index GIN GIN stands for Generalized Inverted Index. It is an index structure storing a set of (key, posting list) pairs, where 'posting list' is a set of documents in which the key occurs. It is generalized in the sense that a GIN index does not need to be aware of the operation that it accelerates. Instead, it uses custom strategies defined for particular data types. One advantage of GIN is that it allows the development of custom data types with the appropriate access methods, by an expert in the domain of the data type, rather than a database expert. This is much the same advantage as using GiST. The GIN implementation in PostgreSQL is primarily maintained by Teodor Sigaev and Oleg Bartunov, and there is more information on their website. The GIN interface has a high level of abstraction, requiring the access method implementer to only implement the semantics of the data type being accessed. The GIN layer itself takes care of concurrency, logging and searching the tree structure. All it takes to get a GIN access method working is to implement four user-defined methods, which define the behavior of keys in the tree. In short, GIN combines extensibility along with generality, code reuse, and a clean interface. There are four methods that an index operator class for GIN must provide: compare extract value extract query consistent The PostgreSQL source distribution includes GIN classes for one-dimensional arrays of all internal types. The following contrib modules also contain GIN operator classes: intarray Enhanced support for int4[] tsearch2 Support for inverted text indexing. This is much faster for very large, mostly-static sets of documents.