Impedance mismatch is not an "Objects vs. Relations" problem
  A problem of impedance mismatch between applications written in OO languages and relational DB is not a problem of discrepancy between object-oriented and relational approaches themselves. Its real causes can be found in usual implementation of the OO approach. Direct comparison of the two approaches cannot be used as a base for the conclusion that they are discrepant or mismatched. Experimental proof of absence of contradiction between object-oriented paradigm and relational data model is also presented 
Grigoriev Evgeny
cs.DB