Using Postgres COPY with Elixir/Ecto

This post is about using Postgres COPY to retrieve data from the database into memory when using Elixir and Ecto. Let’s assume you want to read all the data from a table ‘accounts’ using Postgres COPY. Lets assume the table definition in Postgres is:

Table "public.accounts"
Column    |            Type             |                       Modifiers
-------------+-----------------------------+-------------------------------------------------------
id          | integer                     | not null default nextval('accounts_id_seq'::regclass)
name        | character varying(255)      |
address     | character varying(255)      |
city        | character varying(255)      |
state       | character varying(255)      |
zipcode     | character varying(255)      |
inserted_at | timestamp without time zone | not null
updated_at  | timestamp without time zone | not null

You can define an Ecto schema for that:

defmodule Account do
  use Ecto.Schema

  schema "accounts" do
    field :name, :string
    field :address, :string
    field :city, :string
    field :state, :string
    field :zipcode, :string
    field :inserted_at, :string
    field :updated_at, :string
  end
end

Since the data returned by COPY is String data the timestamps are defined in the schema as :string.

Reading the data is then:

columns = ["id", "name", "address", "city", "state", "zipcode", "inserted_at", "updated_at"]
stream = Ecto.Adapters.SQL.stream(Repo, "COPY (SELECT #{Enum.join(columns, ",")} FROM accounts) to STDOUT WITH CSV DELIMITER ',' ESCAPE '\"'")
{:ok, [result|_t]} = Repo.transaction(fn -> Enum.to_list(stream) end)

The data will be in result.rows. When we issued the COPY we explictly selected the order of the columns in a SELECT statement supplied to COPY. The result.columns will be empty so we can’t use that with COPY. We also asked that the data be CSV with a escape of double-quote. This is necessary if we have newlines within our data.

If we wanted to get into the Account struct to make it easier we’ll want to use a CSV parsing library. The CSV Library available from (hex.pm)[https://hex.pm/packages/csv] will work fine for this. To convert the data into Account struct we could do the following:

cols = Enum.map columns, &(String.to_atom(&1))
a_result = result.rows
|> CSV.decode()
|> Enum.to_list()
|> Enum.filter(fn({result, _val}) -> result == :ok end)
|> Enum.reduce([], fn({:ok, a_list}, acc) -> [a_list | acc] end)
|> Enum.map(fn(row) -> struct(Account, Enum.zip(cols, row)) end)

This isn’t something that you’d ordinarily choose to do. The reverse where you are using COPY to get data into the database as quickly as possible is much more common. I just decided to write something up on this because I hadn’t seen anything that explained how to do this in Elixir. This was what I came up with but I make no guarantees that there isn’t a much easier way of doing this that I’m missing.

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