Post-merge fixes: merge MariaDB's fix for MDEV-5177 and MDEV-5555

with Oracle's fix for Bug#17588348 by reverting Oracle's fix.

sql/ha_partition.cc

@@ -7711,29 +7711,19 @@ uint ha_partition::min_record_length(uint options) const
 
 int ha_partition::cmp_ref(const uchar *ref1, const uchar *ref2)
 {
-  int cmp;
+  uint part_id;
   my_ptrdiff_t diff1, diff2;
+  handler *file;
   DBUG_ENTER("ha_partition::cmp_ref");
 
-  cmp = m_file[0]->cmp_ref((ref1 + PARTITION_BYTES_IN_POS),
-			   (ref2 + PARTITION_BYTES_IN_POS));
-  if (cmp)
-    DBUG_RETURN(cmp);
-
   if ((ref1[0] == ref2[0]) && (ref1[1] == ref2[1]))
   {
-   /* This means that the references are same and are in same partition.*/
-    DBUG_RETURN(0);
+    part_id= uint2korr(ref1);
+    file= m_file[part_id];
+    DBUG_ASSERT(part_id < m_tot_parts);
+    DBUG_RETURN(file->cmp_ref((ref1 + PARTITION_BYTES_IN_POS),
+			      (ref2 + PARTITION_BYTES_IN_POS)));
   }
-
-  /*
-    In Innodb we compare with either primary key value or global DB_ROW_ID so
-    it is not possible that the two references are equal and are in different
-    partitions, but in myisam it is possible since we are comparing offsets.
-    Remove this assert if DB_ROW_ID is changed to be per partition.
-  */
-  DBUG_ASSERT(!m_innodb);
-
   diff1= ref2[1] - ref1[1];
   diff2= ref2[0] - ref1[0];
   if (diff1 > 0)