Split Item_type_holder::get_real_type() into virtual Item::real_type_handler()

Item_type_holder::get_real_type() has a switch(item->type()).

This coding style makes it impossible for user defined data type plugins to create their own Item types (e.g. descendants of Item_basic_constant, Item_func, Item_sum for data type specific literals and functions, etc) with a desired UNION aggregation behavior.

Also, this coding style is error prone, because it's very easy to forget to modify Item_type_holder::get_real_type properly when adding new built-in Items.

We'll do the following:

1. Introduce a new virtual method in Item:

virtual const Type_handler *Item::real_type_handler() const;

2. Move pieces from Item_type_holder::get_real_type() "case"s into virtual implementations of the corresponding Items:

Item::real_type_handler();

Item_field::real_type_handler();

Item_blob::real_type_handler();

Item_ref::real_type_handler();

Item_type_holder::real_type_handler();

Item_sum_hybrid::real_type_handler();

Note, there is no a need for Item_func_get_user_var::real_type_handler(). The "case" corresponding to Item_func_get_user_var is not really needed and is a useless code, because starting from 10.1 Item_func_get_user_var is a descendant of Type_handler_hybrid_field_type, which makes sure that:

Item_func_get_user_var::result_type()

Item_func_get_user_var::field_type()

Item_func_get_user_var::type_handler()

are always in sync to each other. All these methods use Type_handler_hybrid_field_type::m_type_handler to return the result.

In 10.0 and earlier, cmp_type and field_type were stored in separate members:

Item_result cached_result_type;

enum_field_types cached_field_type;

which could get out of sync in some cases.

3. Remove Item_type_holder::get_real_type()

4. Remove the convenience class Type_handler_hybrid_real_field_type.

We'll derive Item_type_holder from Type_handler_hybrid_field_type, because the new method Item::real_type_handler() makes it easy to use Type_handler_hybrid_field_type in Item_type_holder directly.

5. Remove Item_sum::keep_field_type()

The idea of keep_field_type() is that the aggregate functions MAX() and MIN() try to preserve the exact data type (e.g. ENUM or SET) from the argument when they appear as a part of a UNION, e.g in combinations with NULL:

Notice, this script creates a column of the ENUM data type in the table t2:

CREATE OR REPLACE TABLE t1 (a ENUM('a','b'));

CREATE OR REPLACE TABLE t2 AS SELECT MAX(a) FROM t1 UNION SELECT NULL;

SHOW CREATE TABLE t2;

+-------+--------------------------------------------------------------------------------------------------+

| Table | Create Table                                                                                     |

+-------+--------------------------------------------------------------------------------------------------+

| t2    | CREATE TABLE `t2` (

  `MAX(a)` enum('a','b') DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

+-------+--------------------------------------------------------------------------------------------------+

Unlike MIN and MAX, most functions convert ENUM/SET to VARCHAR.

keep_field_type() is not needed after adding of real_type_handler(), which replaces this functionality in a more universal way.