MDEV-16309 Split ::create_tmp_field() into virtual methods in Item

Detailed: changes:
1. Moving Field specific code into new methods on Field:
   - Field *Field::create_tmp_field(...)
   - virtual void init_for_tmp_table(...)

2. Removing virtual Item::create_tmp_field().
   Adding instead a new virtual method Item::create_tmp_field_ex().

   Note, a virtual create_tmp_field() still exists, but only for Item_sum.
   This resembles 10.0 code structure. Perhaps create_tmp_field() should
   be removed from Item_sum and Item_sum descendants should override
   create_tmp_field_ex() directly. This can be done in a separate commit.

3. Adding helper classes Tmp_field_src and Tmp_field_param,
   to make the API for Item::create_tmp_field_ex() smaller
   and easier to extend in the future.

4. Decomposing the public function create_tmp_field() into
   virtual implementations for Item and a number of its descendants:
   - Item_basic_value
   - Item_sp_variable
   - Item_name_const
   - Item_result_field
   - Item_field
   - Item_ref
   - Item_type_holder
   - Item_row
   - Item_func_sp
   - Item_func_user_var
   - Item_sum
   - Item_sum_field
   - Item_proc

5. Adding DBUG_ASSERT-only virtual implementations for
   Item types that should not appear in create_tmp_table_ex(),
   for easier debugging:
   - Item_nodeset_func
   - Item_nodeset_to_const_comparator
   - Item_null_result
   - Item_copy
   - Item_ident_for_show
   - Item_user_var_as_out_param

6. Moving public function create_tmp_field_from_field()
   as a method to Item_field.

7. Removing Item::set_result_field(). It's not needed any more.

8. Cleanup: Removing the enum value "EXPR_CACHE_ITEM",
   as it's not used for a very long time.

1 files changed, 12 insertions, 1 deletions

diff --git a/sql/item_xmlfunc.cc b/sql/item_xmlfunc.cc
index 0b9e515a61f..ab41b496cde 100644
--- a/sql/item_xmlfunc.cc
+++ b/sql/item_xmlfunc.cc
@@ -187,6 +187,12 @@ public:
     nodeset->length(0);
   }
   enum Type type() const { return XPATH_NODESET; }
+  Field *create_tmp_field_ex(TABLE *table, Tmp_field_src *src,
+                             const Tmp_field_param *param)
+  {
+    DBUG_ASSERT(0);
+    return NULL;
+  }
   String *val_str(String *str)
   {
     prepare_nodes();
@@ -592,7 +598,12 @@ public:
   {
     return mark_unsupported_function(func_name(), arg, VCOL_IMPOSSIBLE);
   }
-
+  Field *create_tmp_field_ex(TABLE *table, Tmp_field_src *src,
+                             const Tmp_field_param *param)
+  {
+    DBUG_ASSERT(0);
+    return NULL;
+  }
   longlong val_int()
   {
     Item_func *comp= (Item_func*)args[1];