BUG#920132: Assert trx->n_active_thrs == 1 failed at que0que.c line 1050

- MySQL 5.5 introduced caching of constant items by means of 
  wrapping them in Item_cache_XXX objects. If a subquery was wrapped
  in this cache, it could end up being pushed down by ICP.
- The fix is to add Item_cache::walk() which lets ICP to see that 
  the cache item has a subquery inside it, and disable pushdown for this case.

1 files changed, 6 insertions, 0 deletions

diff --git a/sql/item.h b/sql/item.h
index 6c0d3138b62..4af2038aa55 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -3901,6 +3901,12 @@ public:
     return example->is_expensive_processor(arg);
   }
   virtual void set_null();
+  bool walk(Item_processor processor, bool walk_subquery, uchar *arg)
+  {
+    if (example && example->walk(processor, walk_subquery, arg))
+      return TRUE;
+    return (this->*processor)(arg);
+  }
 };