About when we should setup index?

[Emi Lu <emilu@xxxxxxxxxxxxxxxxx>]

Hello,

I have some questions about index.

For example, we have a table
test(id varchar(7) not null primary key,
      name varchar(15) ,
      sex varchar(1) ,
      birth_date date,
      valid_in_table boolean not null,
      time_stamp timestamp not null default now() );

. id is the primary key, so a default unique index is generated 
automatically ?
. if queries based on name are often, index should be setup for name?
 if there are 30,000 records, 29,000 records' names are different, will 
the index for name still be useful?

. possible values for sex are  F/M and null, should we setup index for sex?

. How about index for date and timestamp?

Basically, I'd like to know is there a percentage of the differences 
among data to decide whether index will help or not? For example,  among 
30,000 records, for a column, its value choices are less than A% and 
greater than B% so that we know index will help a lot?

thanks a lot!
Emi