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Query Optimizer
Query optimization is a feature of many relational database management systems and other databases such as NoSQL and graph databases. The query optimizer attempts to determine the most efficient way to execute a given query by considering the possible query plans. Generally, the query optimizer cannot be accessed directly by users: once queries are submitted to the database server, and parsed by the parser, they are then passed to the query optimizer where optimization occurs. However, some database engines allow guiding the query optimizer with hints. A query is a request for information from a database. It can be as simple as "find the address of a person with Social Security number 123-45-6789," or more complex like "find the average salary of all the employed married men in California between the ages 30 to 39 who earn less than their spouses." The result of a query is generated by processing the rows in a database in a way that yields the requested information. Since databa ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Software Feature
A feature is "a prominent or distinctive user-visible aspect, quality, or characteristic of a software system or systems", as defined by Kang et al. At the implementation level, "it is a structure that extends and modifies the structure of a given software in order to satisfy a stakeholder’s requirement, to implement and encapsulate a design decision, and to offer a configuration option", as defined by Apel et al. Context The term feature means the same for software as it does for any kind of system. For example, the British Royal Navy's HMS Dreadnought (1906) was considered an important milestone in naval technology because of its advanced features that did not exist in pre-dreadnought battleships. Feature also applies to computer hardware. In the early history of computers, devices such as Digital Equipment Corporation's PDP-7 minicomputer (created in 1964) was noted for having a wealth of features, such as being the first version of the PDP minicomputer series to us ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
IBM System R
IBM System R is a database system built as a research project at IBM's San Jose Research Laboratory beginning in 1974, led by Edgar Codd, to implement his ideas on relational databases. System R was a seminal project as the first implementation of SQL, which has since become the standard relational data query language. It was also the first system to demonstrate that a relational database could provide good transaction processing performance. Design decisions in System R, as well as some fundamental algorithm choices (such as the dynamic programming algorithm used in query optimization), influenced many later relational systems. System R's first customer was Pratt & Whitney in 1977.. Not running on Unix hurt its popularity. See also * IBM Db2 * IBM SQL/DS * Ingres (database) * SQL * System/38 The System/38 is a discontinued minicomputer and midrange computer manufactured and sold by IBM. The system was announced in 1978. The System/38 has 48-bit computing, 48-bit address ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Logical Conjunction
In logic, mathematics and linguistics, ''and'' (\wedge) is the Truth function, truth-functional operator of conjunction or logical conjunction. The logical connective of this operator is typically represented as \wedge or \& or K (prefix) or \times or \cdot in which \wedge is the most modern and widely used. The ''and'' of a set of operands is true if and only if ''all'' of its operands are true, i.e., A \land B is true if and only if A is true and B is true. An operand of a conjunction is a conjunct. Beyond logic, the term "conjunction" also refers to similar concepts in other fields: * In natural language, the denotation of expressions such as English language, English "Conjunction (grammar), and"; * In programming languages, the Short-circuit evaluation, short-circuit and Control flow, control structure; * In set theory, Intersection (set theory), intersection. * In Lattice (order), lattice theory, logical conjunction (Infimum and supremum, greatest lower bound). Notati ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Histograms
A histogram is a visual representation of the distribution of quantitative data. To construct a histogram, the first step is to "bin" (or "bucket") the range of values— divide the entire range of values into a series of intervals—and then count how many values fall into each interval. The bins are usually specified as consecutive, non-overlapping intervals of a variable. The bins (intervals) are adjacent and are typically (but not required to be) of equal size. Histograms give a rough sense of the density of the underlying distribution of the data, and often for density estimation: estimating the probability density function of the underlying variable. The total area of a histogram used for probability density is always normalized to 1. If the length of the intervals on the ''x''-axis are all 1, then a histogram is identical to a relative frequency plot. Histograms are sometimes confused with bar charts. In a histogram, each bin is for a different range of values, so ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cardinality
The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thumb is ''pollex'' (compare ''hallux'' for big toe), and the corresponding adjective for thumb is ''pollical''. Definition Thumb and fingers The English word ''finger'' has two senses, even in the context of appendages of a single typical human hand: 1) Any of the five terminal members of the hand. 2) Any of the four terminal members of the hand, other than the thumb. Linguistically, it appears that the original sense was the first of these two: (also rendered as ) was, in the inferred Proto-Indo-European language, a suffixed form of (or ), which has given rise to many Indo-European-family words (tens of them defined in English dictionaries) that involve, or stem from, concepts of fiveness. The thumb shares the following with each of ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Not Exists
Not or NOT may also refer to: Language * Not, the general declarative form of "no", indicating a negation of a related statement that usually precedes * ... Not!, a grammatical construction used as a contradiction, popularized in the early 1990s Science and technology * Negation, a unary operator in logic depicted as ~, ¬, or ! * Bitwise NOT, an operator used in computer programming * NOT gate, a digital logic gate (commonly called an inverter) * Nordic Optical Telescope, an astronomical telescope at Roque de los Muchachos Observatory, La Palma, Canary Islands Other uses * Nottingham railway station (station code NOT) * Polish Federation of Engineering Associations (''Naczelna Organizacja Techniczna'') * Not, Missouri Not is an unincorporated community in Shannon County, Missouri, United States. History A post office called Not was established in 1886, and remained in operation until 1917. The community was so named on account of the knot A knot is an in ..., an uninc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Exists (SQL Operator)
eXist-db (or eXist for short) is an open source software project for NoSQL databases built on XML technology. It is classified as both a NoSQL document-oriented database system and a native XML database (and it provides support for XML, JSON, HTML and Binary documents). Unlike most relational database management systems (RDBMS) and NoSQL databases, eXist-db provides XQuery and XSLT as its query and application programming languages. eXist-db is released under version 2.1 of the GNU LGPL. Features eXist-db allows software developers to persist XML/JSON/Binary documents without writing extensive middleware. eXist-db follows and extends many W3C XML standards such as XQuery. eXist-db also supports REST interfaces for interfacing with AJAX-type web forms. Applications such as XForms may save their data by using just a few lines of code. The WebDAV interface to eXist-db allows users to "drag and drop" XML files directly into the eXist-db database. eXist-db automatically indexes d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
GROUP BY (SQL)
A GROUP BY statement in SQL specifies that a SQL SELECT statement partitions result rows into groups, based on their values in one or several columns. Typically, grouping is used to apply some sort of aggregate function for each group. The result of a query using a GROUP BY statement contains one row for each group. This implies constraints on the columns that can appear in the associated SELECT clause. As a general rule, the SELECT clause may only contain columns with a unique value per group. This includes columns that appear in the GROUP BY clause as well as aggregates resulting in one value per group. Examples Returns a list of Department IDs along with the sum of their sales for the date of January 1, 2000. SELECT DeptID, SUM(SaleAmount) FROM Sales WHERE SaleDate = '01-Jan-2000' GROUP BY DeptID In the following example one can ask "How many ''units'' were sold in each ''region'' for every ''ship date?''": The following code returns the data of the above pi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Database Management System
In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and analyze the data. The DBMS additionally encompasses the core facilities provided to administer the database. The sum total of the database, the DBMS and the associated applications can be referred to as a database system. Often the term "database" is also used loosely to refer to any of the DBMS, the database system or an application associated with the database. Before digital storage and retrieval of data have become widespread, index cards were used for data storage in a wide range of applications and environments: in the home to record and store recipes, shopping lists, contact information and other organizational data; in business to record presentation notes, project research and notes, and contact information; in schools as flash ca ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
