The server-side constraint expressions are evaluated using a two step process. Every constraint expression has two parts, the projection and the selection subexpressions. The projection part of a constraint expression tells which variables to include in any return document describing the data set and the selection subexpression limits the returned data to variables with values that satisfy a set of relational expressions. The projection subexpression is evaluated when the entire constraint expression is parsed; at parse-time the server's copy of the data set's DDS is marked with the variables included in the projection. The selection subexpression, however, is not evaluated until values are read from the data set. One way to classify the projection and selection subexpressions is that projections depend solely on the logical structure of a data set, while selections depend on the values of particular variables within that data set.