The paper presents the concepts related to the design of high availability solutions for Microsoft SQL (MS SQL) Server database servers. MS SQL Server from version 2012 offers a new capability called AlwaysOn which is aimed at reducing downtime of servers or databases during a failure. It includes several mechanisms such as Failover Cluster Instances, Availability Groups, Database mirroring, Log shipping. The paper presents the implementation of some of these concepts in our prototype. We also compare these mechanisms focusing on their aptitude in contemporary information systems.
The paper presents the concepts related to the design of query optimization methods for nested queries. The proposed methods are dedicated for queries having so called weakly dependent subqueries. A subquery is weakly dependent from its direct operator if it depends only on expressions typed by enumerations. We have successfully developed the weakly dependent subqueries method in the context of object-oriented database management systems based on Stack-Based Approach. Systems storing XML data which are queried using XQuery can be considered simplified object-oriented databases. For this reason we try to adopt SBQL query optimization methods to XQuery expressions.
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