Indexes, bibliographies, concordances, and databases may not provide much textual information, but as aggregates of primary and secondary sources, they are often considered tertiary sources.

What type of source is indices?

These are sources that index, abstract, organize, compile, or digest other sources. Some reference materials and textbooks are considered tertiary sources when their chief purpose is to list, summarize or simply repackage ideas or other information.

Is indices primary or secondary?

Difference Between Primary Index and Secondary Index

A primary index is an index on a set of fields that includes the unique primary key and is guaranteed not to contain duplicates. In contrast, a secondary index is an index that is not a primary index and may have duplicates.