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For example, one variant of the block nested loop join reads an entire page of tuples into memory and loads them into a hash table. It then scans S {\displaystyle S} , and probes the hash table to find S {\displaystyle S} tuples that match any of the tuples in the current page of R {\displaystyle R} .
PostgreSQL, MySQL and Oracle support natural joins; Microsoft T-SQL and IBM DB2 do not. The columns used in the join are implicit so the join code does not show which columns are expected, and a change in column names may change the results. In the SQL:2011 standard, natural joins are part of the optional F401, "Extended joined table", package.
In both cases, the features must be part of the language syntax and not an extension such as a library (libraries such as the posix-thread library implement a parallel execution model but lack the syntax and grammar required to be a programming language).
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In a SQL database query, a correlated subquery (also known as a synchronized subquery) is a subquery (a query nested inside another query) that uses values from the outer query. This can have major impact on performance because the correlated subquery might get recomputed every time for each row of the outer query is processed.
Each state is a possibly step for the code execution and each transitions is the reception of a message to change between two steps. And so when all messages are grab, the compiler execute the body join code corresponding to the completed model joint. So in the join-calculus, the basic values are the names like on the example is A,B or C.
Implementations of the fork–join model will typically fork tasks, fibers or lightweight threads, not operating-system-level "heavyweight" threads or processes, and use a thread pool to execute these tasks: the fork primitive allows the programmer to specify potential parallelism, which the implementation then maps onto actual parallel execution. [1]
SQL is a well known query language and data manipulation language for relational databases; XQuery is a query language for XML data sources; XPath is a declarative language for navigating XML documents; YQL is an SQL-like query language created by Yahoo! Search engine query languages, e.g., as used by Google [5] or Bing [6]