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• What is a data structure for quickly finding non-empty intersections of a list of sets?

  - by Andrey Fedorov

  I have a set of N items, which are sets of integers, let's assume it's ordered and call it I[1..N]. Given a candidate set, I need to find the subset of I which have non-empty intersections with the candidate. So, for example, if: I = [{1,2}, {2,3}, {4,5}] I'm looking to define valid_items(items, candidate), such that: valid_items(I, {1}) == {1} valid_items(I, {2}) == {1, 2} valid_items(I, {3,4}) == {2, 3} I'm trying to optimize for one given set I and a variable candidate sets. Currently I am doing this by caching items_containing[n] = {the sets which contain n}. In the above example, that would be: items_containing = [{}, {1}, {1,2}, {2}, {3}, {3}] That is, 0 is contained in no items, 1 is contained in item 1, 2 is contained in itmes 1 and 2, 2 is contained in item 2, 3 is contained in item 2, and 4 and 5 are contained in item 3. That way, I can define valid_items(I, candidate) = union(items_containing[n] for n in candidate). Is there any more efficient data structure (of a reasonable size) for caching the result of this union? The obvious example of space 2^N is not acceptable, but N or N*log(N) would be.

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• question about MySQL database migration

  - by WilliamLou

  Hi there: If I have a MySQL database with several tables on a live server, now I would like to migrate this database to another server. Of course, the migration I mean here involves some database tables, for example: add some new columns to several tables, add some new tables etc.. Now, the only method I can think of is to use some php/python(two scripts I know) script, connect two databases, dump the data from the old database, and then write into the new database. However, this method is not efficient at all. For example: in old database, table A has 28 columns; in new database, table A has 29 columns, but the extra column will have default value 0 for all the old rows. My script still needs to dump the data row by row and insert each row into the new database. Is there any tools or a better method than writing a script yourself? Here, I dont need to worry about multithread writing problems etc.., I mean the old database will be down (not open to public usage etc.., only for upgrade ) for a while. Thanks!!

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• C# String.Replace with a start/index (Added my (slow) implementation)

  - by Chris T

  I'd like an efficient method that would work something like this EDIT: Sorry I didn't put what I'd tried before. I updated the example now. // Method signature, Only replaces first instance or how many are specified in max public int MyReplace(ref string source,string org, string replace, int start, int max) { int ret = 0; int len = replace.Length; int olen = org.Length; for(int i = 0; i < max; i++) { // Find the next instance of the search string int x = source.IndexOf(org, ret + olen); if(x > ret) ret = x; else break; // Insert the replacement source = source.Insert(x, replace); // And remove the original source = source.Remove(x + len, olen); // removes original string } return ret; } string source = "The cat can fly but only if he is the cat in the hat"; int i = MyReplace(ref source,"cat", "giraffe", 8, 1); // Results in the string "The cat can fly but only if he is the giraffe in the hat" // i contains the index of the first letter of "giraffe" in the new string The only reason I'm asking is because my implementation I'd imagine getting slow with 1,000s of replaces.

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• Best way to randomly select rows *per* column in SQL Server

  - by LesterDove

  A search of SO yields many results describing how to select random rows of data from a database table. My requirement is a bit different, though, in that I'd like to select individual columns from across random rows in the most efficient/random/interesting way possible. To better illustrate: I have a large Customers table, and from that I'd like to generate a bunch of fictitious demo Customer records that aren't real people. I'm thinking of just querying randomly from the Customers table, and then randomly pairing FirstNames with LastNames, Address, City, State, etc. So if this is my real Customer data (simplified): FirstName LastName State ========================== Sally Simpson SD Will Warren WI Mike Malone MN Kelly Kline KS Then I'd generate several records that look like this: FirstName LastName State ========================== Sally Warren MN Kelly Malone SD Etc. My initial approach works, but it lacks the elegance that I'm hoping the final answer will provide. (I'm particularly unhappy with the repetitiveness of the subqueries, and the fact that this solution requires a known/fixed number of fields and therefore isn't reusable.) SELECT FirstName = (SELECT TOP 1 FirstName FROM Customer ORDER BY newid()), LastName= (SELECT TOP 1 LastNameFROM Customer ORDER BY newid()), State = (SELECT TOP 1 State FROM Customer ORDER BY newid()) Thanks!

