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This code in item.cc is not friendly to pluggable data types:
void Item_type_holder::get_full_info(Item *item)
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{
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if (Item_type_holder::real_type_handler() == &type_handler_enum ||
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Item_type_holder::real_type_handler() == &type_handler_set)
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{
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if (item->type() == Item::SUM_FUNC_ITEM &&
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(((Item_sum*)item)->sum_func() == Item_sum::MAX_FUNC ||
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((Item_sum*)item)->sum_func() == Item_sum::MIN_FUNC))
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item = ((Item_sum*)item)->get_arg(0);
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/*
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We can have enum/set type after merging only if we have one enum|set
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field (or MIN|MAX(enum|set field)) and number of NULL fields
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*/
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DBUG_ASSERT((enum_set_typelib &&
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item->real_type_handler() == &type_handler_null) ||
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(!enum_set_typelib &&
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item->real_item()->type() == Item::FIELD_ITEM &&
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(item->real_type_handler() == &type_handler_enum ||
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item->real_type_handler() == &type_handler_set) &&
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((Field_enum*)((Item_field *) item->real_item())->field)->typelib;
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if (!enum_set_typelib)
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{
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enum_set_typelib= ((Field_enum*)((Item_field *) item->real_item())->field)
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}
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}
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}
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Unlike ENUM and SET, new TYPELIB-based data types may want to preserve the exact data type hybrid functions like CASE or LEAST.
Instead of having this hard-coded logic that preserves TYPELIB only in UNION and only for fields and aggregate functions MIN(field) and MAX(field), will introduce new methods:
virtual TYPELIB *Field::get_typelib() const;
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virtual TYPELIB *Item::get_typelib() const;
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and decompose this code into virtual implementations in Field, Field_enum, Item_field, Item_ref, Item_sum_hybrid.
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