This document discusses SQL set operators like UNION, INTERSECT, and MINUS. It explains that INTERSECT returns rows common to two result sets, while MINUS returns rows in the first set not in the second. The operators support NULLs and have rules like UNION. Examples demonstrate their usage in views, derived tables, and procedures. Optimization techniques like nested loops and hash joins are covered. Scenarios illustrate uses like finding overlapping or non-overlapping supplier and order IDs. ANSI join improvements like hash joins are also summarized.

1. Useful Business Analytics SQL
operators and more
Ajaykumar Gupte
IBM
1

2. 4/9/15 2
AGENDA

Set Operators

Functionality, Basic Rules

Null Friendly Intersect and Minus

Usage

Execution Plans

Scenarios

ANSI JOIN Query improvements

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SET Operators

UNION Operator - Combines the rows from two or
more result sets into a single result set.

INTERSECT Operator - Computes a result set that
contains the common rows from two result sets.

MINUS/EXCEPT Operator - Evaluates two result sets and
returns all rows from the first set that are not also
contained in the second set.

4. Set Operations Result Sets
• MINUS and EXCEPT are synonyms.

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Functionality Of Intersect and Minus

Extension to the existing UNION/UNION ALL
SET operation

Results are always distinct or unique rows
(eliminate duplicate rows)

Same rules of UNION also applies e.g
- Both query blocks should have exact same number of
columns
- Projection clause should have comparable data types
- Projection clause can not have BYTE or TEXT

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Functionality Of Intersect and Minus
- Order by should be at the end
- Precedence will be from left to right, unless
they are grouped using parentheses
- Existing restrictions for UNION, applies to
these operators too.

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NULL Friendly SET Operators

Both Intersect and Minus are NULL friendly,
means when comparing NULL to NULL they are
considered equal

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Examples
create table t1 (col1 int); create table t2 (col1 int);
insert into t1 values (1); insert into t2 values (1);
insert into t1 values (2); insert into t2 values (3);
insert into t1 values (2); insert into t2 values (4);
insert into t1 values (2); insert into t2 values (4);
insert into t1 values (3); insert into t2 values (NULL);
insert into t1 values (4);
insert into t1 values (4);
insert into t1 values (NULL);
insert into t1 values (NULL);
insert into t1 values (NULL);

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Examples
select col1 from t1 intersect select col1 from t2;
col1
1
3
4
4 row(s) retrieved.
select col1 from t1 minus select col1 from t2;
col1
2
1 row(s) retrieved.
NULL

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Usage
Inside VIEW definitions

create view v1(c1,c2) as
select * from tabp intersect select * from tabr;

create view v55(c1,c2) as
select * from tabp minus
(select * from tabr minus select * from v1)
union (select * from tabp minus select * from
tabr);

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Usage
Inside the Derived Table
select * from
(select tab1.* from tab1 LEFT OUTER JOIN tab2
ON tab1.intcol = tab2.intcol2
intersect
select tab2.* from tab3 FULL OUTER JOIN tab2
ON tab2.charcol2 = tab3.charcol3);

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Usage
Inside the Subquery
select c1,c2,c3,c4,c5 from mtab1 where
exists (select c1,c2,c3,c4,c5 from stab1
group by c2,c3,c4,c5,c1
intersect
select c1,c2,c3,c4,c5 from stab2
group by c2,c3,c4,c5,c1
having count(*) < 3)
and c1 = 1;

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Usage
Inside the Procedure
create procedure p1_1()
returning int;
define ret_val int;
define row_val int;
let ret_val = 0;
foreach select intcol into row_val from tab1
intersect
select intcol2 from tab2
let ret_val = ret_val + 1;
end foreach
return ret_val;
end procedure;

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Usage
Cross database and Cross server
select intcol2, charcol2 from tab2
minus
(select intcol3, charcol3 from db2:tab3
intersect
select intcol, charcol from db3@serv3:tab1);

15. Set Operators Optimization
•INTERSECT – rows common to both arms
– internally transformed into EXISTS subquery with special
NULL handling
•MINUS or EXCEPT – rows in first arm that’s not
in second arm
– internally transformed into NOT EXISTS subquery with
special NULL handling

16. Nested Loop – Semi Join
•Execute subquery as a variation of nested-loop join
•Semi Join- read inner table only until server finds a
match
– for each row in the outer table, the inner table
contributes at most one row
•Anti Semi Join – return all non-matching rows from
inner table

17. Set Operations in explain
QUERY:
------
select intcol from tab1
intersect
select intcol2 from tab2
Estimated Cost: 4
Estimated # of Rows Returned: 1
1) informix.tab1: SEQUENTIAL SCAN
2) informix.tab2: SEQUENTIAL SCAN (First Row)
Filters: informix.tab1.intcol ==
informix.tab2.intcol2
NESTED LOOP JOIN (Semi Join)

18. Set Operations in explain
QUERY:
------
select intcol, charcol from tab1
intersect
select intcol2, charcol2 from tab2
minus
select intcol3, charcol3 from tab3
Estimated Cost: 6
Estimated # of Rows Returned: 1
1) informix.tab1: SEQUENTIAL SCAN
2) informix.tab2: SEQUENTIAL SCAN (First Row)
Filters: (informix.tab1.intcol == informix.tab2.intcol2
AND informix.tab1.charcol == informix.tab2.charcol2 )
NESTED LOOP JOIN (Semi Join)
3) informix.tab3: SEQUENTIAL SCAN (First Row)
Filters: (informix.tab1.charcol == informix.tab3.charcol3
AND informix.tab1.intcol == informix.tab3.intcol3 )
NESTED LOOP JOIN (Anti Semi Join)

19. Scenarios
This INTERSECT query example finds suppliers who have
placed an order.
select supplier_id from suppliers
INTERSECT
select supplier_id from orders;
This MINUS query example finds suppliers who have not
placed any order.
select supplier_id from suppliers
MINUS
select supplier_id from orders;

20. ANSI Join improvements
• Join Directives supported in ANSI queries
– ORDERED directive not allowed.
• HASH Join Support
– Support for Bushy tree and Right deep tree execution.
• Optimizer changes to allow comparison
between Nested Loop and Hash Joins.

21. Hash Join Support in ANSI JOIN
• Without Hash join support, only way to
execute joins on large tables without index is
to create DYNAMIC index followed by Nested
Loop join.
• Hash join can be faster for large joins
• Optimizer costing is adjusted for situation
where build/probe sides for hash join can be
composite

22. Hash Join for ANSI JOIN in sqexplain
QUERY:
------
select * from (t1 left join t2 on t1.a = t2.a )
left join (t3 inner join t4 on t3.a = t4.a) on t4.a = t1.a
1) informix.t1: SEQUENTIAL SCAN
2) informix.t2: INDEX PATH
(1) Index Name: informix.ind2
Index Keys: a (Serial, fragments: ALL)
Lower Index Filter: informix.t1.a = informix.t2.a
ON-Filters:informix.t1.a = informix.t2.a
NESTED LOOP JOIN(LEFT OUTER JOIN)
3) informix.t3: SEQUENTIAL SCAN
4) informix.t4: INDEX PATH
(1) Index Name: informix.ind4
Index Keys: a (Serial, fragments: ALL)
Lower Index Filter: informix.t3.a = informix.t4.a
ON-Filters:informix.t3.a = informix.t4.a
NESTED LOOP JOIN
ON-Filters:informix.t4.a = informix.t1.a
DYNAMIC HASH JOIN (LEFT OUTER JOIN)
Dynamic Hash Filters: informix.t4.a = informix.t1.a

23. Questions?
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