ANSI SQL-92 [MS, ANSI] defines IsolationLevels in terms of phenomena: Dirty Reads, Non-Repeatable Reads, and Phantoms. This paper shows that these phenomena and the ANSI SQL definitions fail to characterize several popular isolation levels, including the standard locking implementations of the levels. Investigating the ambiguities of the phenomena leads to clearer definitions; in addition new phenomena that better characterize isolation types are introduced. An important multiversion isolation type, Snapshot Isolation, is defined.
Regular readers of this column will have become familiar with database language SQL -- indeed, most readers are already familiar with it. We have also discussed the fact that the SQL standard is being published in multiple parts and have even discussed one of those parts in some detail[l].Another standard, based on SQL and its structured user-defined types[2], has been developed and published by the International Organization for Standardization (ISO). This standard, like SQL, is divided into multiple parts (more independent than the parts of SQL, in fact). Some parts of this other standard, known as SQL/MM, have already been published and are currently in revision, while others are still in preparation for initial publication.In this issue, we introduce SQL/MM and review each of its parts, necessarily at a high level.
Not very long ago, we discussed the creation of a new part of SQL, XML-Related Specifications (SQL/XML), in this column [1]. At the time, we referred to the work that had been done as "infrastructure". We are pleased to be able to say that significant progress has been made, and SQL/XML [2] is now going out for the first formal stage of processing, Final Committee Draft (FCD) ballot, in ISO/IEC JTC1.In our previous column, we described the mapping of SQL 〈identifier〉s to XML Names, SQL data types to XML Schema data types, and SQL values to XML values. There have been a few small corrections and enhancements in these areas, but for the most part the descriptions in our previous column are still accurate.Thc new work that we will discuss in this column comes in three parts. The first part provides a mapping from a single table, all tables in a schema, or all tables in a catalog to an XML document. The second of these parts includes the creation of an XML data type in SQL and adds functions that create values of this new type. These functions allow a user to produce XML from existing SQL data. Finally, the "infrastructure" work that we described in our previous article included the mapping of SQL's predefined data types to XML Schema data types. This mapping has been extended to include the mapping of domains, distinct types, row types, arrays, and multisets.The FCD ballot that we mentioned began in early April. This will allow the comments contained in the ballot responses to be discussed at the Editing Meeting in September or October of this year. We expect the Editing Meeting to recommend progression to Final Draft International Status (FDIS) ballot, which suggests that an International Standard will be published by the middle of 2003.
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