As for DB Performance tuning, it's out of Developer's control, but it's of DBA, I guess... I don't think an interviewer will expect such DB performance tuning solutions from you, if you are applying for a developer position...
But as for Web tier and Business tier, developers should be able to reply the interviewer with satisfiable answers...
Co-author of SCMAD Exam Guide, Author of JMADPlus
SCJP1.2, CCNA, SCWCD1.4, SCBCD1.3, SCMAD1.0, SCJA1.0, SCJP6.0