Fix for bug #32202: ORDER BY not working with GROUP BY

The bug is a regression introduced by the fix for bug30596. The problem
was that in cases when groups in GROUP BY correspond to only one row,
and there is ORDER BY, the GROUP BY was removed and the ORDER BY
rewritten to ORDER BY <group_by_columns> without checking if the
columns in GROUP BY and ORDER BY are compatible. This led to
incorrect ordering of the result set as it was sorted using the
GROUP BY columns. Additionaly, the code discarded ASC/DESC modifiers
from ORDER BY even if its columns were compatible with the GROUP BY
ones.

This patch fixes the regression by checking if ORDER BY columns form a
prefix of the GROUP BY ones, and rewriting ORDER BY only in that case,
preserving the ASC/DESC modifiers. That check is sufficient, since the
GROUP BY columns contain a unique index.

sql/sql_select.cc

@@ -1057,10 +1057,19 @@ JOIN::optimize()
         We have found that grouping can be removed since groups correspond to
         only one row anyway, but we still have to guarantee correct result
         order. The line below effectively rewrites the query from GROUP BY
-        <fields> to ORDER BY <fields>. One exception is if skip_sort_order is
-        set (see above), then we can simply skip GROUP BY.
+        <fields> to ORDER BY <fields>. There are two exceptions:
+        - if skip_sort_order is set (see above), then we can simply skip
+          GROUP BY;
+        - we can only rewrite ORDER BY if the ORDER BY fields are 'compatible'
+          with the GROUP BY ones, i.e. either one is a prefix of another.
+          We only check if the ORDER BY is a prefix of GROUP BY. In this case
+          test_if_subpart() copies the ASC/DESC attributes from the original
+          ORDER BY fields.
+          If GROUP BY is a prefix of ORDER BY, then it is safe to leave
+          'order' as is.
        */
-      order= skip_sort_order ? 0 : group_list;
+      if (!order || test_if_subpart(group_list, order))
+          order= skip_sort_order ? 0 : group_list;
       group_list= 0;
       group= 0;
     }