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  • MySQL - Calculating fields on the fly vs storing calculated data

    - by Christian Varga
    Hi Everyone, I apologise if this has been asked before, but I can't seem to find an answer to a question that I have about calculating on the fly vs storing fields in a database. I read a few articles that suggested it was preferable to calculate when you can, but I would just like to know if that still applies to the following 2 examples. Example 1. Say you are storing data relating to a car. You store the fuel tank size in litres, and how many litres it uses per 100km. You also want to know how many KMs it can travel, which can be calculated from the tank size and economy. I see 2 ways of doing this: When a car is added or updated, calculate the amount of KMs and store this as a static field in the database. Every time a car is accessed, calculate the amount of KMs on the fly. Because the cars economy/tank size doesn't change (although it could be edited), the KMs is a pretty static value. I don't see why we would calculate it every single time the car is accessed. Wouldn't this waste cpu time as opposed to simply storing it in a separate field in the database and calculating only when a car is added or updated? My next example, which is almost an entirely different question (but on the same topic), relates to counting children. Let's say we have a app which has categories and items. We have a view where we display all the categories, and a count of all the items inside each category. Again, I'm wondering what's better. To perform a MySQL query to count all the items in each category every single time the page is accessed? Or store the count in a field in the categories table and update when an item is added / deleted? I know it is redundant to store anything that can be calculated, but I worry that calculating fields or counting records might be slow as opposed to storing the data in a field. If it's not then please let me know, I just want to learn about when to use either method. On a small scale I guess it wouldn't matter either way, but apps like Facebook, would they really count the amount of friends you have every time someone views your profile or would they just store it as a field? I'd appreciate any responses to both of these scenarios, and any resource that might explain the benefits of calculating vs storing. Thanks in advance, Christian

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  • runModalForWindow throttles http requests

    - by Nikita Rybak
    I have url connection, which normally works fine NSURLConnection *connection = [[NSURLConnection alloc] initWithRequest:request delegate:delegate]; But when I create a modal window, no request ever receives response: [NSApp runModalForWindow:window]; If I comment this line out, thus creating a 'standard' window, everything works. I tried implementing all methods from NSURLConnectionDelegate, not a single of them called. I suspect this is something about 'run loops', but have little experience in this area. Does anybody have experience in this? Thank you

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  • Can I integrate many sources into one RSS reader?

    - by BenAlabaster
    I'm looking to integrate a bunch of RSS/ATOM sources into a feed for our intranet. This is outside my usual realm of work so I've had very limited exposure to this. I'd like to integrate all the feeds into a single reader such that all the news items are in chronological release order. Is this doable? Do I need a special component? Or can I make the ASP.NET integrated components do all this simply enough?

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  • The PROMOTE TRANSACTION request failed because there is no local transaction active.

    - by Mark J Miller
    Under what circumstances would I see the above message? I have a single call to SQL Server which is wrapped in a call to TransactionScope. In our development and QA environments MSDTC is turned off and the call succeeds fine. However, in our production environment with MSDTC turned on we are failing with this call. Is there something that would cause this when I am sure we are not looking at a distributed transaction call at all?

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  • WinForm-style Invoke() in unmanaged C++

    - by Matt Green
    I've been playing with a DataBus-type design for a hobby project, and I ran into an issue. Back-end components need to notify the UI that something has happened. My implementation of the bus delivers the messages synchronously with respect to the sender. In other words, when you call Send(), the method blocks until all the handlers have called. (This allows callers to use stack memory management for event objects.) However, consider the case where an event handler updates the GUI in response to an event. If the handler is called, and the message sender lives on another thread, then the handler cannot update the GUI due to Win32's GUI elements having thread affinity. More dynamic platforms such as .NET allow you to handle this by calling a special Invoke() method to move the method call (and the arguments) to the UI thread. I'm guessing they use the .NET parking window or the like for these sorts of things. A morbid curiosity was born: can we do this in C++, even if we limit the scope of the problem? Can we make it nicer than existing solutions? I know Qt does something similar with the moveToThread() function. By nicer, I'll mention that I'm specifically trying to avoid code of the following form: if(! this->IsUIThread()) { Invoke(MainWindowPresenter::OnTracksAdded, e); return; } being at the top of every UI method. This dance was common in WinForms when dealing with this issue. I think this sort of concern should be isolated from the domain-specific code and a wrapper object made to deal with it. My implementation consists of: DeferredFunction - functor that stores the target method in a FastDelegate, and deep copies the single event argument. This is the object that is sent across thread boundaries. UIEventHandler - responsible for dispatching a single event from the bus. When the Execute() method is called, it checks the thread ID. If it does not match the UI thread ID (set at construction time), a DeferredFunction is allocated on the heap with the instance, method, and event argument. A pointer to it is sent to the UI thread via PostThreadMessage(). Finally, a hook function for the thread's message pump is used to call the DeferredFunction and de-allocate it. Alternatively, I can use a message loop filter, since my UI framework (WTL) supports them. Ultimately, is this a good idea? The whole message hooking thing makes me leery. The intent is certainly noble, but are there are any pitfalls I should know about? Or is there an easier way to do this?

