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

Part #2: fix the case where table->stat_records()=1 (due to EITS
statistics), but the range returns rows=0.

mysql-test/main/merge.result

@@ -3944,6 +3944,17 @@ 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;
+# Testcase 2:
+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=MERGE UNION = (t1, t2) INSERT_METHOD=FIRST;
+ANALYZE TABLE tm PERSISTENT FOR ALL;
+Table	Op	Msg_type	Msg_text
+test.tm	analyze	status	Engine-independent statistics collected
+test.tm	analyze	note	The storage engine for the table doesn't support analyze
+SELECT DISTINCT a FROM (SELECT * FROM tm WHERE a iS NOT NULL) AS sq;
+a
+DROP TABLE tm, t1, t2;
 #
 # MDEV-30568 Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost
 #

mysql-test/main/merge.test

@@ -2905,6 +2905,14 @@ 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 # Testcase 2:
+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=MERGE UNION = (t1, t2) INSERT_METHOD=FIRST;
+ANALYZE TABLE tm PERSISTENT FOR ALL;
+SELECT DISTINCT a FROM (SELECT * FROM tm WHERE a iS NOT NULL) AS sq;
+DROP TABLE tm, t1, t2;
+
 --echo #
 --echo # MDEV-30568 Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost
 --echo #

sql/opt_range.cc

@@ -14527,6 +14527,17 @@ get_best_group_min_max(PARAM *param, SEL_TREE *tree, double read_time)
       if ((cur_index_tree= tree->keys[cur_param_idx]))
       {
         cur_quick_prefix_records= param->quick_rows[cur_index];
+        if (!cur_quick_prefix_records)
+        {
+          /*
+            Non-constant table has a range with rows=0. Can happen e.g. for
+            Merge tables. Regular range access will be just as good as loose
+            scan.
+          */
+          if (unlikely(trace_idx.trace_started()))
+            trace_idx.add("aborting_search", "range with rows=0");
+          DBUG_RETURN(NULL);
+        }
         if (unlikely(cur_index_tree && thd->trace_started()))
         {
           Json_writer_array trace_range(thd, "ranges");