概括潜在的Hibernate性能问题
学习Hibernate时，经常会遇到Hibernate性能问题，这里将介绍Hibernate性能问题的解决方法。
在使用Hibernate进行分页的过程中，如果你收到如下警告，那么这里就是一个潜在的Hibernate性能问题点：
WARNING: firstResult/maxResults specified with collection fetch; applying in memory!
出现这个警告的直接后果是：无论你想要看第几页的数据，从Hibernate打印出的SQL来看它总是查询了所有满足条件的结果。这是为什么呢？来看看这句警告所在的代码，它位于org.hibernate.hql.ast.QueryTranslatorImpl中，部分摘录如下：
view plaincopy to clipboardprint?
QueryNode query = ( QueryNode ) sqlAst;
boolean hasLimit = queryParameters.getRowSelection() != null &&
queryParameters.getRowSelection().definesLimits();
boolean needsDistincting = ( query.getSelectClause().isDistinct() || hasLimit ) &&
containsCollectionFetches();
QueryParameters queryParametersToUse;
if ( hasLimit && containsCollectionFetches() ) {
log.warn( "firstResult/maxResults specified with collection fetch; applying in memory!" )
RowSelection selection = new RowSelection();
selection.setFetchSize( queryParameters.getRowSelection().getFetchSize() );
selection.setTimeout( queryParameters.getRowSelection().getTimeout() );
queryParametersqueryParametersToUse = queryParameters.createCopyUsing( selection );
}
else {
queryParametersqueryParametersToUse = queryParameters;
}
List results = queryLoader.list( session, queryParametersToUse );
QueryNode query = ( QueryNode ) sqlAst;
boolean hasLimit = queryParameters.getRowSelection() != null &&
queryParameters.getRowSelection().definesLimits();
boolean needsDistincting = ( query.getSelectClause().isDistinct() || hasLimit ) &&
containsCollectionFetches();
QueryParameters queryParametersToUse;
if ( hasLimit && containsCollectionFetches() ) {
log.warn( "firstResult/maxResults specified with collection fetch; applying in memory!" );
RowSelection selection = new RowSelection();
selection.setFetchSize( queryParameters.getRowSelection().getFetchSize() );
selection.setTimeout( queryParameters.getRowSelection().getTimeout() );
queryParametersqueryParametersToUse = queryParameters.createCopyUsing( selection );
}
else {
queryParametersqueryParametersToUse = queryParameters;
}
List results = queryLoader.list( session, queryParametersToUse );