关于mysql中的Query语句

Question

I use this code for paging in mysql (working with Struts 2 + Hibernate):

Query query=getSession().createQuery("from GovOffice");
query.setFirstResult(0);
query.setMaxResult(100);
List<GovOffice> list=query.list();

This will return list of GovOffice which is started from the first record, display 100 records per page.

Suppose i have 100,000 records, is this query "from GovOffice" get all 100,000 records first? By setFirstResult and setMaxReSult it will limit from 100,000 to 100 records.If this is true that means paging must be useless.

If not is there any way to prove that query will not get data from DB until we call query.list() .

Answer 1

Query 's setFirstResult and setMaxResult use the underlying possibilities of the databases.

In the case of MySQL, it's limit .

So, no, it doesn't fetch all records and yes it's efficient.

The proof in the source code ( org.hibernate.dialect.MySQLDialect ):

238 public String getLimitString(String sql, boolean hasOffset) {
239     return new StringBuffer( sql.length()+20 )
240         .append(sql)
241         .append( hasOffset ? " limit ?, ?" : " limit ?")
242         .toString();
243 }

Answer 2

I think the documentation makes it pretty clear that only the max results will be retrieved from the database.

setMaxResults(int)

Seems to me to be the equiv of

SELECT * FROM GovOffice LIMIT 0, 100;