MDEV-30568: Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost

In get_range_limit_read_cost(), handle the case where range_rows=0.

mysql-test/main/merge.result

@@ -3945,5 +3945,14 @@ SELECT DISTINCT a FROM tm WHERE a > 50;
 a
 DROP TABLE tm, t1, t2;
 #
+# MDEV-30568 Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost
+#
+CREATE TABLE t1 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE t2 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE tm (f INT, KEY(f)) ENGINE=MERGE UNION = (t1, t2);
+SELECT DISTINCT f FROM tm WHERE f IN (47, 126, 97, 48, 73, 0);
+f
+DROP TABLE tm, t1, t2;
+#
 # End of 11.0 tests
 #

mysql-test/main/merge.test

@@ -2905,6 +2905,15 @@ 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 # MDEV-30568 Assertion `cond_selectivity <= 1.000000001' failed in get_range_limit_read_cost
+--echo #
+CREATE TABLE t1 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE t2 (f INT, KEY(f)) ENGINE=MyISAM;
+CREATE TABLE tm (f INT, KEY(f)) ENGINE=MERGE UNION = (t1, t2);
+SELECT DISTINCT f FROM tm WHERE f IN (47, 126, 97, 48, 73, 0);
+DROP TABLE tm, t1, t2;
+
 --echo #
 --echo # End of 11.0 tests
 --echo #

sql/sql_select.cc

@@ -30475,7 +30475,15 @@ static bool get_range_limit_read_cost(const POSITION *pos,
         account that using key2 we have to examine much fewer rows.
       */
       best_rows= pos->records_out;      // Best rows with any key/keys
-      double cond_selectivity= best_rows / range_rows;
+      double cond_selectivity;
+      /*
+        We assign "double range_rows" from integer #rows a few lines above
+        so comparison with 0.0 makes sense
+      */
+      if (range_rows > 0.0)
+        cond_selectivity= best_rows / range_rows;
+      else
+        cond_selectivity= 1.0;
       DBUG_ASSERT(cond_selectivity <= 1.000000001);
       set_if_smaller(cond_selectivity, 1.0);