code: func PgXlogLocationDiff(db XODB, v0 pgtypes.PgLsn, v1 pgtypes.PgLsn) (float64, error) {
  	var err error
  
  	// sql query
  	const sqlstr = `SELECT pg_catalog.pg_xlog_location_diff($1, $2)`
  
  	// run query
  	var ret float64
  	XOLog(sqlstr, v0, v1)
  	err = db.QueryRow(sqlstr, v0, v1).Scan(&ret)
  	if err != nil {
  		return 0.0, err
  	}
  
  	return ret, nil
  }
docs: // PgXlogLocationDiff calls the stored procedure 'pg_catalog.pg_xlog_location_diff(pg_lsn, pg_lsn) numeric' on db.