What's different between inverted index and docValues in solr?

Question

I have read many article here. I have concluded that index is for searching, and docValue is for sorting, faceting. I am confused that whether index and docValue are the same data structure or same idea(store column value to get doc id)? If it is not the same, where is the different?

Answer 1

Inverted index ::

Inverted Index is a concept, which is used for building the search library Lucene. The standard way that Solr builds the index is with an inverted index. This style builds a list of terms found in all the documents in the index and next to each term is a list of documents that the term appears in (as well as how many times the term appears in that document). This makes search very fast - since users search by terms, having a ready list of term-to-document values makes the query process faster. This is like retrieving pages in a book related to a keyword by scanning the index at the back of a book, as opposed to searching every word of every page of the book. This type of index is called an inverted index, because it inverts a page-centric data structure (page->words) to a keyword-centric data structure (word->pages). Solr stores this index in a directory called index in the data directory.

DocValue ::

For other features that we now commonly associate with search, such as sorting, faceting, and highlighting, this approach is not very efficient. The faceting engine, for example, must look up each term that appears in each document that will make up the result set and pull the document IDs in order to build the facet list. In Solr, this is maintained in memory, and can be slow to load (depending on the number of documents, terms, etc.).

In Lucene 4.0, a new approach was introduced. DocValue fields are now column-oriented fields with a document-to-value mapping built at index time. This approach promises to relieve some of the memory requirements of the fieldCache and make lookups for faceting, sorting, and grouping much faster.

For docValues, you only need to enable it for a field that you will use it with. As with all schema design, you need to define a field type and then define fields of that type with docValues enabled. Enabling a field for docValues only requires adding docValues="true" to the field. DocValues are only available for specific field types.

<field name="category" type="string" indexed="false" stored="false" docValues="true" />