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• Implementing Model-level caching

  - by Byron

  I was posting some comments in a related question about MVC caching and some questions about actual implementation came up. How does one implement a Model-level cache that works transparently without the developer needing to manually cache, yet still remains efficient? I would keep my caching responsibilities firmly within the model. It is none of the controller's or view's business where the model is getting data. All they care about is that when data is requested, data is provided - this is how the MVC paradigm is supposed to work. (Source: Post by Jarrod) The reason I am skeptical is because caching should usually not be done unless there is a real need, and shouldn't be done for things like search results. So somehow the Model itself has to know whether or not the SELECT statement being issued to it worthy of being cached. Wouldn't the Model have to be astronomically smart, and/or store statistics of what is being most often queried over a long period of time in order to accurately make a decision? And wouldn't the overhead of all this make the caching useless anyway? Also, how would you uniquely identify a query from another query (or more accurately, a resultset from another resultset)? What about if you're using prepared statements, with only the parameters changing according to user input? Another poster said this: I would suggest using the md5 hash of your query combined with a serialized version of your input arguments. This would require twice the number of serialization options. I was under the impression that serialization was quite expensive, and for large inputs this might be even worse than just re-querying. And is the minuscule chance of collision worth worrying about? Conceptually, caching in the Model seems like a good idea to me, but it seems in practicality the developer should have direct control over caching and write it into the controller. Thoughts/ideas? Edit: I'm using PHP and MySQL if that helps to narrow your focus.

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• What is the best database structure for this scenario?

  - by Ricketts

  I have a database that is holding real estate MLS (Multiple Listing Service) data. Currently, I have a single table that holds all the listing attributes (price, address, sqft, etc.). There are several different property types (residential, commercial, rental, income, land, etc.) and each property type share a majority of the attributes, but there are a few that are unique to that property type. My question is the shared attributes are in excess of 250 fields and this seems like too many fields to have in a single table. My thought is I could break them out into an EAV (Entity-Attribute-Value) format, but I've read many bad things about that and it would make running queries a real pain as any of the 250 fields could be searched on. If I were to go that route, I'd literally have to pull all the data out of the EAV table, grouped by listing id, merge it on the application side, then run my query against the in memory object collection. This also does not seem very efficient. I am looking for some ideas or recommendations on which way to proceed. Perhaps the 250+ field table is the only way to proceed. Just as a note, I'm using SQL Server 2012, .NET 4.5 w/ Entity Framework 5, C# and data is passed to asp.net web application via WCF service. Thanks in advance.

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• FIFO dequeueing in python?

  - by Aaron Ramsey

  hello again everybody— I'm looking to make a functional (not necessarily optimally efficient, as I'm very new to programming) FIFO queue, and am having trouble with my dequeueing. My code looks like this: class QueueNode: def __init__(self, data): self.data = data self.next = None def __str__(self): return str(self.data) class Queue: def__init__(self): self.front = None self.rear = None self.size = 0 def enqueue(self, item) newnode = QueueNode(item) newnode.next = None if self.size == 0: self.front = self.rear = newnode else: self.rear = newnode self.rear.next = newnode.next self.size = self.size+1 def dequeue(self) dequeued = self.front.data del self.front self.size = self.size-1 if self.size == 0: self.rear = None print self.front #for testing if I do this, and dequeue an item, I get the error "AttributeError: Queue instance has no attribute 'front'." I guess my function doesn't properly assign the new front of the queue? I'm not sure how to fix it though. I don't really want to start from scratch, so if there's a tweak to my code that would work, I'd prefer that—I'm not trying to minimize runtime so much as just get a feel for classes and things of that nature. Thanks in advance for any help, you guys are the best.

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• Finding Common Phrases in MS SQL TEXT Column

  - by regex

  Hello All, Short Desc: I'm curious to see if I can use SQL Analysis services or some other MS SQL service to mine some data for me that will show commonalities between SQL TEXT fields in a dataset. Long Desc I am looking at a subset of data that consists of about 10,000 rows of TEXT blobs which are used as a notes column in a issue tracking (ticketing) software. I would like to use something out of the box (without having to build something) that might be able to parse through all of the rows and find commonly used byte sequences in the "Notes" column. In other words, I want to find commonly used phrases (two to three word phrases, so 9 - 20 character sections of the TEXT blob). This will help me better determine if associate's notes contain similar phrases (troubleshooting techniques) that we could standardize in our troubleshooting process flow. Closing Note I'd really rather not build an application to do this as my method will probably not be the most efficient way to do it. Hopefully all this makes sense. Please let me know in the comments if anything needs clarification. Thanks in advance for your help.

