MDEV-5814: MySQL Bug#13948247 DIVISION BY 0 IN GET_BEST_DISJUNCT_QUICK WITH FORCE INDEX GROUP BY

- Adopt MySQL's fix: don't run index_merge optimizer if the table statistics 
  reports that the table has 0 rows.

mysql-test/r/blackhole.result

@@ -9,3 +9,11 @@ SELECT 1 FROM t1 WHERE a = ANY (SELECT a FROM t2);
 1
 DROP TABLE t1, t2;
 End of 5.5 tests 
+#
+# Bug#13948247 DIVISION BY 0 IN GET_BEST_DISJUNCT_QUICK WITH FORCE INDEX GROUP BY
+#
+CREATE TABLE t1(a INT, b INT, c INT, KEY(c), UNIQUE(a)) ENGINE = BLACKHOLE;
+SELECT 0 FROM t1 FORCE INDEX FOR GROUP BY(a) WHERE a = 0 OR b = 0 AND c = 0;
+0
+DROP TABLE t1;
+End of 5.6 tests

mysql-test/t/blackhole.test

@@ -19,3 +19,12 @@ DROP TABLE t1, t2;
 
 --echo End of 5.5 tests 
 
+--echo #
+--echo # Bug#13948247 DIVISION BY 0 IN GET_BEST_DISJUNCT_QUICK WITH FORCE INDEX GROUP BY
+--echo #
+
+CREATE TABLE t1(a INT, b INT, c INT, KEY(c), UNIQUE(a)) ENGINE = BLACKHOLE;
+SELECT 0 FROM t1 FORCE INDEX FOR GROUP BY(a) WHERE a = 0 OR b = 0 AND c = 0;
+DROP TABLE t1;
+
+--echo End of 5.6 tests

sql/opt_range.cc

@@ -3168,7 +3168,8 @@ int SQL_SELECT::test_quick_select(THD *thd, key_map keys_to_use,
         }
       }
 
-      if (optimizer_flag(thd, OPTIMIZER_SWITCH_INDEX_MERGE))
+      if (optimizer_flag(thd, OPTIMIZER_SWITCH_INDEX_MERGE) &&
+          head->stat_records() != 0)
       {
         /* Try creating index_merge/ROR-union scan. */
         SEL_IMERGE *imerge;