Re: [boost] A question for folks here

On Tue, Jul 16, 2024 at 2:56 PM Mohammad Nejati [ashtum] via Boost wrote:

On Tue, Jul 16, 2024 at 3:47 PM Ruben Perez wrote:

...

Yes! That's it. Actually, the mapping is one of the points I don't see any clearly after reading the protocol spec (and it looks like libpq doesn't solve it either, leaving you just a bunch of bytes/text to interpret). Do you have any ideas or any resources on how to approach the issue?

I found what is done in https://github.com/yandex/ozo inspiring. It creates a compile-time map for standard types with fixed OIDs and a runtime map for user-defined types, retrieving OID numbers from PostgreSQL upon first use. Additionally, it exposes several type traits for specialization, enabling the use of containers and wrapper types for PostgreSQL types like

Interesting, thanks, I wasn't aware of it. I'll need to check it out. I think I have something similar for the fixed compile-time OIDs in my home-grown libpq wrapper. Thanks to those, I can manipulate basic types easily for binding and retrieval in a typesafe manner, with runtime checks to validate actual and expected OIDs match. In my use case, I do everything using the BINARY format for binding and result-sets. And I use the COPY protocol extensively for performance (also in BINARY format), in both directions. I'm no high-caliber C++ dev, but I'm sharing below an excerpt of what I do, in case that's useful. There's more, but this is not the time nor place. For another interesting C++ libpq wrapper, also check https://github.com/dmitigr/pgfe Thanks, --DD PS: I'd appreciate if you can advertise where you work I github, so I can monitor it, please namespace pq { ... // "Strong-typedefs", to prevent implicit conversions enum class Oid : uint32_t {}; enum class Row : uint32_t {}; enum class Col : uint32_t {}; inline constexpr Oid cInvalidOid{0}; ... static_assert(std::is_same_v); // Immutable "views" over "typed" (in the SQL sense) memory using Bytea = std::span<const std::byte>; using Name = std::span; using Text = std::string_view; using Uuid = std::span; ... struct Type { const Text name_; const int len_ = -1; const Oid oid_ = cInvalidOid; const Oid array_oid_ = cInvalidOid; }; ... template struct OidTraits { // fail at compile-time, not link-time. (see http://goo.gl/mPTYn) static_assert(sizeof(typename T::please_specialize_OidTraits) == 0); }; // For full catalog of available OIDs in PostgreSQL, see // https://github.com/postgres/postgres/blob/master/src/include/catalog/pg_type... template<> struct OidTraits<bool> { // boolean, true/false static constexpr Type type{ "bool", 1, Oid{ 16 }, Oid{ 1000 } }; }; template<> struct OidTraits<Bytea> { // variable-length string, binary values escaped static constexpr Type type{ "bytea", -1, Oid{ 17 }, Oid{ 1001 } }; }; template<> struct OidTraits<char> { // single character static constexpr Type type{ "char", 1, Oid{ 18 }, Oid{ 1002 } }; }; template<> struct OidTraits<Name> { // 63-byte type for storing system identifiers static constexpr Type type{ "name", int(Name::extent), Oid{ 19 }, Oid{ 1003 } }; }; template<> struct OidTraits { // ~18 digit integer, 8-byte storage static constexpr Type type{ "int8", 8, Oid{ 20 }, Oid{ 1016 } }; }; template<> struct OidTraits { // 32 thousand to 32 thousand, 2-byte storage static constexpr Type type{ "int2", 2, Oid{ 21 }, Oid{ 1005 } }; }; using Int2Vector = std::span<const int16_t>; template<> struct OidTraits<Int2Vector> { // array of int2, used in system tables static constexpr Type type{ "int2vector", -1, Oid{ 22 }, Oid{ 1006 } }; }; template<> struct OidTraits { // -2 billion to 2 billion integer, 4-byte storage static constexpr Type type{ "int4", 4, Oid{ 23 }, Oid{ 1007 } }; }; // Skipping regproc (24, 1008) registered procedure template<> struct OidTraits<Text> { // variable-length string, no limit specified static constexpr Type type{ "text", -1, Oid{ 25 }, Oid{ 1009 } }; }; template<> struct OidTraitsstd::string : OidTraits<Text> {}; template<> struct OidTraits<Oid> { // object identifier(oid), maximum 4 billion static constexpr Type type{ "oid", 4, Oid{ 26 }, Oid{ 1028 } }; }; template<> struct OidTraits<float> { // single-precision floating point number, 4-byte storage static constexpr Type type{ "float4", 4, Oid{ 700 }, Oid{ 1021 } }; }; template<> struct OidTraits<double> { // double-precision floating point number, 8-byte storage static constexpr Type type{ "float8", 8, Oid{ 701 }, Oid{ 1022 } }; }; // TODO: bpchar(1042, 1014): char(length), blank-padded string, fixed storage length // TODO: varchar(1043, 1015): varchar(length), non-blank-padded string, variable storage length // TODO: date(1082, 1182): date // TODO: time(1083, 1183): time of day /* ** TODO: How to map the same C++ type to a different SQL type? template<> struct OidTraits<Timestamp> { // date and time static constexpr Type type{ "timestamp", 8, Oid{ 1114 }, Oid{ 1115 } }; }; */ template<> struct OidTraits<Timestamp> { // date and time with time zone static constexpr Type type{ "timestamptz", 8, Oid{ 1184 }, Oid{ 1185 } }; }; // For wire-representations of those datatypes, see below (and co.) // https://github.com/postgres/postgres/blob/master/src/include/utils/date.h template<> struct OidTraits<Uuid> { // UUID datatype static constexpr Type type{ "uuid", 16, Oid{ 2950 }, Oid{ 2951 } }; }; template<> struct OidTraits< acme::core::Guid

: OidTraits< Uuid {};

// Partial specialization for all enums // Warning: Unsigned integer underlying types not supported by PostgreSQL template <typename E> struct OidTraits< E, std::enable_if_t

: OidTraits< std::underlying_type_t<E> {};

//============================================================================ template <typename T> constexpr Oid oid_of() { if constexpr (std::is_array_v<T>) { static_assert(std::rank<T>::value == 1); using elem_t = typename std::remove_all_extents<T>::type; return OidTraits::type.array_oid_; } else { return OidTraits<T>::type.oid_; } } template <typename T> constexpr const Type& type_of() { return OidTraits<T>::type; } inline void ensure_oid(Oid actual, Oid expected) { if (actual != expected) { throw std::runtime_error(fmt::format( "Unexpected Oid: Got {}; Want {}", ::Oid(actual), ::Oid(expected) )); } } template <typename T> void ensure_oid(Oid actual) { ensure_oid(actual, oid_of<T>()); } ... }