This function retrieves the row identity string from a row
string. The identity string contains the name of the table and primary key
of the row, allowing the row to be retrieved.

row is a row string obtained by selecting _ROW from some table.
The row identity string is a regular varchar value that can appear as a
column value. This is a sort of @universal foreign key@ with which a
reference can be made to any part of the database.
