$to_insert_stmt = q{
SELECT * FROM scratch S
WHERE session_id = ? AND NOT EXISTS (
  SELECT 1 FROM real_data
  WHERE key_val1 = S.key_val1 AND ...
)
};
$to_update_stmt = q{
SELECT * FROM scratch S
WHERE session_id = ? AND EXISTS (
  SELECT 1 FROM real_data
  WHERE key_val1 = S.key_val1 AND ...
)
};