Could you explain what a GIN index is and why it is used in databases?

GIN (Generalized Inverted Index) is a type of index in PostgreSQL databases used for fast searching of complex data types such as arrays, JSON, text documents, and other structures with multiple values.

The GIN index stores a list of references to rows for each keyword or element, where they appear, enabling efficient queries with operators, for example, searching for the presence of an element in an array or by key in JSON.

It is used when you need to quickly search for multiple values within a single field, for example, finding all records where an array contains a specific value.