耗時(shí)查詢:
mysql> select * from channel_details where channel like "%" and type=29 and `year`>=2017 limit 10;
+---------+------+-------+---------------+-------------+---------+------+-----+---------------------+
| id | year | month | point_of_time | channel | ratings | type | age | time_at |
+---------+------+-------+---------------+-------------+---------+------+-----+---------------------+
| 7124305 | 2017 | 1 | 02:00 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124306 | 2017 | 1 | 02:01 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124307 | 2017 | 1 | 02:02 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124308 | 2017 | 1 | 02:03 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124309 | 2017 | 1 | 02:04 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124310 | 2017 | 1 | 02:05 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124311 | 2017 | 1 | 02:06 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124312 | 2017 | 1 | 02:07 | ??????????? | 0 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124313 | 2017 | 1 | 02:08 | ??????????? | 0.001 | 29 | 40 | 2017-02-15 15:03:54 |
| 7124314 | 2017 | 1 | 02:09 | ??????????? | 0.001 | 29 | 40 | 2017-02-15 15:03:54 |
+---------+------+-------+---------------+-------------+---------+------+-----+---------------------+
10 rows in set (16.10 sec)
mysql> explain select * from channel_details where channel like "%" and type=29 and `year`>=2017 limit 10;
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
| 1 | SIMPLE | channel_details | NULL | ref | year,type | type | 4 | const | 4969150 | 5.56 | Using where |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
不耗時(shí)查詢:
mysql> explain select * from channel_details where channel like "%" and type=29 limit 10;
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
| 1 | SIMPLE | channel_details | NULL | ref | type | type | 4 | const | 4969150 | 11.11 | Using where |
+----+-------------+-----------------+------------+------+---------------+------+---------+-------+---------+----------+-------------+
1 row in set, 1 warning (0.09 sec)
mysql> select * from channel_details where channel like "%" and type=29 limit 10;
+----+------+-------+---------------+-------------+---------+------+-----+---------------------+
| id | year | month | point_of_time | channel | ratings | type | age | time_at |
+----+------+-------+---------------+-------------+---------+------+-----+---------------------+
| 1 | 2016 | 9 | 02:00 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 2 | 2016 | 9 | 02:01 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 3 | 2016 | 9 | 02:02 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 4 | 2016 | 9 | 02:03 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 5 | 2016 | 9 | 02:04 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 6 | 2016 | 9 | 02:05 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 7 | 2016 | 9 | 02:06 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 8 | 2016 | 9 | 02:07 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 9 | 2016 | 9 | 02:08 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
| 10 | 2016 | 9 | 02:09 | ??????????? | 0 | 29 | 40 | 2016-12-22 22:59:24 |
+----+------+-------+---------------+-------------+---------+------+-----+---------------------+
10 rows in set (0.11 sec)
明明都是走了type索引,為什么時(shí)間差別這么大?
1、兩個(gè)查詢可以創(chuàng)建一個(gè)(type,year)的復(fù)合索引來用
2、查詢時(shí)間差異大是因?yàn)槟愕臈l件不一樣,索引檢索的順序規(guī)則要明白,第一條:順序檢索type索引,找到對(duì)應(yīng)的主鍵,再到表數(shù)據(jù)文件中查找滿足year>=2017的數(shù)據(jù),找到10條為止,由于2017之前還有很多數(shù)據(jù),比如2016的,順序檢索完所有2016才能找到2017的數(shù)據(jù),直到找到10條滿足條件的記錄。第二條:順序檢索type索引,找到10條記錄即可!這就是區(qū)別??!
北大青鳥APTECH成立于1999年。依托北京大學(xué)優(yōu)質(zhì)雄厚的教育資源和背景,秉承“教育改變生活”的發(fā)展理念,致力于培養(yǎng)中國IT技能型緊缺人才,是大數(shù)據(jù)專業(yè)的國家
北大青鳥中博軟件學(xué)院創(chuàng)立于2003年,作為華東區(qū)著名互聯(lián)網(wǎng)學(xué)院和江蘇省首批服務(wù)外包人才培訓(xùn)基地,中博成功培育了近30000名軟件工程師走向高薪崗位,合作企業(yè)超4
中公教育集團(tuán)創(chuàng)建于1999年,經(jīng)過二十年潛心發(fā)展,已由一家北大畢業(yè)生自主創(chuàng)業(yè)的信息技術(shù)與教育服務(wù)機(jī)構(gòu),發(fā)展為教育服務(wù)業(yè)的綜合性企業(yè)集團(tuán),成為集合面授教學(xué)培訓(xùn)、網(wǎng)
達(dá)內(nèi)教育集團(tuán)成立于2002年,是一家由留學(xué)海歸創(chuàng)辦的高端職業(yè)教育培訓(xùn)機(jī)構(gòu),是中國一站式人才培養(yǎng)平臺(tái)、一站式人才輸送平臺(tái)。2014年4月3日在美國成功上市,融資1
曾工作于聯(lián)想擔(dān)任系統(tǒng)開發(fā)工程師,曾在博彥科技股份有限公司擔(dān)任項(xiàng)目經(jīng)理從事移動(dòng)互聯(lián)網(wǎng)管理及研發(fā)工作,曾創(chuàng)辦藍(lán)懿科技有限責(zé)任公司從事總經(jīng)理職務(wù)負(fù)責(zé)iOS教學(xué)及管理工作。
浪潮集團(tuán)項(xiàng)目經(jīng)理。精通Java與.NET 技術(shù), 熟練的跨平臺(tái)面向?qū)ο箝_發(fā)經(jīng)驗(yàn),技術(shù)功底深厚。 授課風(fēng)格 授課風(fēng)格清新自然、條理清晰、主次分明、重點(diǎn)難點(diǎn)突出、引人入勝。
精通HTML5和CSS3;Javascript及主流js庫,具有快速界面開發(fā)的能力,對(duì)瀏覽器兼容性、前端性能優(yōu)化等有深入理解。精通網(wǎng)頁制作和網(wǎng)頁游戲開發(fā)。
具有10 年的Java 企業(yè)應(yīng)用開發(fā)經(jīng)驗(yàn)。曾經(jīng)歷任德國Software AG 技術(shù)顧問,美國Dachieve 系統(tǒng)架構(gòu)師,美國AngelEngineers Inc. 系統(tǒng)架構(gòu)師。