Bug#17588348: INDEX MERGE USED ON PARTITIONED TABLE

               CAN RETURN WRONG RESULT SET

PROBLEM
-------
In ha_partition::cmp_ref() we were only calling the 
underlying cmp_ref() of storage engine if the records
are in the same partiton,else we sort by partition and
returns the result.But the index merge intersect 
algorithm expects first to sort by row-id first and 
then by partition id.

FIX
---
Compare the refernces first using storage engine cmp_ref
and then if references are equal(only happens if 
non clustered index is used) then sort it by partition id.

[Approved by Mattiasj #rb3755]
-

sql/ha_partition.cc

@@ -7485,19 +7485,29 @@ uint ha_partition::min_record_length(uint options) const
 
 int ha_partition::cmp_ref(const uchar *ref1, const uchar *ref2)
 {
-  uint part_id;
+  int cmp;
   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]))
   {
-    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)));
+   /* This means that the references are same and are in same partition.*/
+    DBUG_RETURN(0);
   }
+
+  /*
+    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)