Oracle VS DB2   Oracle的优越多版本读一致性模型允许读和写完全独立完成而不冲突 IBM DB2要求读锁，读锁会引起死锁，影响并发能力，增大管理难度 Oracle仅对写活动要求锁   “Locks are acquired even if your application merely reads rows, so it is still important to commit read-only units of work. This is because shared locks are acquired by repeatable read, read stability, and cursor stability isolation levels in read-only applications. With repeatable read and read stability, all locks are held until a COMMIT is issued, preventing other processes from updating the locked data, unless you close your cursor using the WITH RELEASE clause. In addition, catalog locks are acquired even in uncommitted read applications using dynamic SQL or XQuery statements.” http://publib.boulder.ibm.com/infocenter/db2luw/v9r5/topic/com.ibm.db2.luw.admin.perf.doc/doc/c0005276.html   IBM DB2 does not have rollback segments and thus requires read locks to provide read consistency