All the commercial enterprise database performance consultants are busy spending the money they make from fixing these critical apps and expensive problems ;-)
Edit: Can’t speak much for MSSQL and DB2, but there are lots of similar problems in the Oracle world too, sometimes due to an app design that works against the intended use of the DB/optimizer, sometimes due to a database optimizer bug and sometimes just due to the complexity of estimating the optimal plan based on limited statistical summaries of the “shape of your data” and expectation that the optimization won’t take longer than a few milliseconds.
Edit: Can’t speak much for MSSQL and DB2, but there are lots of similar problems in the Oracle world too, sometimes due to an app design that works against the intended use of the DB/optimizer, sometimes due to a database optimizer bug and sometimes just due to the complexity of estimating the optimal plan based on limited statistical summaries of the “shape of your data” and expectation that the optimization won’t take longer than a few milliseconds.