Explain the following terms: a) Extent b)Temporal database c) Degree of homogeneity of DBMS

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 Extent

Ans: An extent is a logical unit of database storage space allocation made up of a number of contiguous data blocks. One or more extents in turn make up a segment. When the existing space in a segment is completely used, Oracle allocates a new extent for the segment.

When Extents Are Allocated

When you create a table, Oracle allocates to the table's data segment an initial extent of a specified number of data blocks. Although no rows have been inserted yet, the Oracle data blocks that correspond to the initial extent are reserved for that table's rows.

If the data blocks of a segment's initial extent become full and more space is required to hold new data, Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previously allocated extent in that segment. For maintenance purposes, the header block of each segment contains a directory of the extents in that segment.


Temporal database

Ans: A temporal database is a database that has certain features that support time-sensitive status for entries. Where some databases are considered current databases and only support factual data considered valid at the time of use, a temporal database can establish at what times certain entries are accurate.

A temporal database stores data relating to time instances. It offers temporal data types and stores information relating to past, present, and future times. The temporal database has two major notions or attributes. 1. valid time. 2. transaction time. More specifically the temporal aspects usually include valid time and transaction time. These attributes can be combined to form bitemporal data.

  • Valid time is the time period during which a fact is true in the real world.
  •  Transaction time is the time period during which a fact stored in the database was known.
  • Bitemporal data combines both Valid and Transaction Time.

It is possible to have timelines other than Valid Time and Transaction Time, such as Decision Time, in the database. In that case the database is called a multitemporal database as opposed to a bitemporal database. However, this approach introduces additional complexities such as dealing with the validity of (foreign) keys.

Degree of homogeneity of DBMS

Ans: If all servers (or individual local DBMSs) use identical software and all users (clients) use identical software, the DDBMS is called homogeneous; otherwise, it is called heterogeneous. Another factor related to the degree of homogeneity is the degree of local autonomy. If there is no provision for the local site to function as a stand-alone DBMS, then the system has no local autonomy. On the other hand, if direct access by local transactions to a server is permitted, the system has some degree of local autonomy.

At one extreme of the autonomy spectrum, we have a DDBMS that "looks like" a centralized DBMS to the user. A single conceptual schema exists, and all access to the system is obtained through a site that is part of the DDBMS—which means that no local autonomy exists. At the other extreme, we encounter a type of DDBMS called a federated DDBMS (or a multidatabase system). In such a system, each server is an independent and autonomous centralized DBMS that has its own local users, local transactions, and DBA and hence has a very high degree of local autonomy. The term federated database system (FDBS) is used when there is some global view or schema of the federation of databases that is shared by the applications. On the other hand, a multi-database system does not have a global schema and interactively constructs one as needed by the application. Both systems are hybrids between distributed and centralized systems and the distinction we made between them is not strictly followed. We will refer to them as FDBSs in a generic sense.

In a heterogeneous FDBS, one server may be a relational DBMS, another a network DBMS, and a third an object or hierarchical DBMS; in such a case it is necessary to have a canonical system language and to include language translators to translate subqueries from the canonical language to the language of each server.



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