MDEV-12426 Add Field::type_handler() + MDEV-12432

This is a joint patch for:

- MDEV-12426 Add Field::type_handler()
- MDEV-12432 Range optimizer for ENUM and SET does not return "Impossible WHERE" in some case

With the new type handler approach being added to Field, it was easier to fix
MDEV-12432 rather than to reproduce the old ENUM/SET behavior.

The patch does the following:

1. Adds Field::type_handler(), according to the task description.

2. Fixes the asymmetry between Fields and Items of ENUM and SET field types.
   Field_enum::cmp_type() returned INT_RESULT
   Item*::cmp_type() returned STRING_RESULT for ENUM and SET expressions

This asymmetry was originally done for easier coding in the optimizer sources.
However, in 10.1 we moved a lot of code to methods of the class Field:
- test_if_equality_guarantees_uniqueness()
- can_be_substituted_to_equal_item()
- get_equal_const_item()
- can_optimize_keypart_ref()
- can_optimize_hash_join()
- can_optimize_group_min_max()
- can_optimize_range()
- can_optimize_outer_join_table_elimination()

As of 10.2 only a few lines of the code in opt_range.cc, field.cc and field.h
still relayed on the fact that Field_enum::cmp_type() returns INT_RESULT:
- Some asserts in field.cc
- Field_year::get_copy_func()
- Item_func_like::get_mm_leaf()
- Item_bool_func::get_mm_leaf()
These lines have been fixed.

3. Item_bool_func::get_mm_leaf() did not work well for ENUM/SET,
see MDEV-12432. So the ENUM/SET code was rewritten, and the relevant
code in Field_enum::store() and Field_set::store() was fixed to
properly return errors to the caller.

4. The result of Field_decimal::result_type() was changed from REAL_RESULT
to DECIMAL_RESULT. Data type aggregation (e.g. in COALESCE()) is now more
precise for old DECIMAL, because Item::decimal_precision() now goes through
the DECIMAL_RESULT branch. Earlier it went through the REAL_RESULT branch.

sql/sql_type.cc

@@ -21,35 +21,39 @@
 #include "item.h"
 #include "log.h"
 
-static Type_handler_tiny        type_handler_tiny;
-static Type_handler_short       type_handler_short;
-static Type_handler_long        type_handler_long;
-static Type_handler_int24       type_handler_int24;
-static Type_handler_year        type_handler_year;
-static Type_handler_time        type_handler_time;
-static Type_handler_date        type_handler_date;
-static Type_handler_timestamp   type_handler_timestamp;
-static Type_handler_timestamp2  type_handler_timestamp2;
-static Type_handler_olddecimal  type_handler_olddecimal;
-
+Type_handler_row         type_handler_row;
 
 Type_handler_null        type_handler_null;
-Type_handler_row         type_handler_row;
-Type_handler_string      type_handler_string;
-Type_handler_varchar     type_handler_varchar;
+
+Type_handler_tiny        type_handler_tiny;
+Type_handler_short       type_handler_short;
+Type_handler_long        type_handler_long;
+Type_handler_int24       type_handler_int24;
 Type_handler_longlong    type_handler_longlong;
 Type_handler_float       type_handler_float;
 Type_handler_double      type_handler_double;
-Type_handler_newdecimal  type_handler_newdecimal;
-Type_handler_datetime    type_handler_datetime;
 Type_handler_bit         type_handler_bit;
-Type_handler_enum        type_handler_enum;
-Type_handler_set         type_handler_set;
 
+Type_handler_olddecimal  type_handler_olddecimal;
+Type_handler_newdecimal  type_handler_newdecimal;
+
+Type_handler_year        type_handler_year;
+Type_handler_time        type_handler_time;
+Type_handler_date        type_handler_date;
+Type_handler_timestamp   type_handler_timestamp;
+Type_handler_timestamp2  type_handler_timestamp2;
+Type_handler_datetime    type_handler_datetime;
 Type_handler_time2       type_handler_time2;
 Type_handler_newdate     type_handler_newdate;
 Type_handler_datetime2   type_handler_datetime2;
 
+Type_handler_enum        type_handler_enum;
+Type_handler_set         type_handler_set;
+
+Type_handler_string      type_handler_string;
+Type_handler_var_string  type_handler_var_string;
+Type_handler_varchar     type_handler_varchar;
+
 Type_handler_tiny_blob   type_handler_tiny_blob;
 Type_handler_medium_blob type_handler_medium_blob;
 Type_handler_long_blob   type_handler_long_blob;
@@ -245,6 +249,7 @@ const Name Type_handler_null::m_name_null(C_STRING_WITH_LEN("null"));
 
 const Name
   Type_handler_string::m_name_char(C_STRING_WITH_LEN("char")),
+  Type_handler_var_string::m_name_var_string(C_STRING_WITH_LEN("varchar")),
   Type_handler_varchar::m_name_varchar(C_STRING_WITH_LEN("varchar")),
   Type_handler_tiny_blob::m_name_tinyblob(C_STRING_WITH_LEN("tinyblob")),
   Type_handler_medium_blob::m_name_mediumblob(C_STRING_WITH_LEN("mediumblob")),
@@ -326,6 +331,18 @@ const Type_handler *Type_handler_row::type_handler_for_comparison() const
   return &type_handler_row;
 }
 
+/***************************************************************************/
+
+const Type_handler *Type_handler_enum::cast_to_int_type_handler() const
+{
+  return &type_handler_longlong;
+}
+
+
+const Type_handler *Type_handler_set::cast_to_int_type_handler() const
+{
+  return &type_handler_longlong;
+}
 
 /***************************************************************************/