非关系数据库

Question

1. What exactly is non-relational database ??i have heard that web-sites like fb, google are using non relational database what is that

2. Under what condition we should use non-relational database

3. What are the advantage of non-relational data base over relational database ..??

Answer 1

Objectivity, db4o (open source), etc are called object databases which I think you're asking about? Rather than RDBMS they're called ODBMS. There's also the new "cloud databases" but I'm not too familiar with those and I'm not sure they'll be practical/useful for every type of application, even in the distant future. Between those two that's (probably) the two major non-RDBMS db types. (see http://en.wikipedia.org/wiki/NoSQL )

Object dbs are pretty cool, think something like Hibernate, only faster since there isn't a conversion to SQL being done. Object dbs are supposed to be much faster when you have complex/deep data hierarchies. (They're probably also a little faster "in general", but there are some downsides to watch out for.)

One downside to an object database is that it puts more work on developers to sometimes to DBA-like tasks. (I guess this could be good or bad.) Your traditional Oracle DBAs will be standing around clueless and useless if you use an object db. Some issues include - (at least with db4o) no really powerful tools to help manage the db (RDBMS have a ton of these), a different "schema upgrade" process, and generally it's a less mature type of database. Scalability can be an issue if you're talking about a large system, there's no clustering in db4o or anything like that (yet).

Also, just for completeness, older legacy databases like Adabas or VSAM are considered non-relational.