column on partitioned table
      
      
An assertion 'ASSERT_COULUMN_MARKED_FOR_READ' is failed if the query 
is executed with index containing double column on partitioned table.
The problem is that assertion expects all the fields which are read,
to be in the read_set.
      
In this query only the field 'a' is in the readset as the tables in
the query are joined by the field 'a' and so the assertion fails 
expecting other field 'b'.
      
Since the function cmp() is just comparison of two parameters passed, 
the assertion is not required.
      
Fixed by removing the assertion in the double fields comparision
function and also fixed the index initialization to do ordered
index scan with RW LOCK which ensures all the fields from a key are in
the read_set.
 

Note: this bug is not reproducible with other datatypes because the
      assertion doesn't exist in comparision function for other 
      datatypes.

mysql-test/r/partition.result:
  Testcase for BUG#45816
mysql-test/t/partition.test:
  Testcase for BUG#45816
sql/field.cc:
  Removed the assertion ASSERT_COLUMN_MARED_FOR_READ in Field_double::cmp()
  function
sql/ha_partition.cc:
  Fixed index_int() method to make it initialize the read_set properly if
  ordered index scan with RW lock is requested.