MDEV-30088 Assertion `cond_selectivity <= 1.0' failed in get_range_limit_read_cost

Fixed cost calculation for MERGE tables with 0 tables

mysql-test/main/merge.result

@@ -3925,3 +3925,16 @@ drop table tm, t;
 #
 # End of 10.8 tests
 #
+#
+# MDEV-30088 Assertion `cond_selectivity <= 1.0' failed in get_range_limit_read_cost
+#
+CREATE TABLE t1 (a TIMESTAMP, KEY(a)) ENGINE=MRG_MyISAM;
+explain SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	NULL	NULL	NULL	NULL	NULL	NULL	NULL	Impossible WHERE noticed after reading const tables
+SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+a	COUNT(*)
+DROP TABLE t1;
+#
+# End of 11.0 tests
+#

mysql-test/main/merge.test

@@ -2886,3 +2886,16 @@ drop table tm, t;
 --echo #
 --echo # End of 10.8 tests
 --echo #
+
+--echo #
+--echo # MDEV-30088 Assertion `cond_selectivity <= 1.0' failed in get_range_limit_read_cost
+--echo #
+
+CREATE TABLE t1 (a TIMESTAMP, KEY(a)) ENGINE=MRG_MyISAM;
+explain SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
+DROP TABLE t1;
+
+--echo #
+--echo # End of 11.0 tests
+--echo #

storage/myisammrg/ha_myisammrg.cc

@@ -361,7 +361,8 @@ IO_AND_CPU_COST ha_myisammrg::keyread_time(uint index, ulong ranges,
     cost.cpu*= file->tables;
   }
   /* Add the cost of having to do a key lookup in all trees */
-  cost.cpu+= (file->tables-1) * (ranges * KEY_LOOKUP_COST);
+  if (file->tables)
+    cost.cpu+= (file->tables-1) * (ranges * KEY_LOOKUP_COST);
   return cost;
 }