Support for pragma tables

[fmattern]

Using sqlite a statement like
select name from pragma_table_info('mytable')
is supported to get the column names of a database table.

With sqlpp11 I like to use it as a normal table, generating the struct for the columns and for the table like this:

struct TableInfo
	: sqlpp::table_t<TableInfo, TableInfo_::Cid, TableInfo_::Name, TableInfo_::Type, TableInfo_::Notnull, TableInfo_::DfltValue, TableInfo_::Pk> {
	struct _alias_t {
		static constexpr const char _literal[] = "pragma_table_info('mytable')";
		using _name_t = sqlpp::make_char_sequence<sizeof(_literal), _literal>;
		template <typename T>
		struct _member_t {
			T tableinfo;
			T& operator()() { return tableinfo; }
			const T& operator()() const { return tableinfo; }
		};
	};
};

and getting this information as a normal table:

for (const auto& col: db(select(tableInfo.name).from(tableInfo).unconditionally())) {
	std::cout << col.name << std::endl;
}

This almost works, the problem is that in column.h the serialization always generate a "table.column" which does not work in this case.
A quick fix to improve it would be the following:

 template <typename Context, typename Table, typename ColumnSpec>
 Context& serialize(const column_t<Table, ColumnSpec>&, Context& context)
 {
   using T = column_t<Table, ColumnSpec>;
   std::string table = name_of<typename T::_table>::template char_ptr<Context>();
   if (!table.starts_with("pragma_")) {
       context << table << '.';
   }
   context << name_of<T>::template char_ptr<Context>();
   return context;
 }

Do you have a better idea to use the pragma_table_info in sqlpp11 or is it possible to exclude the table prefix on pragma tables like this fix?

[rbock]

Thanks for reaching out and the initial idea.

Given that the table name is known, we should be able to do something like this:

constexpr bool is_pragma_table = ....
if (!is_pragma_table) ...

That would avoid creating a temporary string and a runtime branch.

WDYT?