The document discusses query evaluation plans and optimization in a database management system. It describes how selection and projection operations can be performed on-the-fly without intermediate results. Different join algorithms like nested loops and sort-merge joins are compared and the factors affecting the choice of join algorithm, like indexes and additional selection conditions, are explained. Subquery materialization is also covered briefly.

1. G.H Patel College of Engineering and
Technology
Department Of Computer Engineering
DATABASE MANGMENT SYSTEM (2130703)

2. • QUERY EVALUATION PLAN
G.H Patel College of Engg and Technology, Department Of Computer Engineering 2

3. ename (on the fly)
planeId=100 AND rating>5 (on the fly)
(nested loops)
(file scan) Employees Maintenances (file scan)
Method
to use
Sometimes it might
be possible, to
pipeline the result of
one operator to
another operator
without creating a
temporary table
for the intermediate
result.
This saves in cost.
When the input to a
unary operator (e.g. 
or ) is pipelined into
it, we say the
operator is applied
on-the-fly.
G.H Patel College of Engg and Technology, Department Of Computer Engineering 3

4. •
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5. ename (on the fly)
planeId=100 AND rating>5 (on the fly)
Maintenances
(file scan)
(nested loops join)
Employees
(file scan)
Cost for this plan:
300,000 I/Os for the join.
 and  are done in the fly; no
I/O cost for them.
G.H Patel College of Engg and Technology, Department Of Computer Engineering 5

6. ename (on the fly)
planeId=100 AND rating>5 (on the fly)
Maintenances
(file scan)
(sort merge join)
Employees
(file scan)
Cost for this plan:
7,500 I/Os for the join.
 and  are done in the fly; no
I/O cost for them.
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10. FOR EXAMPLE:
SELECT * FROM STAFF WHERE ID = (SELECT MAX(MANAGER) FROM ORG)
IN THIS STATEMENT, THE SUBQUERY NEEDS TO BE EVALUATED ONLY ONCE.
THIS TYPE OF SUBQUERY MUST RETURN ONLY ONE ROW. IF EVALUATING THE SUBQUERY
CAUSES A CARDINALITY VIOLATION (IF IT RETURNS MORE THAN ONE ROW), AN EXCEPTION IS
THROWN WHEN THE SUBQUERY IS RUN.
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11. SUBQUERY MATERIALIZATION IS DETECTED BEFORE OPTIMIZATION, WHICH ALLOWS THE
DERBY OPTIMIZER TO SEE A MATERIALIZED SUBQUERY AS AN UNKNOWN CONSTANT VALUE.
THE COMPARISON IS THEREFORE OPTIMIZABLE.
THE ORIGINAL STATEMENT IS TRANSFORMED INTO THE FOLLOWING TWO STATEMENTS:
CONSTANT = SELECT MAX(MANAGER) FROM ORG SELECT * FROM STAFF WHERE ID
= CONSTANT
THE SECOND STATEMENT IS OPTIMIZABLE.
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13. G.H Patel College of Engg and Technology, Department Of Computer Engineering 13

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16. Employees (sin INT, ename VARCHAR(20), rating INT, age REAL)
Maintenances (sin INT, planeId INT, day DATE, descCode
CHAR(10))
SELECT ename
FROM Employees NATURAL JOIN Maintenances
WHERE planeId = 100 AND rating > 5;
ename (planeId=100 AND rating>5 (Employees Maintenances))
G.H Patel College of Engg and Technology, Department Of Computer Engineering 16

17. ename
planeId=100 AND rating>5
RA expressions can
are represented by
an expression tree.
Maintenances Employees
An algorithm is
chosen for each
node in the
expression tree.
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18. •
• QUERY EVALUATION PLANS
•
•
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• QUERY OPTIMIZATION.
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19. • CLUSTERING INDEX
• "PRIMARY"
• NON-CLUSTERING INDEX
• "SECONDARY"
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20. EMPLOYEES
MAINTENANCES
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• 1000
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• 500
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21. R.ATTR = VALUE (R)
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R.ATTR < VALUE (R)
• WHY?
•
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22. •
• THAT’S EASY
•
• THIS IS MORE EXPENSIVE
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23. •
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INDEX NESTED LOOPS JOIN
• SIN
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• ANALYSIS
•
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• 300,000 I/OS
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24. SORT-MERGE
•
• ANALYSIS
• MAINTENANCES EMPLOYEES
•
• MAINTENANCES
• EMPLOYEES
•
• (MUCH BETTER!!)
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25. • INDEX NESTED LOOPS JOIN
• SORT-MERGE JOIN
• WHY BOTHER WITH INDEX NESTED LOOPS JOIN?
• INDEX NESTED LOOPS
• MAINTENANCES
•
MAINTENANCES MAINTENANCES
EMPLOYEES
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26. •
• MAINTENANCES EMPLOYEES
• MAINTENANCES
•
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27. • OBSERVE THAT THE CHOICE OF INDEX NESTED LOOPS JOIN IS BASED ON CONSIDERING THE QUERY
AS A WHOLE, INCLUDING THE EXTRA SELECTION ON MAINTENANCES, RATHER THAN JUST THE JOIN
OPERATION BY ITSELF.
• QUERY OPTIMIZATION
•
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