Summary of Application Development and Administration

By -

Summary

• Time plays an important role in database systems. Databases are models of the real world. Whereas most databases model the state of the real world at a point in time (at the current time), temporal databases model the states of the real world across time.

• Facts in temporal relations have associated times when they are valid, which can be represented as a union of intervals. Temporal query languages simplify modeling of time, as well as time-related queries.

• Spatial databases are finding increasing use today to store computer-aided- design data as well as geographic data.

• Design data are stored primarily as vector data; geographic data consist of a combination of vector and raster data. Spatial-integrity constraints are important for design data.

• Vector data can be encoded as first-normal-form data, or can be stored using non-first-normal-form structures, such as lists. Special-purpose index structures are particularly important for accessing spatial data, and for processing spatial queries.

• R-trees are a multidimensional extension of B-trees; with variants such as R+- trees and R∗-trees, they have proved popular in spatial databases. Index structures that partition space in a regular fashion, such as quadtrees, help in processing spatial join queries.

• Multimedia databases are growing in importance. Issues such as similarity- based retrieval and delivery of data at guaranteed rates are topics of current research.

• Mobile computing systems have become common, leading to interest in data- base systems that can run on such systems. Query processing in such systems may involve lookups on server databases. The query cost model must include the cost of communication, including monetary cost and battery-power cost, which is relatively high for mobile systems.

• Broadcast is much cheaper per recipient than is point-to-point communication, and broadcast of data such as stock-market data helps mobile systems to pick up data inexpensively.

• Disconnected operation, use of broadcast data, and caching of data are three important issues being addressed in mobile computing.

Comments

Post a Comment

Popular posts from this blog

XML Document Schema

By -
Image
XML Document Schema Databases have schemas, which are used to constrain what information can be stored in the database and to constrain the data types of the stored information. In contrast, by default, XML documents can be created without any associated schema: An el- ement may then have any subelement or attribute. While such freedom may occasionally be acceptable given the self-describing nature of the data format, it is not generally useful when XML documents must be processesed automatically as part of an application, or even when large amounts of related data are to be formatted in XML. Here, we describe the document-oriented schema mechanism included as part of the XML standard, the Document Type Definition , as well as the more recently defined XML S chema . Document Type Definition The document type definition ( D TD ) is an optional part of an XML document. The main purpose of a DTD is much like that of a schema: to constrain and type the information present in the docume
Read more

Extended Relational-Algebra Operations.

By -
Image
Extended Relational-Algebra Operations The basic relational-algebra operations have been extended in several ways. A simple extension is to allow arithmetic operations as part of projection. An important extension is to allow aggregate operations such as computing the sum of the elements of a set, or their average. Another important extension is the outer-join operation, which allows relational-algebra expressions to deal with null values, which model missing information. Generalized Projection The generalized-projectio n operation extends the projection operation by allowing arithmetic functions to be used in the projection list. The generalized projection operation has the form where E is any relational-algebra expression, and each of F 1 , F 2 ,.. . , F n is an arithmetic expression involving constants and attributes in the schema of E . As a special case, the arithmetic expression may be simply an attribute or a constant. For example, suppose we have a relation c r
Read more

Distributed Databases:Concurrency Control in Distributed Databases

By -
Image
Concurrency Control in Distributed Databases We show here how some of the concurrency-control schemes discussed in Chapter 16 can be modified so that they can be used in a distributed environment. We assume that each site participates in the execution of a commit protocol to ensure global trans- action atomicity. The protocols we describe in this section require updates to be done on all replicas of a data item. If any site containing a replica of a data item has failed, updates to the data item cannot be processed. In Section 19.6 we describe protocols that can continue transaction processing even if some sites or links have failed, thereby providing high availability. Locking Protocols The various locking protocols described in Chapter 16 can be used in a distributed environment. The only change that needs to be incorporated is in the way the lock manager deals with replicated data. We present several possible schemes that are applicable to an environment where data can be rep
Read more