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• Simple description of worker and I/O threads in .NET

  - by Konstantin

  It's very hard to find detailed but simple description of worker and I/O threads in .NET What's clear to me regarding this topic (but may not be technically precise): Worker threads are threads that should employ CPU for their work; I/O threads (also called "completion port threads") should employ device drivers for their work and essentially "do nothing", only monitor the completion of non-CPU operations. What is not clear: Although method ThreadPool.GetAvailableThreads returns number of available threads of both types, it seems there is no public API to schedule work for I/O thread. You can only manually create worker thread in .NET? It seems that single I/O thread can monitor multiple I/O operations. Is it true? If so, why ThreadPool has so many available I/O threads by default? In some texts I read that callback, triggered after I/O operation completion is performed by I/O thread. Is it true? Isn’t this a job for worker thread, considering that this callback is CPU operation? To be more specific – do ASP.NET asynchronous pages user I/O threads? What exactly is performance benefit in switching I/O work to separate thread instead of increasing maximum number of worker threads? Is it because single I/O thread does monitor multiple operations? Or Windows does more efficient context switching when using I/O threads?

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• Database design and foreign keys: Where should they be added in related tables?

  - by Carvell Fenton

  I have a relatively simple subset of tables in my database for tracking something called sessions. These are academic sessions (think offerings of a particular program). The tables to represent a sessions information are: sessions session_terms session_subjects session_mark_item_info session_marks All of these tables have their own primary keys, and are like a tree, in that sessions have terms, terms have subjects, subjects have mark items, etc. So each on would have at least its "parent's" foreign key. My question is, design wise is it a good idea to include the sessions primary key in the other tables as a foreign key to easily select related session items, or is that too much redundency? If I include the session foreign key (or all parent foreign keys from tables up the heirarchy) in all the tables, I can easily select all the marks for a session. As an example, something like SELECT mark FROM session_marks WHERE sessionID=... If I don't, then I would have to combine selects with something like WHERE something IN (SELECT... Which approach is "more correct" or efficient? Thanks in advance!

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• PHP templating challenge (optimizing front-end templates)

  - by Matt

  Hey all, I'm trying to do some templating optimizations and I'm wondering if it is possible to do something like this: function table_with_lowercase($data) { $out = '<table>'; for ($i=0; $i < 3; $i++) { $out .= '<tr><td>'; $out .= strtolower($data); $out .= '</td></tr>'; } $out .= "</table>"; return $out; } NOTE: You do not know what $data is when you run this function. Results in: <table> <tr><td><?php echo strtolower($data) ?></td></tr> <tr><td><?php echo strtolower($data) ?></td></tr> <tr><td><?php echo strtolower($data) ?></td></tr> </table> General Case: Anything that can be evaluated (compiled) will be. Any time there is an unknown variable, the variable and the functions enclosing it, will be output in a string format. Here's one more example: function capitalize($str) { return ucwords(strtolower($str)); } If $str is "HI ALL" then the output is: Hi All If $str is unknown then the output is: <?php echo ucwords(strtolower($str)); ?> In this case it would be easier to just call the function (ie. <?php echo capitalize($str) ?> ), but the example before would allow you to precompile your PHP to make it more efficient

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• PHP templating challenge (optimizing front-end templates)

  - by Matt

  Hey all, I'm trying to do some templating optimizations and I'm wondering if it is possible to do something like this: function table_with_lowercase($data) { $out = '<table>'; for ($i=0; $i < 3; $i++) { $out .= '<tr><td>'; $out .= strtolower($data); $out .= '</td></tr>'; } $out .= "</table>"; return $out; } NOTE: You do not know what $data is when you run this function. Results in: <table> <tr><td><?php echo strtolower($data) ?></td></tr> <tr><td><?php echo strtolower($data) ?></td></tr> <tr><td><?php echo strtolower($data) ?></td></tr> </table> General Case: Anything that can be evaluated (compiled) will be. Any time there is an unknown variable, the variable and the functions enclosing it, will be output in a string format. Here's one more example: function capitalize($str) { return ucwords(strtolower($str)); } If $str is "HI ALL" then the output is: Hi All If $str is unknown then the output is: <?php echo ucwords(strtolower($str)); ?> In this case it would be easier to just call the function (ie. <?php echo capitalize($str) ?> ), but the example before would allow you to precompile your PHP to make it more efficient

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• Passing data between ViewControllers versus doing local Fetch in each VC

