Item_param has two parallel systems to maintain the data type:

• Type_handler_hybrid_field_type, which is used externally, like for a regular Item.
• enum_item_param_state constants INT_VALUE, DECIMAL_VALUE, REAL_VALUE, STRING_VALUE, TIME_VALUE, which are used internally, to maintain the data stored inside Item_param::value.

We'll do the following:

• Remove the data type specific constants from enum_item_param_state
• Add a new value SHORT_DATA_VALUE instead, for all data types.
• Start using Type_handler_hybrid_field_type to maintain Item_param::value internally.

After this change, enum_item_param_state will look like this:

enum enum_item_param_state

{

  NO_VALUE, NULL_VALUE, SHORT_DATA_VALUE, LONG_DATA_VALUE,

  DEFAULT_VALUE, IGNORE_VALUE

} state;

Note, SHORT_DATA_VALUE is needed to distinguish the state from LONG_DATA_VALUE.

These changes are needed to simplify further work for:

• MDEV-14270 Dynamic CREATE TABLE does not preserve the data type of SP variables with NULL value
• MDEV-4912 Add a plugin to field types (column types)