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> The "relational revolution" hinged on getting people to
> think in terms of value based join rather than links/pointers, and on
> RDBMS vendors figuring out how to implement them efficiently for
> ordinary data processing work.
> But the pendulum swung back the other way in the '90's -- the Web has
> no such thing as joins, only hierarchical relationships within (many)
> sites, and hyperlinks across pages. [But it only actually works
> because Google et al build a huge index that lets you search the whole
> thing using a combination of brute force and clever heuristics. ]
> In this decade, there are some signs that the pendulum is swinging
> back again.
> But XQuery at least potentially changes the rules here -- now
> XML users can use hierarchies for the relationships that they are good
> for and value-based joins for the other relationships.
> XQuery may open up new possibilities that were difficult to treat with
> XML when there is not a single hierarchy.
You're expressed several important points.
Hierarchical and CODASYL databases provided speedy inserts and updates, but the
link/pointer model isn't so great for querying -- particularly for ad hoc
queries. That's why the relational model and SQL became important. You tell an
engine what results you want instead of telling it how to navigate to the data.
The same phenomenon applies to the Web.
Serendipity is appealing -- browsing the web and uncovering gems by following
links -- but there's a point when the document collection becomes too large.
That's when using an engine to do the work of finding information becomes more
appealing than specifying the mechanics of navigation. Search engines are an
example of that phenomenon. So is XQuery. We'll need those and other information
retrieval solutions as the universe of searchable documents continues to expand.
XQuery enables us to use declarative logic when searching documents. (Does that
qualify it as a "niche" technology?)