A query is received. The query comprises a plural number of efficiency expressions. A key is generated for the query indicating a first selectivity region in multi-dimensional selectivity space comprising an array of selectivity regions arranged according to a grid with a number of dimensions equal to the plural number. The generated key is used to look up a stored query plan among a plurality of stored query plans. A stored query plan is executed for the query that is associated with a stored key determined to match the generated key.