Indexes have their downsides, though. Optimizers choose the wrong indexes more often than you might think, so you might be surprised how many query performance problems are created by adding indexes to a database. Even if you have complete confidence that a new index will never take the place of a better index in a query execution plan, add indexes with caution and restraint.
In an ideal world, the only performance cost of adding indexes would come when you add, delete, or update rows. On quiet tables, the performance cost of indexes is never a problem, but on a busy, growing table, it can be a major cost. Inserts into an index are usually not a problem, especially on Oracle, which handles index-block locking for uncommitted work especially gracefully.
https://learning.oreilly.com/library/view/sql-tuning/0596005733/ch02.html#sqltuning-CHP-2-SECT-3.2
busy/growing tableでは振る舞いが異なる。
