Adbms 39 algorithms for project and set operations

Advance Database Management Systems : 39
Algorithms for PROJECT and SET Operations
Prof Neeraj Bhargava
Vaibhav Khanna
Department of Computer Science
School of Engineering and Systems Sciences
Maharshi Dayanand Saraswati University Ajmer

2
Algorithms for PROJECT and SET Operations (1)
• Algorithm for PROJECT operations (Figure 15.3b)
 <attribute list>(R)
1. If <attribute list> has a key of relation R, extract all tuples from
R with only the values for the attributes in <attribute list>.
2. If <attribute list> does NOT include a key of relation R,
duplicated tuples must be removed from the results.
• Methods to remove duplicate tuples
1. Sorting
2. Hashing

3
• Algorithm for SET operations
• Set operations:
– UNION, INTERSECTION, SET DIFFERENCE and CARTESIAN
PRODUCT
• CARTESIAN PRODUCT of relations R and S include all possible
combinations of records from R and S. The attribute of the
result include all attributes of R and S.
• Cost analysis of CARTESIAN PRODUCT
– If R has n records and j attributes and S has m records and k
attributes, the result relation will have n*m records and j+k
attributes.
• CARTESIAN PRODUCT operation is very expensive and should
be avoided if possible.

4
• Algorithm for SET operations (contd.)
• UNION (See Figure 15.3c)
– Sort the two relations on the same attributes.
– Scan and merge both sorted files concurrently, whenever the
same tuple exists in both relations, only one is kept in the
merged results.
• INTERSECTION (See Figure 15.3d)
– Sort the two relations on the same attributes.
– Scan and merge both sorted files concurrently, keep in the
merged results only those tuples that appear in both relations.
• SET DIFFERENCE R-S (See Figure 15.3e)
– Keep in the merged results only those tuples that appear in
relation R but not in relation S.

5
Implementing Aggregate Operations and Outer
Joins (1)
• Implementing Aggregate Operations:
• Aggregate operators:
– MIN, MAX, SUM, COUNT and AVG
• Options to implement aggregate operators:
– Table Scan
– Index
• Example
– SELECT MAX (SALARY)
– FROM EMPLOYEE;
• If an (ascending) index on SALARY exists for the employee relation, then
the optimizer could decide on traversing the index for the largest value,
which would entail following the right most pointer in each index node
from the root to a leaf.

6
Joins (2)
• Implementing Aggregate Operations (contd.):
• SUM, COUNT and AVG
• For a dense index (each record has one index entry):
– Apply the associated computation to the values in the index.
• For a non-dense index:
– Actual number of records associated with each index entry must be
accounted for
• With GROUP BY: the aggregate operator must be applied separately to
each group of tuples.
– Use sorting or hashing on the group attributes to partition the file into
the appropriate groups;
– Computes the aggregate function for the tuples in each group.
• What if we have Clustering index on the grouping attributes?

7
Joins (3)
• Implementing Outer Join:
• Outer Join Operators:
– LEFT OUTER JOIN
– RIGHT OUTER JOIN
– FULL OUTER JOIN.
• The full outer join produces a result which is equivalent to the union of the results
of the left and right outer joins.
• Example:
SELECT FNAME, DNAME
FROM (EMPLOYEE LEFT OUTER JOIN DEPARTMENT
ON DNO = DNUMBER);
• Note: The result of this query is a table of employee names and their associated
departments. It is similar to a regular join result, with the exception that if an
employee does not have an associated department, the employee's name will still
appear in the resulting table, although the department name would be indicated
as null.

8
Joins (4)
• Implementing Outer Join (contd.):
• Modifying Join Algorithms:
– Nested Loop or Sort-Merge joins can be modified to implement
outer join. E.g.,
• For left outer join, use the left relation as outer relation and
construct result from every tuple in the left relation.
• If there is a match, the concatenated tuple is saved in the result.
• However, if an outer tuple does not match, then the tuple is still
included in the result but is padded with a null value(s).

9
Joins (5)
• Implementing Outer Join (contd.):
• Executing a combination of relational algebra operators.
• Implement the previous left outer join example
– {Compute the JOIN of the EMPLOYEE and DEPARTMENT tables}
• TEMP1FNAME,DNAME(EMPLOYEE DNO=DNUMBER DEPARTMENT)
– {Find the EMPLOYEEs that do not appear in the JOIN}
• TEMP2   FNAME (EMPLOYEE) - FNAME (Temp1)
– {Pad each tuple in TEMP2 with a null DNAME field}
• TEMP2  TEMP2 x 'null'
– {UNION the temporary tables to produce the LEFT OUTER JOIN}
• RESULT  TEMP1 υ TEMP2
• The cost of the outer join, as computed above, would include the cost of
the associated steps (i.e., join, projections and union).

10
Combining Operations using Pipelining (1)
• Motivation
– A query is mapped into a sequence of operations.
– Each execution of an operation produces a temporary result.
– Generating and saving temporary files on disk is time consuming
and expensive.
• Alternative:
– Avoid constructing temporary results as much as possible.
– Pipeline the data through multiple operations - pass the result
of a previous operator to the next without waiting to complete
the previous operation.

11
Combining Operations using Pipelining (2)
• Example:
– For a 2-way join, combine the 2 selections on the
input and one projection on the output with the
Join.
• Dynamic generation of code to allow for
multiple operations to be pipelined.
• Results of a select operation are fed in a
"Pipeline" to the join algorithm.
• Also known as stream-based processing.

Assignment
• Explain the Algorithms for PROJECT and SET Operations

Adbms 39 algorithms for project and set operations

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