MDEV-18896 Crash in convert_join_subqueries_to_semijoins

If an IN-subquery is used in a table-less select the current code
should never consider it as candidate for semi-join optimizations.
Yet the function check_and_do_in_subquery_rewrites() improperly
checked the property "to be a table-less select". As a result
such select in IN subquery was used in INSERT .. SELECT then
the IN subquery by mistake was registered as a semi-join subquery
and convert_subq_to_sj() was called for it. However the code of
this function does not assume that the parent select of the subquery
could be a table-less select.

mysql-test/r/subselect_sj.result

@@ -3178,5 +3178,12 @@ EXECUTE stmt;
 a
 drop view v3;
 drop table t1,t2,t3;
+#
+# MDEV-18896: IN subquery in WHERE of a table-less query used for INSERT
+#
+create table t1 (a1 varchar(25));
+create table t2 (a2 varchar(25)) ;
+insert into t1 select 'xxx' from dual where 'xxx' in (select a2 from t2);
+drop table t1,t2;
 # End of 5.5 test
 set optimizer_switch=@subselect_sj_tmp;

mysql-test/r/subselect_sj_jcl6.result

@@ -3192,6 +3192,13 @@ EXECUTE stmt;
 a
 drop view v3;
 drop table t1,t2,t3;
+#
+# MDEV-18896: IN subquery in WHERE of a table-less query used for INSERT
+#
+create table t1 (a1 varchar(25));
+create table t2 (a2 varchar(25)) ;
+insert into t1 select 'xxx' from dual where 'xxx' in (select a2 from t2);
+drop table t1,t2;
 # End of 5.5 test
 set optimizer_switch=@subselect_sj_tmp;
 #

mysql-test/t/subselect_sj.test

@@ -2869,6 +2869,16 @@ EXECUTE stmt;
 drop view v3;
 drop table t1,t2,t3;
 
+--echo #
+--echo # MDEV-18896: IN subquery in WHERE of a table-less query used for INSERT
+--echo #
+
+create table t1 (a1 varchar(25));
+create table t2 (a2 varchar(25)) ;
+insert into t1 select 'xxx' from dual where 'xxx' in (select a2 from t2);
+
+drop table t1,t2;
+
 --echo # End of 5.5 test
 
 # The following command must be the last one the file 

sql/opt_subselect.cc

@@ -675,7 +675,7 @@ int check_and_do_in_subquery_rewrites(JOIN *join)
         select_lex->outer_select()->join &&                           // 6
         parent_unit->first_select()->leaf_tables.elements &&          // 7
         !in_subs->has_strategy() &&                                   // 8
-        select_lex->outer_select()->leaf_tables.elements &&           // 9
+        select_lex->outer_select()->table_list.first &&               // 9
         !((join->select_options |                                     // 10
            select_lex->outer_select()->join->select_options)          // 10
           & SELECT_STRAIGHT_JOIN))                                    // 10