MDEV-22462: Item_in_subselect::create_single_in_to_exists_cond(JOIN *, Item **, Item **): Assertion `false' failed.

Item_in_subselect::create_single_in_to_exists_cond() should handle the
case where the subquery is a table-less select but it is not a result
of a UNION.

(Table-less subqueries like "(SELECT 1)" are "substituted" with their select
list, but table-less subqueries with WHERE or HAVING clause, like
"(SELECT 1 WHERE ...)" are not substituted. They are handled with regular
execution path)

1 files changed, 21 insertions, 23 deletions

diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index ed8e5e900a2..1e3c9a77a26 100644
--- a/sql/item_subselect.cc
+++ b/sql/item_subselect.cc
@@ -2249,7 +2249,8 @@ Item_in_subselect::create_single_in_to_exists_cond(JOIN *join,
     */
     Item *item= (Item*) select_lex->item_list.head();
 
-    if (select_lex->table_list.elements)
+    if (select_lex->table_list.elements ||
+        !(select_lex->master_unit()->is_union()))
     {
       Item *having= item;
       Item *orig_item= item;
@@ -2297,31 +2298,28 @@ Item_in_subselect::create_single_in_to_exists_cond(JOIN *join,
     }
     else
     {
-      if (select_lex->master_unit()->is_union())
+      DBUG_ASSERT(select_lex->master_unit()->is_union());
+
+      Item *new_having=
+        func->create(thd, expr,
+                     new (thd->mem_root) Item_ref_null_helper(thd,
+                                                &select_lex->context,
+                                                this,
+                                                &select_lex->ref_pointer_array[0],
+                                                (char *)"<no matter>",
+                                                (char *)"<result>"));
+      if (!abort_on_null && left_expr->maybe_null)
       {
-        Item *new_having=
-          func->create(thd, expr,
-                       new (thd->mem_root) Item_ref_null_helper(thd,
-                                                  &select_lex->context,
-                                                  this,
-                                                  &select_lex->ref_pointer_array[0],
-                                                  (char *)"<no matter>",
-                                                  (char *)"<result>"));
-        if (!abort_on_null && left_expr->maybe_null)
-        {
-          disable_cond_guard_for_const_null_left_expr(0);
-          if (!(new_having= new (thd->mem_root) Item_func_trig_cond(thd, new_having,
-                                                            get_cond_guard(0))))
-            DBUG_RETURN(true);
-        }
-
-        new_having->name= (char*) in_having_cond;
-        if (fix_having(new_having, select_lex))
+        disable_cond_guard_for_const_null_left_expr(0);
+        if (!(new_having= new (thd->mem_root) Item_func_trig_cond(thd, new_having,
+                                                          get_cond_guard(0))))
           DBUG_RETURN(true);
-        *having_item= new_having;
       }
-      else
-        DBUG_ASSERT(false);
+
+      new_having->name= (char*) in_having_cond;
+      if (fix_having(new_having, select_lex))
+        DBUG_RETURN(true);
+      *having_item= new_having;
     }
   }