Bug#49746: Const expression caching led to NDB not using engine condition

pushdown.
      
NDB supports only a limited set of item nodes for use in engine condition
pushdown. Because of this adding cache for const expression effectively
disabled this optimization.
      
The ndb_serialize_cond function is extended to support Item_cache and treat
it as a constant values.
A helper function called ndb_serialize_const is added. It is used to create
Ndb_cond value node from given const item.

1 files changed, 9 insertions, 1 deletions

diff --git a/sql/item.h b/sql/item.h
index c7a97ca716a..d1957c46d25 100644
--- a/sql/item.h
+++ b/sql/item.h
@@ -3266,6 +3266,12 @@ public:
   bool basic_const_item() const
   { return test(example && example->basic_const_item());}
   virtual void clear() { null_value= TRUE; value_cached= FALSE; }
+  Item_result result_type() const
+  {
+    if (!example)
+      return INT_RESULT;
+    return Field::result_merge_type(example->field_type());
+  }
 };
 
 
@@ -3335,7 +3341,9 @@ public:
     is_varbinary(item->type() == FIELD_ITEM &&
                  cached_field_type == MYSQL_TYPE_VARCHAR &&
                  !((const Item_field *) item)->field->has_charset())
-  {}
+  {
+    collation.set(const_cast<DTCollation&>(item->collation));
+  }
   double val_real();
   longlong val_int();
   String* val_str(String *);