MDEV-5846 MySQL Bug #18144 - Cost with FORCE/USE index seems incorrect in some cases.

mysql-test/r/key.result

@@ -612,3 +612,30 @@ SELECT 1 as RES FROM t1 AS t1_outer WHERE
 (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12;
 RES
 DROP TABLE t1;
+#
+# Bug#18144: Cost with FORCE/USE index seems incorrect in some cases.
+#
+# We are interested in showing that the cost for the last plan is higher
+# than for the preceding two plans.
+#
+CREATE TABLE t1( a INT, b INT, KEY( a ) );
+INSERT INTO t1 values (1, 2), (1, 3), (2, 3), (2, 4), (3, 4), (3, 5);
+EXPLAIN SELECT a, SUM( b ) FROM t1 GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using temporary; Using filesort
+SHOW STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	9.212184
+EXPLAIN SELECT a, SUM( b ) FROM t1 USE INDEX( a ) GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	6	Using temporary; Using filesort
+SHOW STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	9.212184
+EXPLAIN SELECT a, SUM( b ) FROM t1 FORCE INDEX( a ) GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index	NULL	a	5	NULL	6	
+SHOW STATUS LIKE 'Last_query_cost';
+Variable_name	Value
+Last_query_cost	14.199000
+DROP TABLE t1;

mysql-test/t/key.test

@@ -561,3 +561,23 @@ SELECT 1 as RES FROM t1 AS t1_outer WHERE
   (SELECT max(b) FROM t1 GROUP BY a HAVING a < 2) > 12;
 
 DROP TABLE t1;
+
+--echo #
+--echo # Bug#18144: Cost with FORCE/USE index seems incorrect in some cases.
+--echo #
+--echo # We are interested in showing that the cost for the last plan is higher
+--echo # than for the preceding two plans.
+--echo #
+CREATE TABLE t1( a INT, b INT, KEY( a ) );  
+INSERT INTO t1 values (1, 2), (1, 3), (2, 3), (2, 4), (3, 4), (3, 5);
+
+EXPLAIN SELECT a, SUM( b ) FROM t1 GROUP BY a;
+SHOW STATUS LIKE 'Last_query_cost';
+
+EXPLAIN SELECT a, SUM( b ) FROM t1 USE INDEX( a ) GROUP BY a;
+SHOW STATUS LIKE 'Last_query_cost';
+
+EXPLAIN SELECT a, SUM( b ) FROM t1 FORCE INDEX( a ) GROUP BY a;
+SHOW STATUS LIKE 'Last_query_cost';
+
+DROP TABLE t1;

sql/handler.h

@@ -2721,6 +2721,18 @@ public:
   }
   virtual double scan_time()
   { return ulonglong2double(stats.data_file_length) / IO_SIZE + 2; }
+
+  /**
+     The cost of reading a set of ranges from the table using an index
+     to access it.
+     
+     @param index  The index number.
+     @param ranges The number of ranges to be read.
+     @param rows   Total number of rows to be read.
+     
+     This method can be used to calculate the total cost of scanning a table
+     using an index by calling it using read_time(index, 1, table_size).
+  */
   virtual double read_time(uint index, uint ranges, ha_rows rows)
   { return rows2double(ranges+rows); }
 

sql/sql_select.cc

@@ -6158,7 +6158,11 @@ best_access_path(JOIN      *join,
     else
     {
       /* Estimate cost of reading table. */
-      tmp= s->scan_time();
+      if (s->table->force_index && !best_key) // index scan
+        tmp= s->table->file->read_time(s->ref.key, 1, s->records);
+      else // table scan
+        tmp= s->scan_time();
+
       if ((s->table->map & join->outer_join) || disable_jbuf)     // Can't use join cache
       {
         /*
@@ -7611,7 +7615,11 @@ best_extension_by_limited_search(JOIN      *join,
         if (join->sort_by_table &&
             join->sort_by_table !=
             join->positions[join->const_tables].table->table)
-          /* We have to make a temp table */
+          /* 
+             We may have to make a temp table, note that this is only a 
+             heuristic since we cannot know for sure at this point. 
+             Hence it may be wrong.
+          */
           current_read_time+= current_record_count;
         if (current_read_time < join->best_read)
         {