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Chapter 25: Problem 7

How is a horizontal partitioning of a relation specified? How can a relation be put back together from a complete horizontal partitioning?

Short Answer

Expert verified
A horizontal partitioning of a relation is specified by associating a boolean-condition with each set of tuples. To reconstruct a relation from horizontal partitions, SQL UNION operator can be utilized providing that each partition maintains the same structure.

Step by step solution

01

Understanding Horizontal Partitioning

Horizontal partitioning is a database design principle which deals with the division of a database into two or more tables. Each table then carries the same column structure but is responsible for maintaining and managing different subsets of the data. In other words, rows of a relation are distributed among different tables which are known as partitions.
02

Specifying Horizontal Partitioning

For specifying horizontal partitioning, a condition that estimates a true or false outcome for each tuple is associated with each partition. These conditions are specified by the designer based on applications needs and system performance. These could be based on range, value or a more complex condition. Let's say we have a relation R that has a partition condition P1, then the partition will include tuples such that each tuple T in R, if P1(T) = True then T is in that partition.
03

Reassembling from Horizontal Partitioning

A relation can be put back together from a complete horizontal partitioning using the SQL UNION operator. The UNION operator is used to combine the result-set of two or more SELECT statements (without returning any duplicate rows). Each partition is treated as a separate SELECT statement, and their result-sets are combined to form the original table. However, when using UNION, it is important to ensure that all SELECT statements have the same structure (same number of columns, same or compatible types, and in the same order).

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