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  • Log4net: seperate log files for each class

    - by Mohsan
    hi.. i want to log messages from multiple classes. the problem is that currently i can log all messages into single file.i want to do this class A should log its messages into ALog.txt and class B should log its messages into BLog.txt please tell me the sample config file for these settings

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  • Sanitising user input using Python

    - by Steve
    What's the best way to sanitise user input for a Python-based web application? Is there a single function to remove HTML characters and any other necessary characters combinations to ensure that an XSS or SQL injection attack isn't possible?

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  • Symfony 1.4: Storing an array in session with SetAttribute

    - by Tom
    Hi, I'm wondering if someone knows how to store an array into a user session? This is the method to store a single attribute: $this->getUser()->setAttribute('something', $something); And the documentation says: "User attributes can store any type of data (strings, arrays, and associative arrays)" ... but doesn't say anything more. The API documentation doesn't have a "SetAttributes" or something similarly obvious. Thank you.

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  • unable to calculate textfield values

    - by user1726508
    i am trying to change the input field when users changes the quantity of items in a text field. Here i am iterating my list from my database. Now i have to make invoice for customer. In my code , if i am changing quantity of a single item, then it is effecting all the other items in the list. I want to change only to the specific items,where its quantity has been change. Below code is giving me error. It is changing all the items value on single change of item quantity. my code; <script type="text/javascript"> $(document).ready(function(){ $(function() { $('input[name="quantity"]').change(function() { var unitprice = $('input[name^="unitprice"]').val(); $(this).parents('tr').find('input[name^="price"]').val($(this).val() * unitprice); }); }); }); </script> <tr> <td height="65%" valign="top" width="100%"> <table width="100%" height="100%" border="0" cellpadding="0" cellspacing="0"> <s:iterator value="#session.BOK" status="userStatus"> <tr style="height: 10px;"> <td width="65%" align="left"><s:property value="bookTitile"/></td> <td width="10%" align="left"><s:textfield name="unitprice" value="%{price}" size="4"/></td> <td width="10%" align="center"><s:textfield name="quantity" value="%{quantity}" size="2"/></td> <td width="15%" align="center"><s:textfield name="price" size="6"></s:textfield> </td> </tr> </s:iterator> </table> </td> </tr> output looks like this image...

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  • Getting started with mvim, how to manage a project?

    - by zengr
    Hi, I have started out with macvim and now fairly comfortable with the navigation (on a single file) and now I need to write a whole project (say rails) using mvim. In textmate, you have mate project_dir which opens the project in a side drawer, so my question is: Is there a similar feature in mvim? How can a complete project managed in traditionally in macvim? Links to some write-ups are welcomed.

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  • Automatically connect to a DB in a swing application

    - by Oussama
    Hello, I m working on a single user swing application that access an hsqldb database. How can i Automatically run the DB server when a user run the application.? for example, after i finish development i will put the application into an exe file. If the exe file is distributed to multiple users. How can the DB server run and the DB be created when the user run the exe file? Thanks

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  • Render similar data in C# by grouping

    - by Ted
    I am trying to create some dynamic html out of some data from db. My data returned from db is: ApplicationName URL AppName1 URL1 AppName2 URL1 AppName2 URL2 AppName1 URL2 I want that all URL's for a single application should be rendered under one heading of ApplicationName. Like: AppName1 URL1 URL2 AppName2 URL1 URL2 Can anyone help me in the code for this?

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  • threading.local equivalent for twisted.web?

    - by defnull
    In asynchronous environments, threading.local is not guaranteed to be context-local anymore, because several contexts may coexist within a single thread. Most asynchronous frameworks (gevent, eventlet) provide a get_current_context() functionality to identify the current context. Some offer a way to monkey-patch threading.local so it is local to 'greenthreads' or other framework-specific contexts. I cannot find such a functionality in the twisted documentation. How do I do this?

