The V$ Views (Developer and DBA)
Senior DBAs often tell junior DBAs that back in version 6 they used to know every V$ view by heart. Version 6 had only 23 V$ views, so the DBAs from the old days had it pretty easy. Oracle 9i had 259 V$ views and almost 400 X$ tables. Oracle 10gR2 (10.2.0.1.0) had 340 V$ views and 530 X$ tables. Oracle 11gR2 has 525 V$ views and 945 X$ tables (this number also depends on the version, OS, and options that you use as well).
Almost every great tuning or DBA product has one aspect in common. Most of them ...