'SELECT DISTINCT a,b FROM t1' should not use temp table if there is unique 
index (or primary key) on a.
There are a number of other similar cases that can be calculated without the
use of a temp table : multi-part unique indexes, primary keys or using GROUP BY 
instead of DISTINCT.
When a GROUP BY/DISTINCT clause contains all key parts of a unique
index, then it is guaranteed that the fields of the clause will be
unique, therefore we can optimize away GROUP BY/DISTINCT altogether.
This optimization has two effects:
* there is no need to create a temporary table to compute the
   GROUP/DISTINCT operation (or the temporary table will be smaller if only GROUP 
   is removed and DISTINCT stays or if DISTINCT is removed and GROUP BY stays)
* this causes the statement in effect to become updatable in Connector/Java
because the result set columns will be direct reference to the primary key of 
the table (instead to the temporary table that it currently references). 

Implemented a check that will optimize away GROUP BY/DISTINCT for queries like 
the above.
Currently it will work only for single non-constant table in the FROM clause.


mysql-test/r/distinct.result:
  Bug #16458: Simple SELECT FOR UPDATE causes "Result Set not updatable" error
    - test case
mysql-test/t/distinct.test:
  Bug #16458: Simple SELECT FOR UPDATE causes "Result Set not updatable" error
    - test case
sql/sql_select.cc:
  Bug #16458: Simple SELECT FOR UPDATE causes "Result Set not updatable" error
    - disable GROUP BY if contains the fields of a unique index.