Fix for bug MDEV-3992, second attempt

The previous fix for MDEV-3992 was incomplete, because it still computed
incorrectly the number of keyparts of the extended secondary key in the
case when columns of the PK participate in the secondary key.

This patch by Monty corrects the above problem.

sql/sql_select.cc

@@ -18139,26 +18139,36 @@ static int test_if_order_by_key(ORDER *order, TABLE *table, uint idx,
           table->s->primary_key != MAX_KEY &&
           table->s->primary_key != idx)
       {
+        KEY_PART_INFO *start,*end;
+        uint pk_part_idx= 0;
         on_pk_suffix= TRUE;
-        key_part= table->key_info[table->s->primary_key].key_part;
-        key_part_end=key_part+table->key_info[table->s->primary_key].key_parts;
+        start= key_part= table->key_info[table->s->primary_key].key_part;
         const_key_parts=table->const_key_parts[table->s->primary_key];
 
         /*
-          Check for constness only those keyparts of the PK suffix, that will
-          be used to extend the secondary key 'idx'. This handles the case when
-          some columns of the PK are used in the secondary index.
+          Calculate true key_part_end and const_key_parts
+          (we have to stop as first not continous primary key part)
         */
-        for (uint pk_part_idx= 0;
-             ((const_key_parts & 1) &&
-              (table->key_info[idx].ext_key_part_map & (1 << pk_part_idx)));
-             const_key_parts>>= 1, pk_part_idx++)
+        for (key_part_end= key_part,
+             end= key_part+table->key_info[table->s->primary_key].key_parts;
+             key_part_end < end; key_part_end++, pk_part_idx++)
+        {
+          /* Found hole in the pk_parts; Abort */
+          if (!(table->key_info[idx].ext_key_part_map &
+                (((key_part_map) 1) << pk_part_idx)))
+            break;
+        }
+        /* Adjust const_key_parts */
+        const_key_parts&= (((key_part_map) 1) << pk_part_idx) -1;
+
+         for (; const_key_parts & 1 ; const_key_parts>>= 1)
            key_part++; 
         /*
-         The primary and secondary key parts were all const (i.e. there's
-         one row).  The sorting doesn't matter.
+          Test if the primary key parts were all const (i.e. there's one row).
+          The sorting doesn't matter.
         */
-        if (key_part == key_part_end && reverse == 0)
+        if (key_part == start+table->key_info[table->s->primary_key].key_parts &&
+            reverse == 0)
         {
           key_parts= 0;
           reverse= 1;
@@ -18178,6 +18188,7 @@ static int test_if_order_by_key(ORDER *order, TABLE *table, uint idx,
     if (reverse && flag != reverse)
       DBUG_RETURN(0);
     reverse=flag;				// Remember if reverse
+    if (key_part < key_part_end)
       key_part++;
   }
   if (on_pk_suffix)
@@ -22940,7 +22951,7 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
   {
     int direction;
     ha_rows select_limit= select_limit_arg;
-    uint used_key_parts;
+    uint used_key_parts= 0;
 
     if (keys.is_set(nr) &&
         (direction= test_if_order_by_key(order, table, nr, &used_key_parts)))
@@ -23017,8 +23028,8 @@ test_if_cheaper_ordering(const JOIN_TAB *tab, ORDER *order, TABLE *table,
                     if rec_per_key[0] != 0.
 	          */
                   DBUG_ASSERT(pkinfo->rec_per_key[i]);
-                  rec_per_key*= (i == 0) ? table_records :
-                                           pkinfo->rec_per_key[i-1];
+                  DBUG_ASSERT(i > 0);
+                  rec_per_key*= pkinfo->rec_per_key[i-1];
                   rec_per_key/= pkinfo->rec_per_key[i];
                 }
 	      }