SELECTname,ageFROMusersWHEREname='John Doe'; 由于查询只需要name和age列的值,而这些值都在索引中,所以MySQL可以使用覆盖索引来执行查询,而不需要回表查询数据。 3. 避免过度索引 不要过度索引表,因为过多的索引会增加写操作的开销(如INSERT、UPDATE和DELETE)。每个额外的索引都需要占用磁盘空间,并可能降低写操作...
mysql> EXPLAIN SELECT username,age,salary FROM employee where username = 'john';+---+---+---+---+---+---+---+---+---+---+---+---+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+---+-...
题目JOHN SNOW DEFEATS "KING CHOLERA" Cholera used to be one of the most(1)___(fear)disease in the world.In the early 19th century,when an outbreak of cholera hit Europe,millions of people died from the disease.John Snow became frustrated (2)___no one knew how ...
1.执行计划中字段解释 SQL> select * from scott.emp a,scott.emp b where a.empno=b.mgr; 已选择13行。 执行计划 Plan hash value: 992080948 | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | | 0 | SELECT STATEMENT | | 13 | 988 | 6 (17)| 00:00:01 | | 1 | ME...
Stephen Colbert chats with John Dickerson about what to expect from Donald Trump's second term in The White House and how to talk to young people about it. #Colbert #JohnDickerson #CBSNews #Election2024 #StephenColbert #TheLateShow Air Date: Nov 6, 2024 ...
SELECT*FROMtestWHEREfirst_name='John';SELECT*FROMtestWHERElast_name='Jones'ORfirst_name='John'; Suppose that you issue the followingSELECTstatement: SELECT*FROMtbl_nameWHEREcol1=val1ANDcol2=val2; If a multiple-column index exists oncol1andcol2, the appropriate rows can be fetched directly. ...
Unit 6 Words and Phrases3 Directions: Complete the sentences with the proper forms of the words given in parentheses. Words and Phrases Section B 1. He made a detailed (compare) between the western and Chinese education systems. 2. Weekends spent at Uncle John’s were the most (entertainment...
更多“You are John Smith who wants to purchase some office equipment (办公用品) . Write a letter of inquiry. T…”相关的问题 第1题 听力原文:M: Good morning. This is John Parker speaking. I'm just ringing to confirm my app ointment with Mr. Smith for this afternoon. W: Yes. Mr....
拍品终止拍卖: 2003年6月3日 载入资料中 关注 分享 试用预计金额计算器細明 细节 LYELL, Charles (1797-1875). Principles of Geology, being an Attempt to Explain the Former Changes of the Earth's Surface, by Reference to Causes Now in Operation, London: John Murray, 1830-1833. 3 volumes,...
Fogleman 1 John Fogleman Psychoanalysis of the Literary Hero The Hero: Our Common Thread Throughout time, human beings have struggled to explain the world around them. Being unknowingly thrust in to an existence filled with uncontrollable phenomena has no doubt caused many men and women throughout...