Fig. 7

Limits of procedure call semantics. The example queries demonstrate a limitation of procedure call parameters passing. In general, all example queries should select compounds that have the role identified by identifier 33281, and optionally should find similar compounds for each compound. Although procedure calls should be transparent from the user’s point of view, there is the requirement that variables used as parameters of the procedure call must be bound to values in the same group pattern before the procedure call is evaluated. For this reason, Query a is not correct, and the translator will report an error, because the MOLFILE variable is used as a parameter but is not bound in the same group pattern. It would seem that Query b solves this issue. However, in this case, variable ATB is not restricted in the group pattern containing the procedure call, so the procedure will be called for all structures in the database, which is very inefficient. Thus, although some patterns are repeated, only Query c can be considered a proper query