DBConnection.handle_close

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

handle_close(query, opts, state)

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Specs

handle_close(query(), opts :: Keyword.t(), state :: any()) ::
  {:ok, result(), new_state :: any()}
  | {:error | :disconnect, Exception.t(), new_state :: any()}

Close a query prepared by handle_prepare/3 with the database. Return {:ok, result, state} on success and to 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.