code: private String buildFixedQuery(GregorianCalendar expireDate) {
          // Query: '{aging field} <= "{expire date}"', fetching the _ID and
          // aging field, up to a batch full at a time.
          StringBuilder fixedParams = new StringBuilder();
          fixedParams.append("q=");
          fixedParams.append(m_agingFieldDef.getName());
          fixedParams.append(" <= \"");
          fixedParams.append(Utils.formatDate(expireDate));
          fixedParams.append("\"");
          
          // Fields: _ID.
          fixedParams.append("&f=_ID");
          
          // Size: QUERY_PAGE_SIZE
          fixedParams.append("&s=");
          fixedParams.append(QUERY_PAGE_SIZE);
          return fixedParams.toString();
      }
docs: Build the fixed part of the query that fetches a batch of object IDs.