Insight: updating a row by primary key setting primary key cause index changes

If one updates a row setting all values in a row including the primary key, does this cause oracle to recalc its indexes even thought the values are the same. I would think oracle would notices this. If this is so one could get better performance by not setting these values. example:
Table x {
 PrimaryKey name,
 ssn
}

update table x set name = 'Joe' , ssn = 10 where name = 'Joe';

Thanks , jcl
jcl_at_lightspeed.net

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