MDEV-9695: Wrong window frame when using RANGE BETWEEN N FOLLOWING AND PRECEDING

Part#1: Frame_n_rows::next_partition() should not assume that the
current table->record[0] points to the first row in the partition.

Since cursor supports move_to() operation, we dont need this.

mysql-test/r/win.result

@@ -658,3 +658,29 @@ part_id	pk	val	CNT
 5678	207	3	9
 5678	208	3	9
 drop table t4;
+#
+# MDEV-9695: Wrong window frame when using RANGE BETWEEN N FOLLOWING AND PRECEDING
+#
+create table t1 (pk int, a int, b int);
+insert into t1 values
+( 1 , 0, 1),
+( 2 , 0, 2),
+( 3 , 1, 4),
+( 4 , 1, 8),
+( 5 , 2, 32),
+( 6 , 2, 64),
+( 7 , 2, 128),
+( 8 , 2, 16);
+select pk, a, b,
+bit_or(b) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) as bit_or
+from t1;
+pk	a	b	bit_or
+1	0	1	3
+2	0	2	3
+3	1	4	12
+4	1	8	12
+5	2	32	96
+6	2	64	224
+7	2	128	208
+8	2	16	144
+drop table t1;

mysql-test/t/win.test

@@ -422,3 +422,22 @@ from t4;
 
 drop table t4;
 
+--echo #
+--echo # MDEV-9695: Wrong window frame when using RANGE BETWEEN N FOLLOWING AND PRECEDING
+--echo #
+create table t1 (pk int, a int, b int);
+insert into t1 values
+( 1 , 0, 1),
+( 2 , 0, 2),
+( 3 , 1, 4),
+( 4 , 1, 8),
+( 5 , 2, 32),
+( 6 , 2, 64),
+( 7 , 2, 128),
+( 8 , 2, 16);
+ 
+select pk, a, b,
+bit_or(b) over (partition by a order by pk ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) as bit_or
+from t1;
+
+drop table t1;

sql/sql_window.cc

@@ -650,16 +650,8 @@ public:
     {
       if (rownum != 0)
       {
-        /* 
-          The cursor in "ROWS n PRECEDING" lags behind by n_rows rows.
-          Catch up.
-          TODO: change this to be a jump forward.
-        */
-        while (!(cursor_eof= (0 != cursor.get_next())))
-        {
-          if (bound_tracker.check_if_next_group())
-            break;
-        }
+        /* The cursor in "ROWS n PRECEDING" lags behind by n_rows rows. */
+        cursor.move_to(rownum);
       }
       n_rows_to_skip= n_rows - (is_top_bound? 0:1);
     }
@@ -717,7 +709,7 @@ private:
     {
       if (!(cursor_eof= (0 != cursor.get_next())))
       {
-        bool new_group= bound_tracker.check_if_next_group();
+        bool new_group= is_preceding? false: bound_tracker.check_if_next_group();
         if (at_partition_start || !new_group)
         {
           if (is_top_bound) // this is frame start endpoint