Question

我想获取每组中的最后一条记录。我使用了非常小的数据库的以下查询，它完美地工作 -

SELECT * FROM logs 
WHERE id IN (
     SELECT max(id) FROM logs 
     WHERE id_search_option = 31 
     GROUP BY items_id
    ) 
ORDER BY id DESC

但是当涉及到具有数百万行（ 80,00000+ rows ）的实际数据库时，系统会被挂起。

我还尝试了另一个查询，平均结果为 6.6sec -

SELECT p1.id, p1.itemtype, p1.items_id, p1.date_mod
FROM logs p1
INNER JOIN (
     SELECT max(id) as max_id, itemtype, items_id, date_mod 
     FROM logs 
     WHERE id_search_option = 31 
     GROUP BY items_id) p2
ON (p1.id = p2.max_id) 
ORDER BY p1.items_id DESC;

请帮忙！

编辑::解释第二个查询

id  select_type     table     type    possible_keys     key     key_len     ref       rows    Extra
1   PRIMARY     <derived2>     ALL     NULL             NULL     NULL       NULL      1177    Using temporary; Using filesort
1   PRIMARY     p1            eq_ref   PRIMARY          PRIMARY   4        p2.max_id    1   
2   DERIVED     logs           ALL     NULL             NULL     NULL      NULL      7930527   Using where; Using temporary; Using filesort

Answer 1

select * from tablename orderby unique_column desc limit 0,1; 试试它会起作用这里0-＆gt;记录，1->一个记录

为大型数据库

1 个答案: