The design of a distributed database introduces three new issues:
• How to partition the database into fragments.
• Which fragments to replicate.
• Where to locate those fragments and replicas.
DATA FRAGMENTATION
Data fragmentation allows you to break a single object into two or more segments or fragments. The object might be a user’s database, a system database, or a table. Each fragment can be stored at any site over a computer network.
Information about data fragmentation is stored in the distributed data catalog (DDC), from which it is accessed by the
TP to process user requests.
There are three types of data fragmentation strategies:
• Horizontal fragmentation refers to the division of a relation into
subsets (fragments) of tuples (rows). Each fragment is stored at a
different node, and each fragment has unique rows. However, the
unique rows all have the same attributes (columns). In short, each
fragment represents the equivalent of a SELECT statement, with the
WHERE clause on a single attribute.
• Vertical fragmentation refers to the division of a relation into attribute
(column) subsets. Each subset (fragment) is stored at a different node,
and each fragment has unique columns—with the exception of the key
column, which is common to all fragments.
• Mixed fragmentation refers to a combination of horizontal and vertical
strategies. In other words, a table may be divided into several horizontal
subsets (rows), each one having a subset of the attributes (columns).
To illustrate the fragmentation strategies, let’s use the CUSTOMER table for the XYZ Company, depicted in Figure bellow.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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