For DBMS, the authors of the APOLLO system found the following [1]:

"Code coverage is a frequently used feedback mechanism in fuzzing engines [22, 27]. We found that this metric is not particularly useful for fuzzing DBMSs, since the core components of DBMS (e.g., query optimizer) already have high coverage (e.g., >95%) after running tens of queries."

[1] APOLLO: Automatic Detection and Diagnosis of Performance Regressions in Database Systems http://www.vldb.org/pvldb/vol13/p57-jung.pdf