hibernate 对 sql server 2005 分页改进

Hibernate 可以实现分页查询 如下

Query q = session.createQuery("from Cat as c");

q.setFirstResult(10000);

q.setMaxResults(20);

List l = q.list();

生成的sql 语句为

select top 10000 .... from Cat as c

分析：Hibernate 获取到 10000条记录过后再筛选出符合条件的20条记录。

再来看看Hibernate 的 dialect 对 sql server 怎样分页的

public String getLimitString(String querySelect, int offset, int limit) {

if ( offset > 0 ) {

throw new UnsupportedOperationException( "sql server has no offset" );

}

return new StringBuffer( querySelect.length() + 8 )

.append( querySelect )

.insert( getAfterSelectInsertPoint( querySelect ), " top " + limit )

.toString();

}

由此可以看出Hibernate对 sql server 分页全是采取 top 方式来处理。如记录有10W条以上的话到后面几页效率会非常低。从getLimitString(String querySelect, int offset, int limit)方法可以看出 sql server 不支持 offset 参数(均为0)！！

然而在sql server2005过后支持ROW_NUMBER() 函数,可用该函数大大提高分页效率。但现有的Hibernate api不支持该函数。我们可以重写public String getLimitString(String querySelect, int offset, int limit)方法来实现 ROW_NUMBER　分页,重写的方法如下：

public String getLimitString(String querySelect, int offset, int limit ){

int lastIndexOfOrderBy = getLastIndexOfOrderBy(querySelect);

//　没有 order by 或第一页的情况下

if(lastIndexOfOrderBy<0 || querySelect.endsWith(")") || offset==0)

return super.getLimitString(querySelect, 0, limit);

else {

//取出 order by 语句

String orderby = querySelect.substring(lastIndexOfOrderBy, querySelect.length());

//取出 from 前的内容

int indexOfFrom = querySelect.toLowerCase().indexOf("from");

String selectFld = querySelect.substring(0,indexOfFrom);

//取出 from 语句后的内容

String selectFromTableAndWhere = querySelect.substring(indexOfFrom, lastIndexOfOrderBy);

StringBuffer sql = new StringBuffer(querySelect.length()+100);

sql.append("select * from (")

.append(selectFld)

.append(",ROW_NUMBER() OVER(").append(orderby).append(") as _page_row_num_hb ")

.append(selectFromTableAndWhere).append(" ) temp ")

.append(" where _page_row_num_hb BETWEEN ")

.append(offset+1).append(" and ").append(limit);

return sql.toString();

}

}

让offset参数有效还应重写 public boolean supportsLimitOffset() 方法

public boolean supportsLimitOffset(){

return true;

}

完整的代码如下：

package org.jac.common;

import org.hibernate.dialect.SQLServerDialect;

public class JacSQLServerDialect extends SQLServerDialect {

static int getLastIndexOfOrderBy(String sql){

return sql.toLowerCase().lastIndexOf("order by ");

}

public String getLimitString(String querySelect, int offset, int limit ){

int lastIndexOfOrderBy = getLastIndexOfOrderBy(querySelect);

//　没有 order by 或第一页的情况下

if(lastIndexOfOrderBy<0 || querySelect.endsWith(")") || offset==0)

return super.getLimitString(querySelect, 0, limit);

else {

//取出 order by 语句

String orderby = querySelect.substring(lastIndexOfOrderBy, querySelect.length());

//取出 from 前的内容

int indexOfFrom = querySelect.toLowerCase().indexOf("from");

String selectFld = querySelect.substring(0,indexOfFrom);

//取出 from 语句后的内容

String selectFromTableAndWhere = querySelect.substring(indexOfFrom, lastIndexOfOrderBy);

StringBuffer sql = new StringBuffer(querySelect.length()+100);

sql.append("select * from (")

.append(selectFld)

.append(",ROW_NUMBER() OVER(").append(orderby).append(") as _page_row_num_hb ")

.append(selectFromTableAndWhere).append(" ) temp ")

.append(" where _page_row_num_hb BETWEEN ")

.append(offset+1).append(" and ").append(limit);

return sql.toString();

}

}

//使offset 参数生效

public boolean supportsLimitOffset(){

return true;

}

最后再 hibernate.cfg.xml 配置 dialect

<property name="dialect">

org.jac.common.JacSQLServerDialect

</property>

测试结果如下：

Query q = session.createQuery("from Cat as c order by c.id asc"); //注意要加 order by　才能用到 ROW_NUMBER分页

q.setFirstResult(10000);

q.setMaxResults(20);

List l = q.list();

生成的sql 语句为

select * from (select ....,ROW_NUMBER() OVER(order by cat0_.id asc) as _page_row_num_hb from

cat as cat0_) temp where _page_row_num_hb BETWEEN 10001 and 10020