Semantic Full-Text Search
The semantic full-text search combines the advantages of the classic full-text search with those of the tag search when dealing with large numbers of documents.
Firstly, one can continue searching for random letter strings intuitively and, secondly, this happens at such a speed, which - in large numbers of documents (>1.000.000) - can normally only be achieved by a tag search.
The price is a slight "fuzziness" of the search results, i.e. the number of hits with the semantic full-text search is approximately 50 % higher, because it also lists "similar" documents.