MDEV-3798: EXPLAIN UPDATE/DELETE

Update the SHOW EXPLAIN code to work with the 
new architecture (part#1):
Before, SHOW EXPLAIN operated on real query plan structures, 
which meant it had to check when SELECTs are created/deleted.
SELECTs would call apc_target->enable() when they got a query 
plan and disable() when their query plan was deleted.

Now, Explain data structure becomes available at once (and we
call apc_target->enable()) and then it stays until it is deleted
(when that happens, we call apc_target->disable()).

1 files changed, 4 insertions, 0 deletions

diff --git a/sql/sql_lex.h b/sql/sql_lex.h
index c5b9c1eada2..0d22e815f5f 100644
--- a/sql/sql_lex.h
+++ b/sql/sql_lex.h
@@ -1031,6 +1031,10 @@ public:
 
   void clear_index_hints(void) { index_hints= NULL; }
   bool is_part_of_union() { return master_unit()->is_union(); }
+  bool is_top_level_node() 
+  { 
+    return (select_number == 1) && !is_part_of_union();
+  }
   bool optimize_unflattened_subqueries(bool const_only);
   /* Set the EXPLAIN type for this subquery. */
   void set_explain_type(bool on_the_fly);