MDEV-11357 Split Item_cache::get_cache() into virtual methods in Type_handler

This patch:
- Adds a new virtual method Type_handler::Item_get_cache
- Splits moves Item_cache::get_cache() into the new method, every
  "case XXX_RESULT" to the corresponding Type_handler_xxx::Item_get_cache.
- Adds Item::get_cache as a convenience wrapper, to make the caller code
  shorter.
- Changes the last argument of Arg_comparator::cache_converted_constant()
  from Item_result to "const Type_handler *".
- Removes subselect_engine::cmp_type, subselect_engine::res_type,
  subselect_engine::res_field_type and derives subselect_engine
  from Type_handler_hybrid_field_type instead.
- Makes Type_handler_varchar public, as it's now needed as the
  default data type handler for subselect_engine.

1 files changed, 4 insertions, 9 deletions

diff --git a/sql/item_subselect.h b/sql/item_subselect.h
index 823dbc6c281..74c03575272 100644
--- a/sql/item_subselect.h
+++ b/sql/item_subselect.h
@@ -301,6 +301,7 @@ public:
   enum Item_result result_type() const;
   enum Item_result cmp_type() const;
   enum_field_types field_type() const;
+  const Type_handler *type_handler() const;
   void fix_length_and_dec();
 
   uint cols();
@@ -747,15 +748,13 @@ public:
 };
 
 
-class subselect_engine: public Sql_alloc
+class subselect_engine: public Sql_alloc,
+                        public Type_handler_hybrid_field_type
 {
 protected:
   select_result_interceptor *result; /* results storage class */
   THD *thd; /* pointer to current THD */
   Item_subselect *item; /* item, that use this engine */
-  enum Item_result res_type; /* type of results */
-  enum Item_result cmp_type; /* how to compare the results */
-  enum_field_types res_field_type; /* column type of the results */
   bool maybe_null; /* may be null (first item in select) */
 public:
 
@@ -766,12 +765,11 @@ public:
 
   subselect_engine(Item_subselect *si,
                    select_result_interceptor *res):
+    Type_handler_hybrid_field_type(&type_handler_varchar),
     thd(NULL)
   {
     result= res;
     item= si;
-    cmp_type= res_type= STRING_RESULT;
-    res_field_type= MYSQL_TYPE_VAR_STRING;
     maybe_null= 0;
   }
   virtual ~subselect_engine() {}; // to satisfy compiler
@@ -808,9 +806,6 @@ public:
   virtual int exec()= 0;
   virtual uint cols()= 0; /* return number of columns in select */
   virtual uint8 uncacheable()= 0; /* query is uncacheable */
-  enum Item_result type() { return res_type; }
-  enum Item_result cmptype() { return cmp_type; }
-  enum_field_types field_type() { return res_field_type; }
   virtual void exclude()= 0;
   virtual bool may_be_null() { return maybe_null; };
   virtual table_map upper_select_const_tables()= 0;