Question

How to use index in a select query in oracle? When creating indexes can enhance performance?

How to use indexed explicitly?

Answer 1

In case oracle comes up with a bad execution plan and you really really have to tell it which index to use (happens very seldom, but it happens), you can user optimizer hints:

SELECT /*+ INDEX(mytable myindex) */ foobar from mytable

But that should be your last resort. Before you even try to do it that way, use

execute dbms_stats.gather_table_stats('myschema','mytable')

to tell Oracle to analyze the table; hopefully, it will find out when to use the index that way.