1. A method for supporting a positioned data modification operation on a grouping of tables in a relational database system that are involved in a table query operation, each of said tables containing one or more records, and wherein said data modification operation is based on the position of a query position iterator with respect to each of said tables comprising: generating at query compile time, an iterator position retrieval function for each table involved in the query operation; generating at query compile time, a data set modification function specific to each table involved in the query operation; invoking at query execution time, each of said iterator retrieval functions until the iterator position retrieval function for a current open table returns the current position of said query execution iterator on that table; and invoking at query execution time, each of said data set modification functions until the data set modification function for said current open table performs a data set modification operation at the current position of said query execution iterator specified by said iterator position retrieval function.