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innodb引擎在統計方面和myisam是不同的,Myisam內置了一個計數器,所以在使用 select count(*) from table 的時候,直接可以從計數器中取出數據.而innodb必須全表掃描一次方能得到總的數量.要初步解決這個問題,需要做不同于myisam的一些工作：
1、使用第二索引(一般不使用主鍵索引),并且添加where條件,如：
代碼如下:
select count(*) from product where comp_id>=0 ;
show index from product ;
id primary key
comp_id index
2、如果只需要粗略統計的話也可使用
show status from product; 來得到大約值
這種辦法可在數據分頁中使用!
3、使用外部計數器,比如建立一個觸發器來計數或者在程序上使用緩存方式定時計數,缺陷是這些辦法會額外消耗一些資源!
參考資料：
mysql高性能：http://www.mysqlperformanceblog.com/2006/12/01/count-for-innodb-tables/
mysql DBA:http://imysql.cn/2008_06_24_speedup_innodb_count
COUNT(*) for Innodb Tables
I guess note number one about MyISAM to Innodb migration is warning what Innodb is very slow in COUNT(*) queries. The part which I often however see omitted is fact it only applies to COUNT(*) queries without WHERE clause.
So if you have query like SELECT COUNT(*) FROM USER It will be much faster for MyISAM (MEMORY and some others) tables because they would simply read number of rows in the table from stored value. Innodb will however need to perform full table scan or full index scan because it does not have such counter, it also can't be solved by simple singe counter for Innodb tables as different transactions may see different number of rows in the table.
If you have query like SELECT COUNT(*) FROM IMAGE WHERE USER_ID=5 this query will be executed same way both for MyISAM and Innodb tables by performing index rage scan. This can be faster or slower both for MyISAM and Innodb depending on various conditions.
In real applications there are much more queries of second type rather than first type so it is typically not as bad problem as it may look. Most typically count of rows is needed by admin tools which may show it in table statistics, it may also be used in application stats to show something like “We have 123.345 users which have uploaded 1.344.656 images” but these are normally easy to remove.
So remember Innodb is not slow for ALL COUNT(*) queries but only for very specific case of COUNT(*) query without WHERE clause. It does not mean I would not like to see it fixed though, it is pretty annoying.
轉自：http://www.sphinxsearch.org/archives/89
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