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前言： 　　一直是想知道一条SQL语句是怎么被执行的，它执行的顺序是怎样的，然后查看总结各方资料，就有了下面这一篇博文了。 　　本文将从MySQL总体架构--->查询执行流程--->语句执行顺序来探讨一下其中的知识。 一、MySQL架构总览： 　　架构最好看图，再配上必要的说明文字。 　　下图根据参考书籍中一图为原本，再在其上添加上了自己的理解。 从上图中我们可以看到，整个架构分为两层，上层是MySQLD的被称为的‘SQL Layer’，下层是各种各样对上提供接口的存储引擎，被称为‘Storage Engine Layer’。其它各个模块和组件，从名字上就可以简单了解到它们的作用，这里就不再累述了。 二、查询执行流程 　　下面再向前走一些，容我根据自己的认识说一下查询执行的流程是怎样的： 1.连接 　　1.1客户端发起一条Query请求，监听客户端的‘连接管理模块’接收请求 　　1.2将请求转发到‘连接进/线程模块’ 　　1.3调用‘用户模块’来进行授权检查 　　1.4通过检查后，‘连接进/线程模块’从‘线程连接池’中取出空闲的被缓存的连接线程和客户端请求对接，如果失败则创建一个新的连接请求 2.处理 　　2.1先查询缓存，检查Query语句是否完全匹配，接着再检查是否具有权限，都成功则直接取数据返回 　　2.2上一步有失败则转交给‘命令解析器’，经过词法分析，语法分析后生成解析树

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 由 翻译 强力驱动 问题: I'm looking for a way to save() a variable under a different name "on the fly" in R (bear with me! I'm pretty sure that's not a duplicate...). Here is an example of what I'd like to achieve: AAA = 1 BBB = 2 XXX = 3 YYY = 4 save ( AAA = XXX , BBB = YYY , file = "tmp.Rdat" ) # does NOT save a variable AAA to file with value 3 in it, which is the aim... Basically I would like the save() function to take the value of XXX and save it to file under a variable named AAA . Note that this is not a question about renaming a variable: I could

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I have a situation where i have to manually merge a label with value and then store in array. For instance aaa 10 , bbb 20, ccc 30 The values are coming from text field and finally i have to bring this in such format... with comma seperated and label's are hardcoded. How to create an Array or a String like this aaa 10 , bbb 20, ccc 30 with Key:Value pair 回答1: I'm not exactly sure what you're asking for, but perhaps this helps //create array var list = []; //get value from input aaa var value1 = document.getElementById("aaa").value; //add

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I have a .net regex which I am testing using Windows Powershell. The output is as follows: > [System.Text.RegularExpressions.Regex]::Match("aaa aaa bbb", "aaa.*?bbb") Groups : {aaa aaa bbb} Success : True Captures : {aaa aaa bbb} Index : 0 Length : 11 Value : aaa aaa bbb My expectation was that using the ? quantifier would cause the match to be aaa bbb , as the second group of a's is sufficient to satisfy the expression. Is my understanding of non-greedy quantifiers flawed, or am I testing incorrectly? Note: this is plainly not the same

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I have a string of words separated by spaces. How to split the string into lists of words based on the words length? Example input: " aa aaa aaaa bb bbb bbbb cc ccc cccc cccc bbb bb aa " output : List 1 = { aa, bb, cc} List 2 = { aaa, bbb, ccc} List 3 = { aaaa, bbbb, cccc} 回答1: Edit: I'm glad my original answer helped the OP solve their problem. However, after pondering the problem a bit, I've adapted it (and I strongly advise against my former solution, which I have left at the end of the post). A simple approach string input = " aa aaa

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I am an inexperienced R user and have been struggling with the By() function and would appreciate your help. The task is simple, I have a longitudinal dataset (How do I declare one?) and need to calculate different metrics by ID. One of the the metrics is a simple percent change (that requires a lag, example below): ID Date Temp %Change AAA 1/1/2003 0.749881714 NA AAA 1/2/2003 0.666661576 -0.110977687 AAA 1/3/2003 0.773079935 0.159628759 AAA 1/4/2003 0.62902364 -0.186340751 AAA 1/5/2003 0.733312374 0.165794619 BBB 1/1/2003 0.707339766 NA BBB

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: In my snippet below, the non-capturing group "(?:aaa)" should be ignored in matching result, so the result should be "_bbb" only. However, I get "aaa_bbb" in matching result; only when I specify group(2) does it show "_bbb" . import re string1 = "aaa_bbb" print(re.match(r"(?:aaa)(_bbb)", string1).group()) >>> aaa_bbb 回答1: group() and group(0) will return the entire match. Subsequent groups are actual capture groups. >>> print (re.match(r"(?:aaa)(_bbb)", string1).group(0)) aaa_bbb >>> print (re.match(r"(?:aaa)(_bbb)", string1).group(1)) _bbb

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 由 翻译 强力驱动 问题: I have some code that I'm trying to run to get an existing element however it keeps returning null despite it existing in the developer console. I've read some stuff about waiting until the window loads before making a call but the code where I am trying to put this does not run on startup and waits until a user action. I have even run the following and it returns null: massText = document . createElement ( 'mass_div' ); console . log ( document . getElementById ( 'mass_div' )); Can anyone tell me why this returns null? I just

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I have the following table: column1 column2 1 aaa^bbb 2 aaa^bbb|ccc^ddd I would like to have a output file as follows: column1 column2 column3 1 aaa bbb 2 aaa bbb 3 ccc ddd Could you mind to let me know if there are smart way of doing this? Update: I was trying to do two things; For ^, I want to separate the context to the column 2 and column 3. For |, I want to separate it to the next row, but keeping the same number in column1 (the column1 is the same for row 2 and 3. Sorry that I make a mistake here. To rewrite, input is as follows:

可以将文章内容翻译成中文,广告屏蔽插件可能会导致该功能失效(如失效，请关闭广告屏蔽插件后再试): 问题: I know this. Calling C function from C++: If my application was in C++ and I had to call functions from a library written in C. Then I would have used //main.cpp extern "C" void C_library_function(int x, int y);//prototype C_library_function(2,4);// directly using it. This wouldn't mangle the name C_library_function and linker would find the same name in its input *.lib files and problem is solved. Calling C++ function from C??? But here I'm extending a large application which is written in C and I need to use a library which is written in C