AddAsColumn inefficiency

[yb303]

Hi,

This check, on every addition of an array to a column, is very expensive for enums:

//---- columns/array.cpp:
void ColumnArray::AppendAsColumn(ColumnRef array) {
    if (!data_->Type()->IsEqual(array->Type())) {
        throw std::runtime_error(...

//---- types/types.h:
bool IsEqual(const TypeRef& other) const { return this->GetName() == other->GetName(); }
                                                   -----^

EnumType::GetName() is walking a std::map, on every call, to generate the full enum description string: "A"=1, "B"=2, ...
I believe a 64 bit hash of whatever type info would make this comparison properly fast.

Thanks,
Yakov

[yb303]

The same technique would also be useful for sending on the wire if using the same hash function :)