MySQL 查询中的分页思路的优化
mysql|分页|优化
作者:steeven
似乎讨论分页的人很少,难道大家都沉迷于limit m,n?
在有索引的情况下,limit m,n速度足够,可是在复杂条件搜索时,
where somthing order by somefield+somefield
mysql会搜遍数据库,找出“所有”符合条件的记录,然后取出m,n条记录。
如果你的数据量有几十万条,用户又搜索一些很通俗的词,
然后要依次读最后几页重温旧梦。。。mysql该很悲壮的不停操作硬盘。
所以,可以试着让mysql也存储分页,当然要程序配合。
(这里只是提出一个设想,欢迎大家一起讨论)
ASP的分页:在ASP系统中有Recordset对象来实现分页,但是大量数据放在内存中,而且不知道什么时候才失效(请ASP高手
指点).
SQL数据库分页:用存储过程+游标方式分页,具体实现原理不是很清楚,设想如果用一次查询就得到需要的结果,或者是
id集,需要后续页时只要按照结果中的IDs读出相关记录。这样只要很小的空间保留本次查询的所有IDs. (SQL中的查询结
果不知道怎样清楚过期垃圾?)
这样,可以让mysql模拟存储分页机制:
1. select id from $table where $condition order by $field limit $max_pages*$count;
查询符合条件的IDs.
限定最大符合条件的记录数量,也可以不加。
2. 因为php在执行结束后所有变量都要lost,所以可以考虑:
方案a. 在mysql建立临时表,查询结果用一个时间或随机数作为唯一标志插入。
其中建立page1~pagen个字段,每个字段保存该页中需要的ids, 这样一个id对一条记录.
方案b. 如果打开session,也可以放在session中保存,实际上是放在文件中保存。
建立一个$IDs数组,$IDs[1]~$IDs[$max_pages]. 考虑到有时候用户会开几个
窗口同时查询,要为$ids做一个唯一标志,避免查询结果相互覆盖。二维数组
和$$var都是好办法。
3. 在每页页的请求中,直接找到对应的IDs,中间以","间隔:
select * from $table where id in ($ids); 速度绝对快
4. 收尾要考虑查询结果的自动清除,可以设置定时或者按比例随机清楚。如果用mysql临时表要加上一个时间标志字段,
session中要加入$IDs["time"]=time(); 在一定时间以后不操作视为过期数据。
5. 如果要优化,可以考虑用把1和2.a中的语句合并成select ...... into ....
Note:
1.以上只是针对mysql的修补方案,希望mysql哪天能把这些功能加进去
2.其它数据库也可以套用。
3.如果其它数据库还有更先进的分页方式,请告诉我或mailto: steeven@kali.com.cn
4.如果真的有很多数据要查询,还是和mysql再见吧,sql,oracle都提供了更先进的关键词索引查询。
精益求精,以上只是抛砖引玉,欢迎共同探讨分页问题。(也可关于其它数据库)
希望有一天能把各种分页方式整理出来供新手参考。

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