MDEV-30525: Assertion `ranges > 0' fails in IO_AND_CPU_COST handler::keyread_time

Make get_best_group_min_max() exit early if the table has
table->records()=0. Attempting to compute loose scan over 0
groups eventually causes an assert when trying to get the
cost of reading 0 ranges.

mysql-test/main/merge.result

@@ -3936,5 +3936,14 @@ SELECT a, COUNT(*) FROM t1 WHERE a >= '2000-01-01 00:00:00' GROUP BY a;
 a	COUNT(*)
 DROP TABLE t1;
 #
+# MDEV-30525: Assertion `ranges > 0' fails in IO_AND_CPU_COST handler::keyread_time
+#
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE t2 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE tm (a INT, KEY(a)) ENGINE=MRG_MyISAM UNION=(t1,t2);
+SELECT DISTINCT a FROM tm WHERE a > 50;
+a
+DROP TABLE tm, t1, t2;
+#
 # End of 11.0 tests
 #

mysql-test/main/merge.test

@@ -2896,6 +2896,15 @@ 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 # MDEV-30525: Assertion `ranges > 0' fails in IO_AND_CPU_COST handler::keyread_time
+--echo #
+CREATE TABLE t1 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE t2 (a INT, KEY(a)) ENGINE=MyISAM;
+CREATE TABLE tm (a INT, KEY(a)) ENGINE=MRG_MyISAM UNION=(t1,t2);
+SELECT DISTINCT a FROM tm WHERE a > 50;
+DROP TABLE tm, t1, t2;
+
 --echo #
 --echo # End of 11.0 tests
 --echo #

sql/opt_range.cc

@@ -14049,6 +14049,8 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree, double read_time)
   else if (join->conds && join->conds->used_tables()
           & OUTER_REF_TABLE_BIT) // Cannot execute with correlated conditions.
     cause= "correlated conditions";
+  else if (table->stat_records() == 0)
+    cause= "Empty table"; // Exit now, records=0 messes up cost computations
 
   if (cause)
   {