mysql - 优化查询 : Select in Select

Question

假设我有一个表来存储每个用户的博客(表名是 blogs)。 而且还要有一个表来存储用户喜欢的其他用户的博客(表名是 likmedBlogs)，是吗？

所以为了检索它们，我只是这样做: -选择用户的博客。 -添加他喜欢的博客。

    $q = mysql_query("
SELECT id 
FROM blogs 
WHERE (iduser = $id) 
      OR id IN 
       (SELECT idblog 
        FROM likedBlogs 
        WHERE iduser='$id') 
      AND (id > 0) 
ORDER BY id DESC 
LIMIT 20
") or die(mysql_error());

我可以做得更好吗？ (您如何评价这个查询的性能？)

谢谢

Answer 1

我相信您可以使用 EXISTS 而不是 IN 更好地重构它。

IN 要求返回整个结果集，然后开始搜索一个值，但是 EXISTS 逐行检查并在找到第一次出现时中断内部查询。

SELECT id  
FROM blogs  
WHERE (iduser = $id)        
OR EXISTS
      (SELECT idblog         
       FROM likedBlogs          
       WHERE iduser='$id' AND idblog = id)
AND (id > 0)  
ORDER BY id 
DESC  LIMIT 20 

参见 Optimizing IN/=ANY Subqueries

very useful optimization is to “inform” the subquery that the only rows of interest are those where the inner expression inner_expr is equal to outer_expr. This is done by pushing down an appropriate equality into the subquery's WHERE clause. That is, the comparison is converted to this: EXISTS (SELECT 1 FROM ... WHERE subquery_where AND outer_expr=inner_expr)