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  • print multiple cheque

    - by rishikesh
    I have a grid-view having details of customers,now I want to print cheques for all customers of the grid with single click of button instead of clicking individual print button for each and every customer.I want to implement this in asp.net MVC 1.0.Please temme hw should i proceed for this..Is iTextSharp will do any help to me or there is any other such pdf libraries which will help me to accomplish my task.

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  • Getting rewrite to work with SSL in a MVC Zend Framework app

    - by cappuccino
    I am following the Zend Framework quickstart document and got stuck on the .htaccess rewrite rules. I am using this: RewriteEngine On RewriteCond %{REQUEST_FILENAME} -s [OR] RewriteCond %{REQUEST_FILENAME} -l [OR] RewriteCond %{REQUEST_FILENAME} -d RewriteRule ^.*$ - [NC,L] RewriteRule ^.*$ index.php [NC,L] This works find when I access through HTTP, however, nothing is served when accessing through HTTPS. I am using a single directory for HTTP and HTTPS content. I would not want to force HTTPS either. How can I fix this?

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  • How can I write an installer for 32 and 64 bit

    - by tom greene
    I have a .NET app which works in 32 and 64 bit. I would like to write one single installer that installs to c:\program files, not c:\program files(x86) regardless of the platform. From this link: http://msdn.microsoft.com/en-us/library/w1behyzx(v=VS.90).aspx it doesn't look possible. Is it the case?

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  • Blit Queue Optimization Algorithm

    - by martona
    I'm looking to implement a module that manages a blit queue. There's a single surface, and portions of this surface (bounded by rectangles) are copied to elsewhere within the surface: add_blt(rect src, point dst); There can be any number of operations posted, in order, to the queue. Eventually the user of the queue will stop posting blits, and ask for an optimal set of operations to actually perform on the surface. The task of the module is to ensure that no pixel is copied unnecessarily. This gets tricky because of overlaps of course. A blit could re-blit a previously copied pixel. Ideally blit operations would be subdivided in the optimization phase in such a way that every block goes to its final place with a single operation. It's tricky but not impossible to put this together. I'm just trying to not reinvent the wheel. I looked around on the 'net, and the only thing I found was the SDL_BlitPool Library which assumes that the source surface differs from the destination. It also does a lot of grunt work, seemingly unnecessarily: regions and similar building blocks are a given. I'm looking for something higher-level. Of course, I'm not going to look a gift horse in the mouth, and I also don't mind doing actual work... If someone can come forward with a basic idea that makes this problem seem less complex than it does right now, that'd be awesome too. EDIT: Thinking about aaronasterling's answer... could this work? Implement customized region handler code that can maintain metadata for every rectangle it contains. When the region handler splits up a rectangle, it will automatically associate the metadata of this rectangle with the resulting sub-rectangles. When the optimization run starts, create an empty region handled by the above customized code, call this the master region Iterate through the blt queue, and for every entry: Let srcrect be the source rectangle for the blt beng examined Get the intersection of srcrect and master region into temp region Remove temp region from master region, so master region no longer covers temp region Promote srcrect to a region (srcrgn) and subtract temp region from it Offset temp region and srcrgn with the vector of the current blt: their union will cover the destination area of the current blt Add to master region all rects in temp region, retaining the original source metadata (step one of adding the current blt to the master region) Add to master region all rects in srcrgn, adding the source information for the current blt (step two of adding the current blt to the master region) Optimize master region by checking if adjacent sub-rectangles that are merge candidates have the same metadata. Two sub-rectangles are merge candidates if (r1.x1 == r2.x1 && r1.x2 == r2.x2) | (r1.y1 == r2.y1 && r1.y2 == r2.y2). If yes, combine them. Enumerate master region's sub-rectangles. Every rectangle returned is an optimized blt operation destination. The associated metadata is the blt operation`s source.

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  • iPhone SDK - How is data shared in a unversal app

    - by norskben
    Stack overflow I want to make a universal version of my app available, but I am wondering how is data managed between the iPad and the iPhone versions? -Are they completely independent? or if I have a plist in the iPad app, does it also appear in the iPhone app. If so, is there any syncing etc etc. I have a few months experience with single iPad or iPhone apps, but never a universal. Thanks again.

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  • UITableView Reusable cells

    - by Michael
    Can someone explain me the way reusable cells works for single table view? How many reusable cells a datasource should create? So far in all samples I've seen only one. Would one even need more?

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