[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: XML-enabled databases, XQuery APIs
> I don't understand this nearly well enough. In a > relational database, what are the characteristics > of an "XML type"? In the mid-1980s, SQL databases stored tabular data and the SQL standard included alphanumeric types (numbers and characters). Fast forward ten years and databases had to support more complex types. Applications required geo-spatial data, multidimensional data, time series, images, audio, video and text. Object-relational technology emerged and SQL platforms began supporting user-defined types, user-defined functions and content-based queries. A classic example was the sunset query at the California Department of Water Resources (DWR). By 1995, DWR had a half-million 35mm slides. It classified them for an SQL database using keywords and text descriptions. That classification and indexing scheme was inadequate. Clients requested photos based on content, such as a search for a reservoir with a low water level. To provide content-based SQL queries, DWR digitized the images in Photo-CD format and put them in the database. That enabled them to run content-based SQL queries such as finding images of sunsets (based on detecting orange at the top of the image). Fast forward to 2003 and now the SQL:2003 standard includes nested collections, multisets and an XMLType. Implementation of the standard is a bit more work than simply updating the parser. Supporting a data type means the SQL DBMS provides a data definition syntax, type checking, constraints, rules and access methods. It knows how to store, index, and optimize queries involving that type. The optimizer determines the best access plan for a query and it uses indexes for performance. For queries with alphanumeric types, a DBMS can use b-tree indexes and b-tree based data access. For complex types, the SQL platform must use different access methods and indexing techniques such as r-trees, quadtrees, KDB-trees and so on. So an SQL column of XMLType is a hierarchically-nested collection of elements. There's a structure to the data, order must be preserved, and it's understood by SQL DDL and DML. ======== Ken North =========== www.WebServicesSummit.com www.SQLSummit.com www.GridSummit.com
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