GIN indexes

Generalized inverted (GIN) indexes are a good way to index text. Suppose you want to index a million text documents. A certain word may occur millions of times. In a normal B-tree this would mean that the key is stored millions of times. Not so in a GIN. Each key (or word) is stored once and assigned to a document list. Keys are organized in a standard B-tree. Each entry will have a document list pointing to all entries in the table having the same key. A GIN index is very small and compact. However, it lacks an important feature found in B-trees—Sorted data. In a GIN, the list of item pointers associated with a certain key is sorted by the position of the row in the table and not by some arbitrary criteria.