Forward Index vs Inverted index Why?
I was reading about inverted index (used by the text search engines like Solr, Elastic Search etc) and as I understand (if we take "Person" as an example):
The attribute to Person relationship is inverted:
John -> PersonId(1), PersonId(2), PersonId(3)
London -> PersonId(1), PersonId(2), PersonId(5)
I can now search the person records for 'John who lives in London'
Doesn't this solve all the problems? Why do we have the forward (or regular database index) at all? Or in other words, in what cases the regular indexing is useful? Please explain. Thanks.
Answer
The point that you're missing is that there is no real technical distinction between a forward index and an inverted index. "Forward" and "inverted" in this case are just descriptive terms to distinguish between:
- A list of words contained in a document.
- A list of documents containing a word.
The concept of an inverted index only makes sense if the concept of a regular (forward) index already exists. In the context of a search engine, a forward index would be the term vector; a list of terms contained within a particular document. The inverted index would be a list of documents containing a given term.
When you understand that the terms "forward" and "inverted" are really just relative terms used to describe the nature of the index you're talking about - and that really an index is just an index - your question doesn't really make sense any more.