On the size of projections: I (Q791330)

The paper deals with the problem of estimating the number of distinct 'rows' in the projection of a 'table' onto a subset of its 'columns' (the words marked by apostrophes roughly stand for 'tuples', 'relation', 'attributes' respectively, in the usual relation model). The number of tables whose projection along one column contains exactly a given number of rows, is found; hence, the probability that the projection will have a fixed number of rows; a similar approach is undertaken for the case where the projection is to be made onto more columns. Finally, more computationally efficient forms of these results are discussed.