我有一张表
articles
,有 500k 行。一篇文章有一个作者列表。我正在尝试创建一个查询来获取作者列表的最新发表的文章。
我使用了以下 HQL 查询,它得到了我想要的结果,但运行速度相当慢(~4s)
select author, article
from Article article inner join article.authors author
where (author.id, article.publishedAt) in
(select author.id, max(article.publishedAt)
from Article article join article.authors author
where author.id in (authors_list))
group by author.id
纯 sql 中可能更好的查询是:
select * from (
select articles.id, author.id
from articles, article_authors, authors
where articles.id = article_authors.article_id and
article_authors.author_id=authors.id
and author.id in (author_list)
order by articles.publishedAt desc
) b
group by authors.id;
但是 Hibernate 文档指出 HQL 子查询只能出现在 select 或 where 子句中。 http://docs.jboss.org/hibernate/orm/3.3/reference/en/html/queryhql.html#queryhql-subqueries
有没有办法使用HQL来模拟这种查询或其他方法来增强查询的性能?
在任何一种情况下,您都希望尝试隔离用于比较的数据(如果数据很大)。在上面的第一个查询中,您有:
in
(select author.id, max(article.publishedAt)
from Article article join article.authors author
where author.id in (authors_list))
尝试先将该语句放入临时表中,然后使用该小数据集以提高效率。 所以它看起来像:
select author.id, max(article.publishedAt) into #temp1
from Article article join article.authors author
where author.id in (authors_list))
select author, article
from Article article inner join article.authors author
where (author.id, article.publishedAt) in
(select author.id, article.publishedAt
from #temp1)
group by author.id
因为计算完成后数据集较小,所以应该会提高性能。