You're seeing just the callback handle_declare, go back to DBConnection module for more information.
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handle_declare(query, params, opts, state)

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handle_declare(query(), params(), opts :: Keyword.t(), state :: any()) ::
  {:ok, query(), cursor(), new_state :: any()}
  | {:error | :disconnect, Exception.t(), new_state :: any()}

Declare a cursor using a query prepared by handle_prepare/3. Return {:ok, query, cursor, state} to return altered query query and cursor cursor for a stream and continue, {:error, exception, state} to return an error and continue or {:disconnect, exception, state} to return an error and disconnect.

This callback is called in the client process.