什么是更好/更快：加入许多表或选择一个大表

We are using Oracle 11, our application written on Java. Once a day, usually afternoon our database freezing because of many big sql queries. I want optimize this queries somehow. This queries consists of many joins of different tables. My question is: is it better for performance to use left join, or it is better put all information in one table and use one select? Suppose I will build a good indexes for this table.We are using Oracle 11, our application written