Bug#52517: Regression in ROW level replication performance with partitions

In bug-28430 HA_PRIMARY_KEY_REQUIRED_FOR_POSITION
was disabled in the partitioning engine in the first patch,
That bug was later fixed a second time, but that flag
was not removed.

No need to disable this flag, as it leads to bad
choise in row replication.

sql/ha_partition.h:
  Not disabling HA_PRIMARY_KEY_REQUIRED_FOR_POSITION flag.
  Updated comment (has nothing to do with hidden key.
sql/handler.h:
  Updated comments to about HA_PRIMARY_KEY_REQUIRED_FOR_POSITION.

sql/ha_partition.h

@@ -53,8 +53,7 @@ typedef struct st_ha_data_partition
                                         HA_CAN_FULLTEXT | \
                                         HA_DUPLICATE_POS | \
                                         HA_CAN_SQL_HANDLER | \
-                                        HA_CAN_INSERT_DELAYED | \
-                                        HA_PRIMARY_KEY_REQUIRED_FOR_POSITION)
+                                        HA_CAN_INSERT_DELAYED)
 class ha_partition :public handler
 {
 private:
@@ -766,9 +765,6 @@ public:
 
     HA_PRIMARY_KEY_REQUIRED_FOR_POSITION:
     Does the storage engine need a PK for position?
-    Used with hidden primary key in InnoDB.
-    Hidden primary keys cannot be supported by partitioning, since the
-    partitioning expressions columns must be a part of the primary key.
     (InnoDB)
 
     HA_FILE_BASED is always set for partition handler since we use a

sql/handler.h

@@ -93,7 +93,10 @@
 #define HA_PRIMARY_KEY_IN_READ_INDEX (1 << 15)
 /*
   If HA_PRIMARY_KEY_REQUIRED_FOR_POSITION is set, it means that to position()
-  uses a primary key. Without primary key, we can't call position().
+  uses a primary key given by the record argument.
+  Without primary key, we can't call position().
+  If not set, the position is returned as the current rows position
+  regardless of what argument is given.
 */ 
 #define HA_PRIMARY_KEY_REQUIRED_FOR_POSITION (1 << 16) 
 #define HA_CAN_RTREEKEYS       (1 << 17)
@@ -1446,10 +1449,9 @@ public:
   virtual int rnd_next(uchar *buf)=0;
   virtual int rnd_pos(uchar * buf, uchar *pos)=0;
   /**
-    One has to use this method when to find
-    random position by record as the plain
-    position() call doesn't work for some
-    handlers for random position.
+    This function only works for handlers having
+    HA_PRIMARY_KEY_REQUIRED_FOR_POSITION set.
+    It will return the row with the PK given in the record argument.
   */
   virtual int rnd_pos_by_record(uchar *record)
     {
@@ -1467,6 +1469,12 @@ public:
     { return HA_ERR_WRONG_COMMAND; }
   virtual ha_rows records_in_range(uint inx, key_range *min_key, key_range *max_key)
     { return (ha_rows) 10; }
+  /*
+    If HA_PRIMARY_KEY_REQUIRED_FOR_POSITION is set, then it sets ref
+    (reference to the row, aka position, with the primary key given in
+    the record).
+    Otherwise it set ref to the current row.
+  */
   virtual void position(const uchar *record)=0;
   virtual int info(uint)=0; // see my_base.h for full description
   virtual void get_dynamic_partition_info(PARTITION_INFO *stat_info,