DBConnection.handle_fetch

You're seeing just the callback handle_fetch, go back to DBConnection module for more information.
Link to this callback

handle_fetch(query, cursor, opts, state)

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Specs

handle_fetch(query(), cursor(), opts :: Keyword.t(), state :: any()) ::
  {:cont | :halt, result(), new_state :: any()}
  | {:error | :disconnect, Exception.t(), new_state :: any()}

Fetch the next result from a cursor declared by handle_declare/4. Return {:cont, result, state} to return the result result and continue using cursor, {:halt, result, state} to return the result result and close the cursor, {:error, exception, state} to return an error and close the cursor, {:disconnect, exception, state} to return an error and disconnect.

This callback is called in the client process.