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A hierarchical query is a type of SQL query that handles hierarchical model data. They are special cases of more general recursive fixpoint queries, which compute transitive closures . In standard SQL:1999 hierarchical queries are implemented by way of recursive common table expressions (CTEs).
In such cases, alternative query plans must be compared in terms of their execution time but also in terms of the precision or reliability of the data they generate. Multi-objective query optimization [10] models the cost of a query plan as a cost vector where each vector component represents cost according to a different cost metric. Classical ...
A query optimizer will not always choose the most efficient query plan for a given query. In some databases the query plan can be reviewed, problems found, and then the query optimizer gives hints on how to improve it. In other databases, alternatives to express the same query (other queries that return the same results) can be tried.
There are three different but related forms of fragmentation: external fragmentation, internal fragmentation, and data fragmentation, which can be present in isolation or conjunction. Fragmentation is often accepted in return for improvements in speed or simplicity. Analogous phenomena occur for other resources such as processors; see below.
Database partitioning emerged in the 1980s with systems like Teradata and NonStop SQL.The approach was later adopted by NoSQL databases and Hadoop-based data warehouses.. While implementations vary between transactional and analytical workloads, the core principles of partitioning remain consistent across both use c
In the duplication process, users may change only the master database. This ensures that local data will not be overwritten. Both replication and duplication can keep the data current in all distributive locations. [2] Besides distributed database replication and fragmentation, there are many other distributed database design technologies.
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.
Isolation is typically enforced at the database level. However, various client-side systems can also be used. It can be controlled in application frameworks or runtime containers such as J2EE Entity Beans [2] On older systems, it may be implemented systemically (by the application developers), for example through the use of temporary tables.