Regular indexes are optimized for the case where a row has a single key value associated with it, so that the mapping between rows and keys is generally simple. The Generalized Inverted Index (GIN) is useful for a different sort of organization. GIN stores a list of keys with what's called a posting list of rows, each of which contain that key. A row can appear on the posting list of multiple keys too.
With the right design, GIN allows efficient construction and search of some advanced key/value data structures that wouldn't normally map well to a database structure. It leverages the ability to customize the comparison operator class used for an index, while relying on a regular B-tree structure to actually store the underlying index ...
