MySQL count(*) , Group BY 和 INNER JOIN

Question

我在 MySQL 5.1 上查询时遇到了非常糟糕的情况。我简化了我在其上进行 JOIN 的 2 个表:

CREATE TABLE  `jobs` (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`title` VARCHAR( 255 ) NOT NULL
) ENGINE = MYISAM ;

和

CREATE TABLE `jobsCategories` (
 `jobID` int(11) NOT NULL,
 `industryID` int(11) NOT NULL,
 KEY `jobID` (`jobID`),
 KEY `industryID` (`industryID`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

查询很简单:

SELECT count(*) as nb,industryID 
FROM  jobs J 
INNER JOIN jobsCategories C ON C.jobID=J.id 
GROUP BY industryID 
ORDER BY nb DESC;

我在 jobs 表中有大约 150000 条记录，在 jobsCategories 表中有 350000 条记录，我有 30 个行业；

执行查询大约需要 50 秒!!!

你知道为什么要花这么长时间吗？我怎样才能优化这个数据库的结构？分析查询显示 99% 的执行时间花在了 tmp 表上的复制上。

EXPLAIN <query> gives me : 


*************************** 1. row ***************************
       id: 1
select_type: SIMPLE
    table: J
     type: index
possible_keys: PRIMARY
      key: PRIMARY
  key_len: 4
      ref: NULL
     rows: 178950
    Extra: Using index; Using temporary; Using filesort
*************************** 2. row ***************************
       id: 1
 select_type: SIMPLE
    table: C
     type: ref
possible_keys: jobID
      key: jobID
  key_len: 8
      ref: J.id
     rows: 1
    Extra: Using where
2 rows in set (0.00 sec)

关于内存:

free -m  : 

total       used       free     shared    buffers     cached
Mem:          2011       1516        494          0          8       1075
-/+ buffers/cache:        433       1578
Swap:         5898        126       5772

使用下面建议的 FORCE INDEX

select count(*) as nb, industryID 
from 
    jobs J 
    inner join jobsCategories C force index (industryID) on (C.jobID = J.id )
group by industryID 
order by nb DESC;

SHOW PROFILE;

给我:

+----------------------+----------+
| Status               | Duration |
+----------------------+----------+
| starting             | 0.000095 |
| Opening tables       | 0.000014 |
| System lock          | 0.000008 |
| Table lock           | 0.000007 |
| init                 | 0.000032 |
| optimizing           | 0.000011 |
| statistics           | 0.000032 |
| preparing            | 0.000016 |
| Creating tmp table   | 0.000031 |
| executing            | 0.000003 |
| Copying to tmp table | 3.301305 |
| Sorting result       | 0.000028 |
| Sending data         | 0.000024 |
| end                  | 0.000003 |
| removing tmp table   | 0.000009 |
| end                  | 0.000004 |
| query end            | 0.000003 |
| freeing items        | 0.000029 |
| logging slow query   | 0.000003 |
| cleaning up          | 0.000003 |
+----------------------+----------+

我想我的 RAM (2Gb) 不够大。我如何确定是这种情况？

Answer 1

首先，我认为您不需要加入表 jobs 来获得相同的结果(除非您在表 jobsCategories 中有一些垃圾数据):

select count(*) as nb, industryID 
from jobsCategories
group by industryID 
order by nb DESC;

否则你可以尝试在 industryID 上强制建立索引:

select count(*) as nb, industryID 
from 
    jobs J 
    inner join jobsCategories C force index (industryID) on (C.jobID = J.id )
group by industryID 
order by nb DESC;