In
query language
A query language, also known as data query language or database query language (DQL), is a computer language used to make queries in databases and information systems. In database systems, query languages rely on strict theory to retrieve informa ...
s, path expressions identify an object by describing how to navigate to it
in some
graph
Graph may refer to:
Mathematics
*Graph (discrete mathematics), a structure made of vertices and edges
**Graph theory, the study of such graphs and their properties
*Graph (topology), a topological space resembling a graph in the sense of discret ...
(possibly
implicit
Implicit may refer to:
Mathematics
* Implicit function
* Implicit function theorem
* Implicit curve
* Implicit surface
* Implicit differential equation
Other uses
* Implicit assumption, in logic
* Implicit-association test, in social psychology
* ...
) of objects. For example, the path expression
''p''.Manager.Home.City might refer the city of residence of someone's manager.
Path expressions have been extended to support
regular expression
A regular expression (shortened as regex or regexp), sometimes referred to as rational expression, is a sequence of characters that specifies a match pattern in text. Usually such patterns are used by string-searching algorithms for "find" ...
-like flexibility.
XPath
XPath (XML Path Language) is an expression language designed to support the query or transformation of XML documents. It was defined by the World Wide Web Consortium (W3C) in 1999, and can be used to compute values (e.g., strings, numbers, or ...
is an example of a path expression language.
In
concurrency control
In information technology and computer science, especially in the fields of computer programming, operating systems, multiprocessors, and databases, concurrency control ensures that correct results for concurrent operations are generated, whil ...
, path expressions are a mechanism for expressing permitted sequences of execution. For example, a path expression like "
, write" might specify that either multiple simultaneous executions of
read or a single execution of
write but not both are allowed at any point in time.
It is important to know that the path expressions are a mechanism for the synchronization of processes at the monitor level in the software. That provides a clear and structured approach to the description of shared data and the coordination and communication between concurrent processes. This method is flexible in its ability to express timing, and can be used in different ways.
In addition, path expressions are useful for process synchronization for two reasons: first, the close relationship between stream expressions and regular expressions that simplify the task of writing and reasoning about programs that use this synchronization mechanism. Second, synchronization in many concurrent programs in a finite state, and therefore can be adequately described by regular expressions. For precisely the same reasons, path expressions are useful for controlling the behavior of complicated asynchronous circuits. In fact, the finite state assumption may be even more reasonable at the hardware level than at the monitor level.
Path expressions provide a high level of descriptive synchronization that aids in the prevention and detection of design errors in complex systems and overcomes some of the dangers, such as certain forms of coding errors.
See also
*
Object database
An object database or object-oriented database is a database management system in which information is represented in the form of objects as used in object-oriented programming. Object databases are different from relational databases which are ...
References
*
*
*
*
Concurrency control
Programming constructs
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