Lucene is a document indexing engine, that is its sole goal, and it does so beautifully. The interesting bit about using Lucene is that it probably wouldn’t be your main data store, but it is likely to be an important piece of your architecture.
The major shift in thinking with Lucene is that while indexing is relatively expensive, querying is free (well, not really, but you get my drift). Compare that to a relational database, where it is usually the inserts that are cheap, but queries are usually what cause us issues. RDBMS are also very good in giving us different views on top of our existing data, but the more you want from them, the more they have to do. We hit that query performance limit again. And we haven’t started talking about locking, transactions or concurrency yet.
Lucene doesn’t know how to do things you didn’t set it up to do. But what it does do, it does very fast.
Add to that the fact that in most applications, reads happen far more often than write, and you get a different constraint system. Because queries are expensive on the RDBMS, we try to make few of them, and we try to make a single query do most of the work. That isn’t necessarily the best strategy, but it is a very common one.
With Lucene, it is cheap to query, so it makes a lot more sense to perform several individual queries and process their results together to get the final result that you need. It may require somewhat more work (although there are things like Solr that would do it for you), but it is results in a far faster system performance overall.
In addition to that, since the Lucene index is important, but can always be re-created from source data (it may take some time, though), it doesn’t require all the ceremony associated with DB servers. Instead of buying an expensive server, get a few cheap ones. Lucene scale easily, after all. And since you only use Lucene for indexing, your actual DB querying pattern shift. Instead of making complex queries in the database, you make them in Lucene, and you only hit the DB with queries by primary key, which are the fastest possible way to get the data.
In effect, you outsourced your queries from the expensive machines to the cheap ones, and for a change, you actually got better performance overall