`telemetry_event` Overrides Repo Query Event

Ecto gives you a single telemetry event out of the box, [:my_app, :repo, :query], where the [:my_app, :repo] is the telemetry prefix option for ecto.

This event is called whenever any request to the database is made:

  handler = fn _, measurements, _, _ ->
    send(self(), :test_message)
  end

  :telemetry.attach(
    "query",
    [:test_telemetry, :repo, :query],
    handler,
    %{}
  )

  Repo.all(TestTelemetry.Colour)

  assert_receive(:test_message)

This event is overriden when using the the :telemetry_event option, a shared option for all Repo query functions.

  handler = fn _, measurements, _, _ ->
    send(self(), :test_message)
  end

  custom_handler = fn _, measurements, _, _ ->
    send(self(), :custom_message)
  end

  :telemetry.attach(
    "query",
    [:test_telemetry, :repo, :query],
    handler,
    %{}
  )

  :telemetry.attach(
    "custom",
    [:custom],
    custom_handler,
    %{}
  )

  Repo.all(TestTelemetry.Colour, telemetry_event: [:custom])

  assert_receive(:custom_message)
  refute_receive(:test_message)

Which means for any given query you can only broadcast one event. If you have a system that keeps track of expensive queries but you also need to debug a particular query in production, you will take that query out of the system to track expensive queries.