1. A computer-implemented method comprising: (A) converting a plurality of atomic tuples, representing contents of a plurality of cells stored in a grid, into a plurality of schema tuples, comprising: (A)(1) assigning a plurality of catalog-defined types to the plurality of atomic tuples; (A)(2) assigning a role of locator to a first subset of the plurality of cells based on the plurality of catalog-defined logical types; and (A)(3) determining whether a locator tuple slice in the plurality of cells contains a missing value.