MDEV-35072: Assertion with optimizer_join_limit_pref_ratio and 1-table select

Variant for 11.2+:

In recompute_join_cost_with_limit(), do not subtract the cost of checking
the WHERE:
   pos->records_read* WHERE_COST_THD(join->thd)

It is already included in pos->read_time.

Also added comments about difference between this fix and the pre-11.2
variant.

mysql-test/main/order_by_limit_join.result

@@ -114,7 +114,7 @@ JS
         "full_join_cost": "REPLACED",
         "risk_ratio": 10,
         "shortcut_join_cost": "REPLACED",
-        "shortcut_cost_with_risk": 0.492018616,
+        "shortcut_cost_with_risk": 0.495218616,
         "use_shortcut_cost": true
     }
 ]
@@ -166,7 +166,7 @@ JS
         "full_join_cost": "REPLACED",
         "risk_ratio": 10,
         "shortcut_join_cost": "REPLACED",
-        "shortcut_cost_with_risk": 19.4241863,
+        "shortcut_cost_with_risk": 16.2273863,
         "use_shortcut_cost": true
     }
 ]
@@ -254,7 +254,7 @@ JS
         "full_join_cost": "REPLACED",
         "risk_ratio": 10,
         "shortcut_join_cost": "REPLACED",
-        "shortcut_cost_with_risk": 199.426608,
+        "shortcut_cost_with_risk": 197.826608,
         "use_shortcut_cost": false
     }
 ]
@@ -375,7 +375,7 @@ JS
         "full_join_cost": "REPLACED",
         "risk_ratio": 10,
         "shortcut_join_cost": "REPLACED",
-        "shortcut_cost_with_risk": 0.532449162,
+        "shortcut_cost_with_risk": 0.535649162,
         "use_shortcut_cost": true
     }
 ]
@@ -463,11 +463,21 @@ JS
         "full_join_cost": "REPLACED",
         "risk_ratio": 10,
         "shortcut_join_cost": "REPLACED",
-        "shortcut_cost_with_risk": 0.492018616,
+        "shortcut_cost_with_risk": 0.495218616,
         "use_shortcut_cost": true
     }
 ]
 set optimizer_search_depth=@tmp_osd;
 set optimizer_trace=@tmp_os;
-set optimizer_join_limit_pref_ratio=default;
 drop table t1, t10, t11;
+#
+# MDEV-35072: Assertion failure with optimizer_join_limit_pref_ratio and 1-table select
+#
+SET optimizer_join_limit_pref_ratio=1;
+CREATE TABLE t1 (c1 INT, INDEX(c1));
+INSERT INTO t1  VALUES (1),(2);
+SELECT * FROM t1  ORDER BY c1 LIMIT 1;
+c1
+1
+DROP TABLE t1;
+set optimizer_join_limit_pref_ratio=default;

mysql-test/main/order_by_limit_join.test

@@ -208,6 +208,15 @@ select json_detailed(json_extract(@trace, '$**.join_limit_shortcut_choice')) as
 
 set optimizer_search_depth=@tmp_osd;
 set optimizer_trace=@tmp_os;
-set optimizer_join_limit_pref_ratio=default;
 drop table t1, t10, t11;
 
+--echo #
+--echo # MDEV-35072: Assertion failure with optimizer_join_limit_pref_ratio and 1-table select
+--echo #
+SET optimizer_join_limit_pref_ratio=1;
+CREATE TABLE t1 (c1 INT, INDEX(c1));
+INSERT INTO t1  VALUES (1),(2);
+SELECT * FROM t1  ORDER BY c1 LIMIT 1;
+DROP TABLE t1;
+
+set optimizer_join_limit_pref_ratio=default;

sql/sql_select.cc

@@ -11626,6 +11626,8 @@ double recompute_join_cost_with_limit(const JOIN *join, bool skip_sorting,
   POSITION *pos= join->best_positions + join->const_tables;
   /*
     Generally, we assume that producing X% of output takes X% of the cost.
+    (Note: before 11.0, we subtracted COST_EPS here. In 11.0+, there's no need
+     to do this)
   */
   double partial_join_cost= join->best_read * fraction;
 
@@ -11646,10 +11648,11 @@ double recompute_join_cost_with_limit(const JOIN *join, bool skip_sorting,
     {
       /*
         Subtract the remainder of the first table's cost we had in
-        join->best_read:
+        join->best_read.
+        (Before 11.0, we also subtracted pos->records_read/TIME_FOR_COMPARE.
+         In 11.0+, that time is already included in pos->read_time)
       */
       partial_join_cost -= pos->read_time*fraction;
-      partial_join_cost -= pos->records_read*fraction * WHERE_COST_THD(join->thd);
 
       /* Add the cost of the new access method we've got: */
       partial_join_cost= COST_ADD(partial_join_cost, *first_table_cost);
@@ -11666,12 +11669,7 @@ double recompute_join_cost_with_limit(const JOIN *join, bool skip_sorting,
       table.  Do the same for costs of checking the WHERE.
     */
     double extra_first_table_cost= pos->read_time * (1.0 - fraction);
-    double extra_first_table_where= pos->records_read * (1.0 - fraction) *
+    partial_join_cost= COST_ADD(partial_join_cost, extra_first_table_cost);
-                                    WHERE_COST_THD(join->thd);
-
-    partial_join_cost= COST_ADD(partial_join_cost,
-                            COST_ADD(extra_first_table_cost,
-                                     extra_first_table_where));
   }
   return partial_join_cost;
 }