSFM_SET_TO_JOIN SET_TO_JOIN
273 hints on 11g
332 hints on 12c

84. Oracle12c Join Techniques
Carl Dudley
Tradba Ltd
Oracle ACE Director