1. A method in a computer system for providing a matrix interface to a data store of entries, the entries having a first field, a second field, and a third field, the method comprising: receiving a request to retrieve an element from a matrix for a source first dimension and a source second dimension, each entry of the data store representing an element of the matrix, the first field representing a first dimension of the matrix, the second field representing a second dimension of the matrix, and the third field representing a value for the element of the matrix at the represented first dimension and second dimension; determining whether an entry of the data store has a value for the first field that matches the source first dimension and a value for the second field that matches the source second dimension; upon determining that an entry of the data store has a value for the first field that matches the source first dimension and a value for the second field that matches the source second dimension, providing the value of the third field of that entry as the value of the element of the matrix for the source first dimension and the source second dimension; and upon determining that no entry of the data store has a value for the first field that matches the source first dimension and a value for the second field that matches the source second dimension, providing an indication that no entry of the data store has a value for the first field that matches the source first dimension and a value for the second field that matches the source second dimension.