*In response to the original question, inverted index vs btree indexes*
Lets look at what these 2 types of indexes are
Forward Index/Index
Here the search key(attribute on which the index is built) is the name of the document. Consider a telephone book, where names are sorted alphabetically. While searching the phone number for a person whose name starts with N, you directly go the page where names from N start and then continue searching till you find the name you are looking for.
Inverted Index
Here the search key is a part of the content of any of the document. Take for instance the index at the back of a reference book. The keywords are listed alphabetically and the index show the page numbers on which a given keyword is present. In this case too if you want to find where a keyword starting with P occurs, you would directly go to the page where keywords for P are listed and then search further.
Knowing both these ideas, you might think, can one use a forward index to search the pages where a keyword occurs, or can you use an inverted index to find the phone number of a person, and the answer is yes, you can search anything using any type of index theoretically. However, the main purpose of an index is to make the lookup/search take less time, and this depends on what you are trying to search and what indexes you are using to search.
Consider a database having reviews for a movie stored area wise.
Assume it has 3 columns id, area_code, review. Id is the primary key,
areacode is a unique integer representing some area and review is a
text review.
Lets say you want to find out what the users of area 007 think about
your movie, in this case you create an index on the column movie and
easily find out the reviews for that area.
Now consider that you want to find out the number of users who gave a
positive review to your movie. A review would be a text, of highly
variable length and content and very high redundant content. But you
assume that certain terms like "excellent", "mind bending",
"masterpiece" etc could be found in a positive review. So here you use
an inverted index, which basically would tell you in which reviews
there were positive terms used, so you can get a rough estimate of the
count.
And finally you can combine both scenarios where you want to find out
the good reviews of a particular area, so you use both the indexes.
This is a theoretical description of what those indexes are and how they are to be used. But when you need to implement them, you need to think about how it would perform (reducing disk accesses or as in my example at the beginning, you need some way to reduce the number of pages you have to go through) and one way to approach this is to use B-trees.
You can read upon B-trees anywhere, but to answer your question, a both the forward and inverted index can use B-trees, because in both the cases you have an index file which needs to be accessed efficiently, and B-tree lets you do exactly that.