  - by Tofrizer

  Hi All, I'm developing an iPhone app using Core Data and I'm looking for some general advice and recommendations on whether its acceptable to pass data between ViewControllers versus doing a local fetch in each ViewController as you navigate to it. Ordinarily I would say it all depends on various factors (e.g. performance etc) but the passing data approach is so prevalent in my app and I'm spooked by all the stories about Apple rejecting apps because of not conforming to their standard guidelines. So let me put another way -- is it non-standard to pass data between VC's? The reason I pass data so much is because each ViewController is just another view on to data present in my object model / graph. Once I have a handle on my first object in the first view controller (which I of course do have to fetch), I can use the existing object composition / relationships to drill down into the next level of detail into data and so I just pass these objects to the next VC. Separately, one possible downside with this passing-data-to-each-VC approach is I don't benefit from (what I perceive to be) the optimisation/benefits that NSFetchedResultsController provides in terms of efficient memory usage and section handling. My app is read-only but I do have one table with 5000 rows and I'm curious if I am missing out on NSFetchedResultsController benefits. Any thoughts on this as well? Can I somehow still benefit from NSFetchedResultsController goodness without having to do a full fetch (as I would have already passed in the data from my previous VC)? Thanks a lot.

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• Automatic tracking algorithm

  - by nico

  Hi everyone, I'm trying to write a simple tracking routine to track some points on a movie. Essentially I have a series of 100-frames-long movies, showing some bright spots on dark background. I have ~100-150 spots per frame, and they move over the course of the movie. I would like to track them, so I'm looking for some efficient (but possibly not overkilling to implement) routine to do that. A few more infos: the spots are a few (es. 5x5) pixels in size the movement are not big. A spot generally does not move more than 5-10 pixels from its original position. The movements are generally smooth. the "shape" of these spots is generally fixed, they don't grow or shrink BUT they become less bright as the movie progresses. the spots don't move in a particular direction. They can move right and then left and then right again the user will select a region around each spot and then this region will be tracked, so I do not need to automatically find the points. As the videos are b/w, I though I should rely on brigthness. For instance I thought I could move around the region and calculate the correlation of the region's area in the previous frame with that in the various positions in the next frame. I understand that this is a quite naïve solution, but do you think it may work? Does anyone know specific algorithms that do this? It doesn't need to be superfast, as long as it is accurate I'm happy. Thank you nico

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• Java: split a List into two sub-Lists?

  - by Chris Conway

  What's the simplest, most standard, and/or most efficient way to split a List into two sub-Lists in Java? It's OK to mutate the original List, so no copying should be necessary. The method signature could be /** Split a list into two sublists. The original list will be modified to * have size i and will contain exactly the same elements at indices 0 * through i-1 as it had originally; the returned list will have size * len-i (where len is the size of the original list before the call) * and will have the same elements at indices 0 through len-(i+1) as * the original list had at indices i through len-1. */ <T> List<T> split(List<T> list, int i); [EDIT] List.subList returns a view on the original list, which becomes invalid if the original is modified. So split can't use subList unless it also dispenses with the original reference (or, as in Marc Novakowski's answer, uses subList but immediately copies the result).

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• Running an existing LINQ query against a dynamic object (DataTable like)

  - by TomTom

  Hello, I am working on a generic OData provider to go against a custom data provider that we have here. Thsi is fully dynamic in that I query the data provider for the table it knows. I have a basic storage structure in place so far based on the OData sample code. My problem is: OData supports queries and expects me to hand in an IQueryable implementation. On the lowe rside, I dont have any query support. Not a joke - the provider returns tables and the WHERE clause is not supported. Performance is not an issue here - the tables are small. It is ok to sort them in the OData provider. My main problem is this. I submit a SQL statement to get out the data of a table. The result is some sort of ADO.NET data reader here. I need to expose an IQueryable implementation for this data to potentially allow later filtering. Any ide ahow to best touch that? .NET 3.5 only (no 4.0 planned for some time). I was seriously thinking of creating dynamic DTO classes for every table (emitting bytecode) so I can use standard LINQ. Right now I am using a dictionary per entry (not too efficient) but I see no real way to filter / sort based on them.

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• How to do this Python / MySQL manipulation (match) more efficiently?

