MYSQL解释Rows_examined不匹配

我有在mysql_slow_queries查询日志如下：MYSQL解释Rows_examined不匹配

Query_time：4.642323 Lock_time：1.821996 Rows_sent：14 Rows_examined：27099

SET TIMESTAMP=1356068688; 
SELECT gw.id website_id, gw.name,gw.url,gw.language,gw.title,gw.nickname, gd.id, 
     gd.deal_title, gd.cdeal_title, gd.deal_details, gd.cdeal_details, 
     gd.discount_price, gd.original_price, gd.savings, gd.expiry, gd.shop, 
     gd.location, gd.clocation, gd.limited_offer, gd.contact, gd.url website, 
     gd.affiliate_url, gd.tags, gd.pic_url, gd.featured, gd.top_pos, 
     gd.sub_pos, gd.appeal, gd.redeem_until, gd.noofpurchased 
FROM groupon_deals gd 
INNER JOIN groupon_websites gw ON gw.id=gd.groupon_websites_id 
WHERE gd.tags LIKE '%technology-and-gadgets%' AND gd.pubDate >= SYSDATE() - INTERVAL 24 HOUR AND 
     gd.hidden = 0 AND gd.pubDate < SYSDATE() AND 
     gd.id NOT IN (1,30079,30090,30116,30118,30070,30136,30137,30138,30156,30103,30157,30038,30044,30084,30025,30013,30111,30030,30020,30059,30087,30026,30016,30112,30031,30021,30005,30092,30027,30017,30113,30049,30032,30023,30006,30096,30040,30028,30018,30120,30033,30024,30008,30110,30029,30019,30128,30131,30129,30100,30004,29995,30076,30126,30069,30078,30071,30034,30080,30065,30073,30082,29987,30074,30117,30068,29981,30098,30102,30088,30119,30135,30155,30107,29997,30041,30046,30077,30003,29992,30058,30097,30014,29999,30066,30127,30009,30081,29993,30060,30015,30114,30000,29985,30099,30010,30083,29994,30061,30022,30115,30001,30072,29986,30011,30086,30062,30123,30002,30075,29990,30054,30160,30094,30012,29998,30064,30125,30039,30130,30134,29982,30159,30048,30047,30158,30043,30101,30104,30106,30122,30056,30057,30063,30161,30053,29984,30132,30109,30036,30108,30037,30121,30045,30124) AND 
     (gw.language = 'C' OR gw.language = 'B') 
ORDER BY gd.sub_pos,gd.noofpurchased DESC

现在，当我去phpMyAdmin的使用EXPLAIN运行相同的查询，我来到这里的输出： http://algaryeung.com/temp/explain-output.jpg

我有2个问题：

1）日志中的rows_examined如何与27099中的rows_examined相比，756是不同的？我是否需要在EXPLAIN中多取2个值才能检查真实的行？

2）我知道这是一种开放式，但我会如何去改善现有的查询？我已经将字段groupon_deals.groupon_websites_id编入索引，并且我认为可能有某种方法可以改进查询中的NOT IN部分。不期待在这里有完整的答案，但任何想法开始挖掘/学习？

来源

2012-12-21 Algar

MySQL EXPLAIN提供了对每个步骤结果返回的行数的预测，即预测。

EXPLAIN真正为您提供的是执行计划，即将要使用的访问路径，操作顺序以及将使用哪些索引。它实际上并没有处理语句以获得准确的行计数，它仅基于其关于表中的行数的信息以及列中的值的基数和分布来预测将检索多少行。

根据您提供的EXPLAIN输出，该查询正在对groupon_websites表进行全面扫描。对于检索到的每个id值（不由谓词消除），MySQL正在groupon_deals表的groupon_websites_id列上执行索引查找。

对于此查询，性能可能会提高一点与索引

... ON groupon_deals (groupon_websites_id, hidden, pubDate, id)

我认为一个良好的开端“挖”是理解EXPLAIN声明。

如果您对MySQL如何处理SQL语句，MySQL可以执行哪些操作以及哪些操作可以使用合适的索引有一定了解，那么这是理解来自MySQL的输出的基础。 EXPLAIN。

我建议从这里开始，MySQL的文档：Understanding the Query Execution Plan

来源

2012-12-21 06:07:18 spencer7593

感谢您的回答，知道EXPLAIN并没有真正处理语句，而是给出了一个计划，解释了很多（没有双关语意）。 – Algar

MYSQL解释Rows_examined不匹配

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