Bug#56814 Explain + subselect + fulltext crashes server

create_sort_index() function overwrites original JOIN_TAB::type field.
At re-execution of subquery overwritten JOIN_TAB::type(JT_ALL) is
used instead of JT_FT. It misleads test_if_skip_sort_order() and
the function tries to find suitable key for the order that should
not be allowed for FULLTEXT(JT_FT) table.
The fix is to restore JOIN_TAB strucures for subselect on re-execution
for EXPLAIN.
Additional fix:
Update TABLE::maybe_null field which
affects list_contains_unique_index() behaviour as it
could have the value(maybe_null==TRUE) based on the
assumption that this join is outer
(see setup_table_map() func).


mysql-test/r/explain.result:
  test case
mysql-test/t/explain.test:
  test case
sql/item_subselect.cc:
  Make subquery uncacheable in case of EXPLAIN. It allows to keep
  original JOIN_TAB::type(see JOIN::save_join_tab) and restore it
  on re-execution.
sql/sql_select.cc:
  -restore JOIN_TAB strucures for subselect on re-execution for EXPLAIN
  -Update TABLE::maybe_null field as it could have
   the value(maybe_null==TRUE) based on the assumption
   that this join is outer(see setup_table_map() func).
   This change is not related to the crash problem but
   affects EXPLAIN results in the test case.

1 files changed, 14 insertions, 0 deletions

diff --git a/sql/sql_select.cc b/sql/sql_select.cc
index a260b78f131..11acd0685a8 100644
--- a/sql/sql_select.cc
+++ b/sql/sql_select.cc
@@ -2490,6 +2490,13 @@ mysql_select(THD *thd, Item ***rref_pointer_array,
 	{
 	  DBUG_RETURN(TRUE);
 	}
+        /*
+          Original join tabs might be overwritten at first
+          subselect execution. So we need to restore them.
+        */
+        Item_subselect *subselect= select_lex->master_unit()->item;
+        if (subselect && subselect->is_uncacheable() && join->reinit())
+          DBUG_RETURN(TRUE);
       }
       else
       {
@@ -8825,6 +8832,13 @@ simplify_joins(JOIN *join, List<TABLE_LIST> *join_list, COND *conds, bool top)
         that reject nulls => the outer join can be replaced by an inner join.
       */
       table->outer_join= 0;
+      /*
+        Update TABLE::maybe_null field as it could have
+        the value(maybe_null==TRUE) based on the assumption
+        that this join is outer(see setup_table_map() func).
+      */
+      if (table->table)
+        table->table->maybe_null= FALSE;
       if (table->on_expr)
       {
         /* Add on expression to the where condition. */