Thoughts on getting Postgresql Multiranges added to the postgrex library?

Questions / Help
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Postgresql added support for multiranges in version 14

I’m looking to add a multirange field to my project but I’m not 100% sure what to do, I think this could be a decent library or an addition to the postgrex library but I’m not certain which is more appropriate. Based off of my reading it looks like I will have to create a new Extension like the library is doing internally.

However, I’m not certain how to determine how these types are encoded and decoded from Postgresql itself. (Can’t seem to find those docs). If anyone has any experience with multiranges in their application, or if there are any of the Postgrex library authors around I’d be happy to collaborate.

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The wire format of postgres types are unfortunately not documented, instead you have to read the postgres C code. Look for functions named TYPE_send and TYPE_recv for encode and decode respectively.

Here is link to the multirange functions: postgres/multirangetypes.c at master · postgres/postgres · GitHub.

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So I have the following extension written, but I can’t seem to figure out why I’m getting a type error.

defmodule Postgrex.Extensions.Multirange do 
  import Postgrex.BinaryUtils, warn: false

  @behaviour Postgrex.SuperExtension

  def init(_), do: nil

  def matching(_state), do: [send: "multirange_send", receive: "multirange_recv"]

  def format(_), do: :super_binary

  def oids(%Postgrex.TypeInfo{array_elem: elem_oid}, _), do: [elem_oid]

  def encode(_) do
    quote location: :keep do
      list, [oid], [type] when is_list(list) -> 
        # encode_list/2 defined by TypeModule
        encoder = &encode_list(&1, type)
        unquote(__MODULE__).encode(list, oid, encoder)

      other, _, _ ->
        raise DBConnection.EncodeError, Postgrex.Utils.encode_msg(other, "a list")
    end
  end

  def decode(_) do 
    quote location: :keep do
      <<len::int32(), binary::binary-size(len)>>, [oid], [type] ->
        <<ndims::int32(), _has_null::int32(), ^oid::uint32(), dims::size(ndims)-binary-unit(64),
          data::binary>> = binary

        # decode_list/2 defined by TypeModule
        flat = decode_list(data, type)

        unquote(__MODULE__).decode(dims, flat)
    end
  end

  ## Helpers

  # Delegate to the Array extension
  def encode(list, elem_oid, encoder) do 
    Postgrex.Extensions.Array.encode(list, elem_oid, encoder)
  end

  # Delegate to the Array extension
  def decode(dims, elems) do 
    Postgrex.Extensions.Array.decode(dims, elems)
  end
end

There’s a failure in Postgrex.Protocol.fetch_type_info/4. type timemultirange can not be handled by the types module MyApp.PostgresTypes. Here is the TypeInfo struct.

%Postgrex.TypeInfo{
   oid: 1431039,
   type: "timemultirange",
   send: "multirange_send",
   receive: "multirange_recv",
   output: "multirange_out",
   input: "multirange_in",
   array_elem: 0,
   base_type: 0,
   comp_elems: []
 }
5

I’m not quite certain why Postgrex fails when it is given this type, it was my understanding that the matching function should work as the type has “multirange_send” and “multirange_recv”. However, it looks like the library cannot find the corresponding extension for some reason. I have defined the extension as follows in a separate file and it does look like it is getting picked up.

Postgrex.Types.define(MyApp.PostgresTypes, [Postgrex.Extensions.Multirange])