【20180329】MySQL優化SQL一則以及思考

線網環境

1. MySQL 5.6.21-log MySQL Community Server
2. innodb_buffer_pool_size 1G
3. 關閉QC

4. 表存在分區

表結構和待優化的SQL

mysql> show create table articles \G
*************************** 1. row ***************************
       Table: articles
Create Table: CREATE TABLE `articles` (
  `id` int(11) NOT NULL,
  `user_id` int(11) NOT NULL,
  `pic_urls` varchar(512) NOT NULL,
  `vote_id` int(11) NOT NULL DEFAULT '0',
  `topic_id` int(11) NOT NULL DEFAULT '0',
  `type` int(11) NOT NULL,
  `content` varchar(3000) DEFAULT NULL,
  `location` varchar(32) NOT NULL DEFAULT '',
  `longitude` varchar(12) NOT NULL,
  `latitude` varchar(12) NOT NULL,
  `status` int(11) NOT NULL,
  `created_at` datetime NOT NULL,
  `user_ip` int(11) unsigned NOT NULL DEFAULT '0',
  `review` tinyint(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  KEY `topic_id` (`topic_id`),
  KEY `user_id` (`user_id`,`status`,`created_at`),
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
/*!50100 PARTITION BY HASH (id)
PARTITIONS 16 */

SELECT COUNT(1) AS c FROM articles WHERE topic_id = 114 AND status = 1 AND id NOT IN (17080670,17079098,17109846,17110577,17126923,17117892)  AND user_id NOT IN (33824249,33828294,32833143,33834489,33841022,33841121,33841156,33844937,33847129,33827047,33848560,32720750,33833941,33848573,33810491,33838021,33847010,33851748,33853093,33853111,33853156,33841943,33853408,33857041,33854233,33846742,33846748,33854255,33857064,33885581,33864476,33934278,34280131,34185367,34280088,34346710,34385075,34385237,34077681,34280171,34346732,34346738,34332490,34335274,34181694,34386339,34250847,34346757,34346752,34346813,34346945,7490639,34346927,34010356,34347049,33940626,34341461,34347061,34347081,34341381,34347085,34347126,34332537,34347117,34394447,34327303,31607732,34173783,34353368,34353330,34150466,34070811,34353413,34353371,34392543,34110905,34332506,34353421,34068582,34368645,33711510,34353619,33888980,34353584,34353617,34366773,34068768,34367479,34370650,34353698,34353721,34360131,34360154,34237758,34366508,34360254,34360246,34376763,34360248,34360276,33965629,34360269,34392397,34327406,34360307,34360402,34360431,27855780,34360467,34029547,34360448,34392561,34360453,34397244,34453559,34453531,34360480,34224620,34397270,34340166,34453534,34462520,1060754,34070596,34335225,33464686,34360505,34360611,34413592,20244349,34367391,34367528,34367558,34496904,34494346,32992426,34367521,34367506,34397284,34075167,34397236,34367508,34502838,34367511,34367671,34367672,34256721,34510895,34511335,34220701,34511556,34370188,34520818,34521120,34521121,34521099,34452468,34530058,34534558,34533294,34541331,34060856,34543797,34082416,34500366,34545190,34515713,34417773,34367802,34553311,34367862,34367832,34127282,34083939,33691298,34373694,34559793,34373689,34545953,34373783,34500833,34573568,34525236,34576425,34373688,34585516,34415308,13164849,34574639,34568298,34558301,34583203,34580301,12135076,5485129,34574974,34552389,34567808,34589428,34586104,34596708,34587929,34589613,34592242,32710709,33996135,34589622,34590003,34579522,34603444,33471864,34104130,34494174,34026755,34335083,34637732,34608155,31270655,34313290,34373760,34373757,32717182,34396802,32568247,31832477,34210659,34653651,34653681,34102064,34414116,34422038,34574228,34422010,34584690,34156339,34422218,34413816,34413818,34554514,30371762,34405772,34397948,34398306,33934125,34404648,34085531,34670672,32578081,34633739,34768345,34454576,34093578,34671375,34790367,34369280,34799915,34799934,34791716,34791710,34791767,34169856,34911914,34887431,34887393,34853976,34915775,34993982,34961184,34993983,35005710,34913662,32555445,35093709,35097894,35146899,35147077,35170956,31163038,33971212,35160092,35183839,35256282,35196189,34399233,35092356,34405546,35168649,31809004,35270433,35168810,35666711,35300151,35710861,18054773,35905428,35534509,35301706,35741153,35367969,30990616,3204057,35970676,33681952,35970746,35369209,35369417,35985122,36020270,35900701,9316493,35553853,35633504,35633617,36232496,35191850,35880221,36415016,36296697,36529238,35786342,36659949,36405256,36422717,36661338,36661297,36614751,36440561,35861881,14752030,32736870,36458571,36689504,36461801,36795506,36796410,37054128,37054123,37054054,37054399,37074768,37090630,37090391,36680809,37231542,37110709,36681004,37158764,37158828,37158887,37167540,37167539,37175339,37175275,37175465,37279948,32873902,37286818,37317499,37345029,37344970,4222498,36483504);

優化思路

1. 第一次： 在topic_id和status上面添加聯合索引。
   • 創建聯合索引在topic_id status
     
   • 查看SQL的執行計劃
     
   • SQL第一次請求耗時
     
   • SQL第二次請求耗時
     
2. 第二次：在topic_id，status和id上面添加聯合索引。
   • 創建聯合索引在 topic_id status id
     
   • 查看SQL的執行計劃
     
   • 第一次SQL請求耗時
     
   • 第二次SQL請求耗時
     
3. 在topic_id，status，id和user_id上面創建聯合索引
   • 創建聯合索引 topic_id status id user_id
     
   • 查看SQL執行計劃
     
   • 第一次SQL請求耗時
     
   • 第二次SQL請求耗時
     
4. 第四次： 在topic_id，status和user_id上面創建聯合索引
   • 創建聯合索引 topic_id status user_id
     
   • 查看SQL執行計劃
     
   • 第一次SQL請求耗時
     
   • 第二次SQL請求耗時
     

思考

1. 第一次和第二次優化和第三次，第四次的優化爲什麼相差那麼大？
   • 主要原因是因爲第一次和第二次優化user_id的鍵值數據需要回表導致優化不是很顯著。第三次和第四次user_id不需要回表，所以優化很明顯。
2. 爲什麼第四次優化的第一次SQL和第二次執行SQL耗時這麼慢？
   • 因爲innodb_buffer_pool_size緩存麼？