Retrieval Data Structure
In computer science, a retrieval data structure, also known as static function, is a space-efficient dictionary-like data type composed of a collection of (key, value) pairs that allows the following operations: * Construction from a collection of (key, value) pairs * Retrieve the value associated with the given key or anything if the key is not contained in the collection * Update the value associated with a key (optional) They can also be thought of as a function for an universe and the set of keys where retrieve has to return for any value and an arbitrary value from otherwise. In contrast to static functions, support (probabilistic) membership queries and dictionaries additionally allow operations like listing keys or looking up the value associated with a key and returning some other symbol if the key is not contained. As can be derived from the operations, this data structure does not need to store the keys at all and may actually use less space than would be needed for a simple list of the key value pairs. This makes it attractive in situations where the associated data is small (e.g. a few bits) compared to the keys because we can save a lot by reducing the space used by keys. To give a simple example suppose video game names annotated with a boolean indicating whether the game contains a dog that can be petted are given. A static function built from this database can reproduce the associated flag for all names contained in the original set and an arbitrary one for other names. The size of this static function can be made to be only bits for a small which is obviously much less than any pair based representation. (Wikipedia).
