Bug#54488 crash when using explain and prepared statements with subqueries

The crash happens because original join table is replaced with temporary table
at execution stage and later we attempt to use this temporary table in
select_describe. It might happen that
Item_subselect::update_used_tables() method which sets const_item flag
is not called by some reasons (no where/having conditon in subquery for example).
It prevents JOIN::join_tmp creation and breaks original join.
The fix is to call ::update_used_tables() before ::const_item() check.

mysql-test/r/ps.result

@@ -3021,4 +3021,22 @@ Warnings:
 Note	1003	select 1 AS `1` from `test`.`t1` `t2` left join `test`.`t1` on(1) where 1
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
+#
+# Bug#54488 crash when using explain and prepared statements with subqueries
+#
+CREATE TABLE t1(f1 INT);
+INSERT INTO t1 VALUES (1),(1);
+PREPARE stmt FROM 'EXPLAIN SELECT 1 FROM t1 WHERE (SELECT (SELECT 1 FROM t1 GROUP BY f1))';
+EXECUTE stmt;
+id     select_type     table   type    possible_keys   key     key_len ref     rows    Extra
+1      PRIMARY t1      ALL     NULL    NULL    NULL    NULL    2       
+2      SUBQUERY        NULL    NULL    NULL    NULL    NULL    NULL    NULL    No tables used
+3      SUBQUERY        t1      ALL     NULL    NULL    NULL    NULL    2       Using temporary; Using filesort
+EXECUTE stmt;
+id     select_type     table   type    possible_keys   key     key_len ref     rows    Extra
+1      PRIMARY t1      ALL     NULL    NULL    NULL    NULL    2       
+2      SUBQUERY        NULL    NULL    NULL    NULL    NULL    NULL    NULL    No tables used
+3      SUBQUERY        t1      ALL     NULL    NULL    NULL    NULL    2       Using temporary; Using filesort
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
 End of 5.1 tests.

mysql-test/t/ps.test

@@ -3090,4 +3090,15 @@ EXECUTE stmt;
 DEALLOCATE PREPARE stmt;
 DROP TABLE t1;
 
+--echo #
+--echo # Bug#54488 crash when using explain and prepared statements with subqueries
+--echo #
+CREATE TABLE t1(f1 INT);
+INSERT INTO t1 VALUES (1),(1);
+PREPARE stmt FROM 'EXPLAIN SELECT 1 FROM t1 WHERE (SELECT (SELECT 1 FROM t1 GROUP BY f1))';
+EXECUTE stmt;
+EXECUTE stmt;
+DEALLOCATE PREPARE stmt;
+DROP TABLE t1;
+
 --echo End of 5.1 tests.

sql/item_subselect.cc

@@ -1914,18 +1914,22 @@ int subselect_single_select_engine::exec()
     }
     if (!select_lex->uncacheable && thd->lex->describe && 
         !(join->select_options & SELECT_DESCRIBE) && 
-        join->need_tmp && item->const_item())
+        join->need_tmp)
     {
-      /*
-        Force join->join_tmp creation, because this subquery will be replaced
-        by a simple select from the materialization temp table by optimize()
-        called by EXPLAIN and we need to preserve the initial query structure
-        so we can display it.
-       */
-      select_lex->uncacheable|= UNCACHEABLE_EXPLAIN;
-      select_lex->master_unit()->uncacheable|= UNCACHEABLE_EXPLAIN;
-      if (join->init_save_join_tab())
-        DBUG_RETURN(1);                        /* purecov: inspected */
+      item->update_used_tables();
+      if (item->const_item())
+      {
+        /*
+          Force join->join_tmp creation, because this subquery will be replaced
+          by a simple select from the materialization temp table by optimize()
+          called by EXPLAIN and we need to preserve the initial query structure
+          so we can display it.
+        */
+        select_lex->uncacheable|= UNCACHEABLE_EXPLAIN;
+        select_lex->master_unit()->uncacheable|= UNCACHEABLE_EXPLAIN;
+        if (join->init_save_join_tab())
+          DBUG_RETURN(1);                        /* purecov: inspected */
+      }
     }
     if (item->engine_changed)
     {