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大约在两年前，我写了 一篇关于MySQL索引的文章 。最近有同学在文章的评论中对文章的内容提出质疑，质疑主要集中在联合索引的使用方式上。在那篇文章中，我说明联合索引是将各个索引字段做字符串连接后作为key，使用时将整体做前缀匹配。 而这名同学在 这个页面 找到了如下一句话：index condition pushdown is usually useful with multi-column indexes: the first component(s) is what index access is done for, the subsequent have columns that we read and check conditions on。从而认为联合索引的使用方式与文中不符。 实际上，这个页面所讲述的是在MariaDB 5.3.3（MySQL是在5.6）开始引入的一种叫做Index Condition Pushdown（以下简称ICP）的查询优化方式。由于本身不是一个层面的东西，前文中说的是Index Access，而这里是Query Optimization，所以并不构成对前文正确性的影响。在写前文时，MySQL还没有ICP，所以文中没有涉及相关内容，但考虑到新版本的MariaDB或MySQL中ICP的启用确实影响了一些查询行为的外在表现

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: string str = "Student_123_"; I need to replace the last character "_" with ",". I did it like this. str.Remove(str.Length -1, 1); str = str + ","; However, is it possible to achieve it more efficiently. may be one line of code.?? BTW, last character can be any character. So Replace wont work here. 回答1: No. In C# strings are immutable and thus you can not change the string "in-place". You must first remove a part of the string and then create a new string. In fact, this is also means your original code is wrong, since str.Remove(str.Length -1

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'm trying to set as default the first occurrence in this example: plunkr getting the following error: Unhandled Promise rejection: Template parse errors: TypeError: Cannot read property 'toUpperCase' of undefined ("dButtonToggleGroup"> <md-button-toggle [ERROR ->]*ngFor="let indicador of indicadores; #first = first" value="indicador.id" [checked]="first"> "): ng:///AppModule/HomeComponent.html@35:78 Parser Error: Unexpected token #, expected identifier, keyword, or string at column 31 in [let indicador of indicadores; #first = first] in ng:

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 accidentally deleted some huge no. of rows from a table.. How can I roll it back ? I executed the query using the putty ... I'll be grateful if any of you can guide me safely out of this.. 回答1: if you haven't made a backup, you are pretty much fudged. 回答2: If you didn't commit the transaction yet, try rollback . If you have already committed the transaction (by commit or by exiting the command line client), you must restore the data from your last backup. 回答3: A "rollback" only works if you used transactions . That way you can group

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: Free function allocate_shared can be used with any standard compliant allocator. But what about shared_ptr's constructor and reset method. template<class Y, class D, class A> shared_ptr(Y * p, D d, A a); template<class Y, class D, class A> void reset(Y * p, D d, A a); The manual says that D should provide a call operator which will be used to delete the pointer and A must be a standard compliant allocator. If so, why D is needed? Can't A do both allocation and delocation? Don't you think that the requirement to provide a deleter for every

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 am developing an application that sits in the system tray and can perform actions on the active window. But when the icon in the system tray is clicked, GetForegroundWindow() returns the taskbar. I need to get the window that was active before the taskbar was. I've tried enumerating the desktop window with EnumWindows and GetWindow , but this is often turning up desktop gadgets and other top items that where not active last. Is it even possible, or the information completely lost when the window is deactivated? 回答1: I think the

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 mean, I malloc a segment of memory, maybe 1k maybe 20bytes.. assume the pointer is pMem How can I know the content pMem refered is all Zero or \0 . I know memcmp but the second parameter should another memory address... thanx 回答1: As others have already suggested you probably want memset or calloc . But if you actually do want to check if a memory area is all zero, you can compare it with itself but shifted by one. bool allZero = pMem[0] == '\0' && !memcmp(pMem, pMem + 1, length - 1); where length is the number of bytes you want to be zero

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: How do I find the last weekday (e.g., Wednesday) of the month using R? In the code below, I calculate the month, day of the month, week of the month, and weekday. There are 5 Wednesdays in January 2014, but only 4 Wednesdays in February 2014, so I cannot use max(week of the month) as a filter. Any help is appreciated although I prefer to use the base R functions. DF <- data.frame(DATE = seq(as.Date("2014-01-01"), as.Date("2014-12-31"), "day")) DF$MONTH <- as.numeric(format(DF$DATE, "%m")) DF$DAY_OF_MONTH <- as.numeric(format(DF$DATE, "%d"))

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: Assuming my model looks like this (this is a simplified example): class Person ( Model ): first_name = CharField (...) last_name = CharField (...) def name (): return first_name + ' ' + last_name Displaying the name as a single column in the admin change list is easy enough. However, I need a single, editable "name" field that is editable from the list page, which I can then parse to extract and set the model field values. The parsing isn't a concern. I am just wondering how to have an editable form field on the list page that

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 a vector vec of structures. Such a structure has elements int a, int b, int c . I would like to assign to some int var the element c, from the last structure in a vector. Please can you provide me with this simple solution? I'm going circle in line like this: var = vec . end (). c ; 回答1: The immediate answer to your question as to fetching access to the last element in a vector can be accomplished using the back() member. Such as: int var = vec . back (). c ; Note: If there is a possibility your vector is empty, such a call