Does oracle use an index on col1 ?
It depends on your optimizer mode. If it is Rule then, It definately use the Index on it. If it is Cost bases then, It evaluates each operation internally for the cost and choose the least expensive.
Does oracle always execute select to create view and then performes other operations ?
No, Views are created very first time when you execute CREATE VIEW statemnet but executed when one executes it with appropriate grant on it instead of TABLE itself.
Does operation 'union' permit using index on column col1 ?
I cannot say, It depends on the Number of Blocks accessed by this Query. If it returns more than 14-20% of total Blocks, It will use FTS instead Index scan.
Does this operation have to be perform before condition col1 = 'aaa'??
First of all, Oracle evaluates the WHERE condition.
You should read Oracle SQL Performace and Tuning Documents. It helps you more for extra detail.
MS Computer Science
OCP DBA 9i/8i