rand()函數埋的一個坑，你們注意了

1、背景

在開發博客網站隨機文章的時候，剛開始沒有注意到這個問題，隨便加了一個rand()函數，數據量小的時候性能還能夠，一旦數據達到幾十萬的時候，就會產生性能問題。

2、問題分析

剛開始的代碼以下：

if(!StringUtils.isEmpty(postParam.getSortType())){            if(postParam.getSortType().equals(PostConstant.SORTTYPE_COMMMENT)){                example.setOrderByClause(" comment_count desc ");            }else if(postParam.getSortType().equals(PostConstant.SORTTYPE_DATE)){                example.setOrderByClause(" post_date desc ");            }else if(postParam.getSortType().equals(PostConstant.SORTTYPE_RANDOM)){                example.setOrderByClause(" RAND() ");                log.info("開始加載隨機文章列表。。。。");            }else if(postParam.getSortType().equals(PostConstant.SORTTYPE_VIEW)){                example.setOrderByClause(" post_date desc ");            }        }else{            example.setOrderByClause(" post_date desc ");        }        Page<WpPosts>   page =(Page<WpPosts>) wpPostsMapper.selectByExample(example);複製代碼

啓動程序，查看隨機文章，後臺日誌報錯

INFO  | 2018-11-19 18:43:53,040 |  JWordpres-v2.0 | [http-nio-80-exec-17-47] (o.s.b.f.x.XmlBeanDefinitionReader:317) | Loading XML bean definitions from class path resource [org/springframework/jdbc/support/sql-error-codes.xml]INFO  | 2018-11-19 18:43:53,202 |  JWordpres-v2.0 | [http-nio-80-exec-17-47] (o.s.j.s.SQLErrorCodesFactory:126) | SQLErrorCodes loaded: [DB2, Derby, H2, HSQL, Informix, MS-SQL, MySQL, Oracle, PostgreSQL, Sybase, Hana]ERROR | 2018-11-19 18:43:53,231 |  JWordpres-v2.0 | [http-nio-80-exec-17-47] (o.a.c.c.C.[.[.[.[dispatcherServlet]:181) | Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.jdbc.UncategorizedSQLException: ### Error querying database. Cause: java.sql.SQLException: Incorrect key file for table '/tmp/#sql_7c6d_0.MYI'; try to repair it### The error may exist in cn/liuhaihua/web/mapper/WpPostsMapper.java (best guess)### The error may involve cn.liuhaihua.web.mapper.WpPostsMapper.selectByExample-Inline### The error occurred while setting parameters### SQL: SELECT id,post_author,post_date,post_date_gmt,post_content,post_title,post_excerpt,post_status,comment_status,ping_status,post_password,post_name,to_ping,pinged,post_modified,post_modified_gmt,post_content_filtered,post_parent,guid,menu_order,post_type,post_mime_type,comment_count FROM wp_posts WHERE ( post_type = ? and post_status = ? ) order by RAND() LIMIT 10### Cause: java.sql.SQLException: Incorrect key file for table '/tmp/#sql_7c6d_0.MYI'; try to repair it; uncategorized SQLException for SQL []; SQL state [HY000]; error code [126]; Incorrect key file for table '/tmp/#sql_7c6d_0.MYI'; try to repair it; nested exception is java.sql.SQLException: Incorrect key file for table '/tmp/#sql_7c6d_0.MYI'; try to repair it] with root causejava.sql.SQLException: Incorrect key file for table '/tmp/#sql_7c6d_0.MYI'; try to repair it複製代碼

查看sql 發現以下代碼：

SELECT    id,    post_author,    post_date,    post_date_gmt,    post_content,    post_title,    post_excerpt,    post_status,    comment_status,    ping_status,    post_password,    post_name,    to_ping,    pinged,    post_modified,    post_modified_gmt,    post_content_filtered,    post_parent,    guid,    menu_order,    post_type,    post_mime_type,    comment_countFROM    wp_postsWHERE    (        post_type = "post"        AND post_status = "publish"    )ORDER BY    RAND()LIMIT 10複製代碼

這個sql會形成嚴重的性能問題，rand()形成在系統文件上來回排序。很是損耗性能

[Err] 126 - Incorrect key file for table '/tmp/#sql_7c6d_0.MYI'; try to repair it複製代碼

3、優化方案

知道問題所在，優化方案其實也蠻簡單的。原理以下

1首先 select count(*) from test where $where; （計算所須要的數據的總條數）2而後 $id=rand($a[0],$a[1]); 產生一個隨機數; 3最後 SELECT * FROM tablename WHERE id>='$id' LIMIT 1 將上面產生的隨機數帶入查詢; 複製代碼

修改代碼以下：

if(!StringUtils.isEmpty(postParam.getSortType())){            if(postParam.getSortType().equals(PostConstant.SORTTYPE_COMMMENT)){                example.setOrderByClause(" comment_count desc ");            }else if(postParam.getSortType().equals(PostConstant.SORTTYPE_DATE)){                example.setOrderByClause(" post_date desc ");            }else if(postParam.getSortType().equals(PostConstant.SORTTYPE_RANDOM)){                //example.setOrderByClause(" RAND() ");                /***                 * 首先 select count(*) from test where $where; （計算所須要的數據的總條數）                 *而後 $id=rand($a[0],$a[1]); 產生一個隨機數;                  *最後 SELECT * FROM tablename WHERE id>='$id' LIMIT 1 將上面產生的隨機數帶入查詢;                  */                log.info("開始加載隨機文章列表。。。。");                Random  random = new Random();                int randId =random.nextInt(count);                criteria.andGreaterThan("id", randId);            }else if(postParam.getSortType().equals(PostConstant.SORTTYPE_VIEW)){                example.setOrderByClause(" post_date desc ");            }        }else{            example.setOrderByClause(" post_date desc ");        }        Page<WpPosts>   page =(Page<WpPosts>) wpPostsMapper.selectByExample(example);複製代碼

這樣修改後以後，系統完美運行，sql查詢時間縮短到0.058秒

4、總結

其實mysql官網也說明這種狀況了，意思是說當記錄超過30萬，rand這種方法就不可用,須要更換方案。

works for small tables, but once the tables grow larger than 300,000 records or so this will be very slow because MySQL will have to process ALL the entries from the table, order them randomly and then return the first row of the ordered result,and this sorting takes long time. Instead you can do it like this (atleast if you have an auto_increment PK): 複製代碼

官方建議修改爲這樣

SELECT MIN(id), MAX(id) FROM tablename; Fetch the result into $a $id=rand($a[0],$a[1]); SELECT * FROM tablename WHERE id>='$id' LIMIT 1 複製代碼