content: /*
   * This library is part of OpenCms -
   * the Open Source Content Management System
   *
   * Copyright (c) Alkacon Software GmbH & Co. KG (https://www.alkacon.com)
   *
   * This library is free software; you can redistribute it and/or
   * modify it under the terms of the GNU Lesser General Public
   * License as published by the Free Software Foundation; either
   * version 2.1 of the License, or (at your option) any later version.
   *
   * This library is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
   * Lesser General Public License for more details.
   *
   * For further information about Alkacon Software, please see the
   * company website: https://www.alkacon.com
   *
   * For further information about OpenCms, please see the
   * project website: https://www.opencms.org
   *
   * You should have received a copy of the GNU Lesser General Public
   * License along with this library; if not, write to the Free Software
   * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   */
  
  package org.opencms.db;
  
  /**
   * Wrapper for {@link CmsSelectQuery} objects which adds SQL code for results paging.<p>
   *
   * The wrapper can either use the window function approach to paging or append a LIMIT/OFFSET clause.
   *
   * @see "http://troels.arvin.dk/db/rdbms/#select-limit-offset"
   *
   * @since 8.0.0
   */
  public class CmsPagingQuery implements I_CmsQueryFragment {
  
      /** Flag which indicates whether subqueries should be named. */
      private boolean m_nameSubquery;
  
      /** The current page. */
      private int m_page;
  
      /** The page size. */
      private int m_pageSize;
  
      /** The wrapped query. */
      private CmsSelectQuery m_select;
  
      /** If true, use window functions, else use a LIMIT/OFFSET clause. */
      private boolean m_useWindowFunctions;
  
      /**
       * Creates a new instance.<p>
       *
       * @param select the wrapped query
       */
      public CmsPagingQuery(CmsSelectQuery select) {
  
          m_select = select;
      }
  
      /**
       * Enables or disables the naming of subqueries.<p>
       *
       * @param nameSubquery if true, enables naming of subqueries
       */
      public void setNameSubquery(boolean nameSubquery) {
  
          m_nameSubquery = nameSubquery;
      }
  
      /**
       * Sets both the page size and current page to use for the query.<p>
       *
       * @param pageSize the page size
       * @param page the current page (counting starts at 1)
       */
      public void setPaging(int pageSize, int page) {
  
          m_pageSize = pageSize;
          m_page = page;
      }
  
      /**
       * Enables the use of window functions.<p>
       *
       * @param useWindowFunctions if true, enables window functions
       */
      public void setUseWindowFunctions(boolean useWindowFunctions) {
  
          m_useWindowFunctions = useWindowFunctions;
      }
  
      /**
       * @see org.opencms.db.I_CmsQueryFragment#visit(org.opencms.db.CmsStatementBuilder)
       */
      public void visit(CmsStatementBuilder builder) {
  
          if (m_useWindowFunctions) {
              I_CmsQueryFragment order = m_select.getOrdering();
              assert order != null;
              CmsCompositeQueryFragment rownumFragment = new CmsCompositeQueryFragment();
              rownumFragment.add(new CmsSimpleQueryFragment("ROW_NUMBER() OVER (ORDER BY "));
              rownumFragment.add(order);
              rownumFragment.add(new CmsSimpleQueryFragment(") AS rownumber"));
              m_select.addColumn(rownumFragment);
              builder.add("SELECT * FROM ( ");
              m_select.visit(builder);
              int start = 1 + (m_pageSize * (m_page - 1));
              int end = (start + m_pageSize) - 1;
              builder.add(")");
              if (m_nameSubquery) {
                  builder.add(" AS rnsq ");
              }
              builder.add(" WHERE rownumber BETWEEN " + start + " AND " + end);
          } else {
              m_select.visit(builder);
              int offset = (m_page - 1) * m_pageSize;
              builder.add("\nLIMIT " + m_pageSize + " OFFSET " + offset);
          }
      }
  }
  