MDEV-36323: Split Materialized code: last_refills is never set in 11.0+

SplM_opt_info::last_refills is never set, so remove it. It was used in
In JOIN_TAB::choose_best_splitting(), use local variable "refills" there,
instead.
The logic in the function is that we compute

    startup_cost= refills * spl_plan->cost;

only when we have a splitting query plan which implies "refills" was set
accordingly.

Also
- Added a comment about what "refills" is.
- Updated derived_cond_pushdown.result: the change makes the
  Split-Materialized plans more expensive, so the join order changes from
  t3, <split-materialized(outer_ref)> to <split-materialized>, t3

Author: spetrunia

mysql-test/main/derived_cond_pushdown.result

@@ -22725,8 +22725,8 @@ where dt.a=t1.a and t3.a < 3
 from t1 limit 5;
 id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
 1	PRIMARY	t1	ALL	NULL	NULL	NULL	NULL	1000	
+2	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where
 2	DEPENDENT SUBQUERY	<derived3>	ref	key0	key0	5	test.t1.a	1	
-2	DEPENDENT SUBQUERY	t3	ALL	NULL	NULL	NULL	NULL	2	Using where; Using join buffer (flat, BNL join)
 3	LATERAL DERIVED	t2	ref	a	a	5	test.t1.a	10	
 select
 a,

sql/opt_split.cc

@@ -291,7 +291,6 @@ class SplM_opt_info : public Sql_alloc
   double unsplit_oper_cost;
   /* Cardinality of T when nothing is pushed */
   double unsplit_card;
-  double last_refills;
 
   SplM_plan_info *find_plan(TABLE *table, uint key, uint parts);
 };
@@ -1056,6 +1055,25 @@ SplM_plan_info * JOIN_TAB::choose_best_splitting(uint idx,
   if (best_table)
   {
     *spl_pd_boundary= this->table->map;
+    /*
+      Compute "refills" - how many times we'll need to refill the split-
+      materialized temp. table. Split-materialized table has references to
+      preceding table(s). Suppose the join prefix is (t1, t2, t3) and
+      split-materialized refers to table t2:
+
+        t1  t2  t3  <split_materialized>
+            ^             |
+            +------------ +
+
+      If we do not use join buffer for table t3, then we'll need to refill
+      the split-materialized table partial_join_cardinality({t1, t2}) times.
+      (this assumes that fanout of table t3 is greater than 1, which is
+      typically true).
+      If table t3 uses join buffer, then every time we get a record combination
+      of {t1.row,t2.row,t3.row} the t2.row may be different and so we will need
+      to refill <split_materialized> every time, that is,
+      partial_join_cardinality(t1,t3,t3) times.
+    */
     if (!best_param_tables)
       refills= 1;
     else
@@ -1176,7 +1194,7 @@ SplM_plan_info * JOIN_TAB::choose_best_splitting(uint idx,
       The best plan that employs splitting is cheaper than
       the plan without splitting
     */
-    startup_cost= spl_opt_info->last_refills * spl_plan->cost;
+    startup_cost= refills * spl_plan->cost;
     records= (ha_rows) (spl_opt_info->unsplit_card * spl_plan->split_sel);
     if (unlikely(thd->trace_started()) && ! already_printed)
     {