Question

我正在使用codeigniter和mysql并应用于具有200 MB内存和1 GB CPU的服务器中，获得了40.000行表a（index：idx_cat）和44行（index：primary）在类别中，我需要在每个类别中获得前4个，这是我的查询

SELECT id,title,preview,created,image,dummy,name 
FROM
 (
 select news.id AS id,news.title AS title,preview,created,news.image,category_id ,
         @num := if(@cat = category_id, @num + 1, 1) as row_number,
         @cat := category_id as dummy,
        name,d_order
 from news use index (idx_cat) inner join category b on category_id=b.id
 where 
    news.category_id in (9,8,3,35,57,56,15,58,41,42,43,44,34,52,37,38,36,11) and
    news.status = 1 
 having row_number <= 4 
 order by dummy ASC,news.created desc
 ) as a 
order by d_order ASC,created DESC



id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   PRIMARY     <derived2>  ALL     NULL    NULL    NULL    NULL    **29639**   Using filesort
2   DERIVED     b   ALL     PRIMARY,id  NULL    NULL    NULL    **44**  Using where; Using temporary; Using filesort
2   DERIVED     news    ref     idx_cat     idx_cat     **4**   b.id    846     Using where

并获得了其他6个简单的连接，如

   select id,name from others a inner join b on a.cat_id = b.id

网站加载速度相当快，大约是1s或2s，但是如果在另一个标签中打开，那么加载它的位就像5-7s一样慢。

奇怪的是cpu使用率达到100％，内存使用量达到+ _ 40 MB完成一个视图（我确定没有其他打开）但CI PROFILING它只说用户4MB。

我还有按需加载模型，助手和库，只有两个（网址和表单）放在自动加载文件中。

如果我打开5到10个窗口它会说内存不足，请问你们有什么建议这件事会发生什么事情，这会让我发疯 - -

Answer 1

要缩小内部选择结果的大小，请将字段name和d_order的选择和连接移动到外部选择

SELECT id,title,preview,created,image,dummy,name,d_order FROM
     (
 select news.id AS id,news.title AS title,preview,created,news.image,category_id ,
         @num := if(@cat = category_id, @num + 1, 1) as row_number,
         @cat := category_id as dummy
 from news use index (idx_cat) 
 where 
    news.category_id in (9,8,3,35,57,56,15,58,41,42,43,44,34,52,37,38,36,11) and
    news.status = 1 
 having row_number <= 4 
 order by dummy ASC,news.created desc
 ) as a inner join category b on category_id=b.id
order by d_order ASC,created DESC

可疑的是，行数仍然很大，但我们减少了内存。您选择的方式需要处理整个新闻表，然后以非常昂贵的计算删除不需要的行。如果你只使用news.id，cat.id和minimun字段进行新闻预选，可能会更有效率，所以新闻机构的重字段不在重选之中。

SELECT id,c.title,c.preview,created,c.image,dummy,name,d_order FROM
     (
 select news.id AS id,category_id ,
         @num := if(@cat = category_id, @num + 1, 1) as row_number,
         @cat := category_id as dummy
 from news use index (idx_cat) 
 where 
    news.category_id in (9,8,3,35,57,56,15,58,41,42,43,44,34,52,37,38,36,11) and
    news.status = 1 
 having row_number <= 4 
 order by dummy ASC,news.created desc
 ) as a 
    inner join category b on category_id=b.id
    inner join news c on a.id = c.id
    order by d_order ASC,created DESC

可能我有一些sintax错误，但在这里写一点有点困难，我没有要测试的数据。希望你能明白这个问题。

codeigniter，mysql查询性能问题

1 个答案: