Correct evaluation of select-project-join queries under PWS reduces to three fundamental operations on pdfs:
floor, marginalize, and product (see Section 3 for
details). These operations use the information maintained by dependency sets and histories
to detect any correlations and handle them appropriately.
The standard relational operations remain unchanged for
the certain attributes in the database.
The usual definition of a pdf requires that it sums up (or
integrates) to 1. We remove this restriction in our model in
order to represent missing tuples with partial pdfs, as mentioned
earlier in Section 2.2.