SQL Server

Distributed Databases with Client Server Architectures Part – 3

Vertical Fragmentation: – Every single site might not need every columns or attributes of a relation or tables, which would specify the need for a dissimilar type of fragmentation. Vertical fragmentation divides a table or relation "vertically" by means of columns or attributes. A vertical fragment of an attributes or relation keeps only sure columns or attributes of the relation or tables. For an instance, an individual might want to fragment the EMPLOYEE relation or table into two (2) vertical fragments. The first fragment contains personal information like FName, MName, LName, DOB, Address and Gender and the second fragment contains work related information like ManagerID, Salary, EmpID, DeptID. This vertical fragmentation is not fairly correct for the reason that, if the two (2) fragments are kept distinctly, an individual cannot put the unique employee rows or tuples back together, as there is no common column or attribute between the two (2) fragments. It is essential to comprise the primary key or a number of candidate key columns or attribute in every single vertical fragment as a result that the full table or relation can be recreated from the fragments. Therefore, an individual should add the EmpID column or attribute to the personal information fragment.

It can be noted over here that every single horizontal fragment on a table or relation R can be quantified by means of operation in the relational algebra. A group of horizontal fragments whose conditions C1, C2, C3, ….., Cn contains every single rows or tuples in the table or relation R which is, every single row or tuple in table or relation R satisfies (C1 or C2 or C3 … or Cn) is recognized as a whole horizontal fragmentation of table or relation R. In numerous circumstances a whole horizontal fragmentation is also disjoint; which is, no row or tuple in table or relation R satisfies (Ci and Cj) for any i ≠ j. The two (2) previous instances of horizontal fragmentation for the EMPLOYEE as well as PROJECT tables or relations were both comprehensive and separate. To rebuild the table or relation R from a comprehensive horizontal fragmentation, an individual is required to apply the UNION operation to the fragments.

The vertical fragment on a table or relation R can be quantified by means of an operation in the relational algebra. The group of vertical fragments whose projection lists L1, L2, L3, …, Ln include every columns or attributes in table or relation R however share only the primary key column or attribute of table or relation R which is known as a complete vertical fragmentation of table or relation R. In this circumstance the projection lists satisfy the subsequent two (2) conditions:

• L1 U L2 U L3 … U Ln = ATTRS (R)

LiLj = PK (R) for any i ≠ j, where ATTRS (R) is the group of columns or attributes of table or relation R and PK (R) is the primary key (PK) of table or relation R.

To rebuild the table or relation R from a full vertical fragmentation, an individual can apply the OUTER UNION operation to the vertical fragments assuming that no horizontal fragmentation is castoff. One thing can be noted over here that an individual can apply a FULL OUTER JOIN operation too as well as will get the same outcome for a full vertical fragmentation, even when a number of horizontal fragmentations might have been applied too. The two (2) vertical fragments of the EMPLOYEE table or relation with projection lists L1 = {EmpID, FName, MName, LName, Gender} and L2 = {EmpID, Salary, ManagerID, DeptID} create a full vertical fragmentation of EMPLOYEE. Two (2) horizontal fragments which are neither full nor split are those defined on the EMPLOYEE relation or table by the conditions (SALARY > 10000) as well as (DeptID = 5); they may not comprise of all EMPLOYEE tuples or rows, as well as they may contain common rows or tuples. Two (2) vertical fragments which are not complete are those defined by means of the columns or attribute lists L1 = { FName, MName, LName, Gender} as well as L2 = {EmpID, FName, MName, LName, SALARY}; these lists violate both conditions of a full vertical fragmentation.

Mixed (Hybrid) Fragmentation: – An individual can merge the two (2) types of fragmentation, producing a mixed fragmentation. For an instance, an individual may combine the horizontal as well as vertical fragmentations of the EMPLOYEE table or relation given earlier into a mixed fragmentation which comprises of six (6) fragments. In this circumstance the unique table or relation can be rebuilt by means of applying UNION in addition to OUTER UNION (or OUTER JOIN) processes in the suitable order.

In overall, a fragment of a table or relation R can be itemized by means of a SELECT or PROJECT grouping of operations . If C = TRUE (which means that every rows or tuples are selected) as well as L ≠ ATTRS (R), an individual get a vertical fragment, in addition to if C ≠ TRUE plus L = ATTRS (R), an individual get a horizontal fragment. As a final point, if C ≠ TRUE as well as L ≠ ATTRS (R), an individual get a mixed fragment. It can be noted that a table or relation can itself be considered as a fragment with C = TRUE as well as L = ATTRS (R). In the subsequent discussion, the term fragment is castoff to refer to a relation or table or to any of the previous kinds of fragments.

A fragmentation schema of a database is a description of a group of fragments which contains every columns or attributes as well as rows or tuples in the database as well as fulfills the condition that the whole database can be recreated from the fragments by means of relating a number of sequence of OUTER UNION or OUTER JOIN in addition to UNION operations. It is also from time to time useful even though not essential to have every fragments be split except for the duplication of Primary Keys (PK) among vertical or mixed fragments. In the last case, every repetition as well as dissemination of fragments is obviously identified at a succeeding phase, individually from fragmentation.

An allocation schema defines the distribution of fragments to sites of the Distributed Databases (DDBs); therefore, it is a plotting which stipulates for every single fragment on the sites at which it is kept. If a fragment is kept at more than one site, it is supposed to be simulated. The data duplication as well as allocation is discussed at next.

In the upcoming part we will be going through Data Duplication and Allocation and Types of Distributed Database Systems.

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