RDBMS trade-offs
Mature technology, with existing & emerging industry standards
Very efficient indexing on attributes (1);
CPU and I/O intensive to join tables(2);
SQL not well-suited for more than 3-4 levels of relationships using joins (3);
Costly to maintain referential integrity(4)
Cannot support dynamic schema (5)
Notes:
(1) This is a short-term advantage, OODB vendors just need to implement the well-known solutions.
(2) Use of parallel processing and very-large-memory architectures can address this issue, but only up to a point.
(3) SQL-3 due to be published in 1998 alleviates this to some extent.
(4) This depends on the RDBMS vendor and CASE tool.
(5) It is almost inconceivable that RDBMSs can ever do this; but that is not to say that it is impossible to write application software using an RDBMS which appears to offer a dynamic schema.