  - by NJTechie

  Following is my data : Company Table : ID Company Address City State Zip Phone 1 ABC 123 Oak St Philly PA 17542 7329878901 2 CDE 111 Joe St Newark NJ 08654 3 GHI 211 Foe St Brick NJ 07740 7321178901 4 JAK 777 Wall Ocean NJ 07764 7322278901 5 KLE 87 Ilk St Plains NY 07654 7376578901 6 AB 1 W.House SField PA 87656 7329878901 Branch Office Table : ID Address City State Zip Phone 1 323 Alk St Philly PA 17542 7329832221 1 171 Joe St Newark NJ 08654 3 287 Foe St Brick NJ 07740 7321178901 3 700 Wall Ocean NJ 07764 7322278901 1 89 Blk St Surrey NY 07154 7376222901 File to be Matched (In MySQL): ID Company Address City State Zip Phone 1 ABC 123 Oak St Philly PA 17542 7329878901 2 AB 171 Joe St Newark NJ 08654 3 GHI 211 Foe St Brick NJ 07740 7321178901 4 JAK 777 Wall Ocean NJ 07764 7322278901 5 K 87 Ilk St Plains NY 07654 7376578901 Resulting File : ID Company Address City State Zip Phone appendedID 1 ABC 123 Oak St Philly PA 17542 7329878901 [Original record, field always empty] 1 ABC 171 Joe St Newark NJ 08654 1 [Company Table] 1 ABC 323 Alk St Philly PA 17542 7329832221 1 [Branch Office Table] 1 AB 1 W.House SField PA 87656 7329878901 6 [Partial firm and State, Zip match] 2 CDE 111 Joe St Newark NJ 08654 3 GHI 211 Foe St Brick NJ 07740 7321178901 3 GHI 700 Wall Ocean NJ 07764 7322278901 3 3 GHI 287 Foe St Brick NJ 07740 7321178901 3 4 JAK 777 Wall Ocean NJ 07764 7322278901 5 KLE 87 Ilk St Surrey NY 07654 7376578901 5 KLE 89 Blk St Surrey NY 07154 7376222901 5 Requirement : 1) I have to match each firm on the 'File to be Matched' to that of Company and Branch Office tables (MySQL). 2) If there are multiple exact/partial matches, then the ID from Company, Branch Office table is inserted as a new row in the resulting file. 3) Not all the firms will be matched perfectly, in that case I have to match on partial Company names (like 5/8th of the company name) and any of the address fields and insert them in the resulting file. Please help me out in the most efficient solution for this problem.

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• tips for fixing bad coding/dev habits ?

  - by dfafa

  i want to become a better coder....so i have decided to sign up for computing science program...maybe a formal education can assist me. i started working on smaller projects to learn but currently i have really bad coding/dev habits which is hindering my productivity as the codebase increases.... i have highlighted them and perhaps someone could make suggestions (or redirect to resources) or a more efficient method. most stuff that i made in the past were web apps. i usually develop with putty + nano...i just love the minimalist feel i use winscp and develop directly on my private web server...too lazy to do it on localhost and upload it later. i dont use subversion control...which one do i need ? sometimes ctrl +z doesn't work well. when i run out of ideas for naming variable, i use swear words instead. i swear a lot when i get stuck....how to deal with anger issue ? my codes look ugly with comments everywhere. would rather use procedural coding finds "thinking" in OO difficult and time consuming i "write first think later". refactors code only if i am getting paid for it. dislikes configuring linux distro, Apache, MySQL, scaling, designing graphics and layouts. does not like writing tests likes working alone. does not like sharing codes. has an econ degree dislikes reading other people's code would rather write it on my own it seems my only true desire is to translate my ideas to a working prototype as fast as possible....it seems like i am very uninterested in the other details...could it be that i am not cut out to be a coder after all ? is going back to study comp sci a bad idea ?

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• Help making userscript work in chrome

  - by Vishal Shah

  I've written a userscript for Gmail Pimp.my.Gmail & i'd like it to be compatible with Google Chrome too. Now i have tried a couple of things, to the best of my Javascript knowledge (which is very weak) & have been successful up-to a certain extent, though im not sure if it's the right way. Here's what i tried, to make it work in Chrome: The very first thing i found is that contentWindow.document doesn't work in chrome, so i tried contentDocument, which works. BUT i noticed one thing, checking the console messages in Firefox and Chrome, i saw that the script gets executed multiple times in Firefox whereas in Chrome it just executes once! So i had to abandon the window.addEventListener('load', init, false); line and replace it with window.setTimeout(init, 5000); and i'm not sure if this is a good idea. The other thing i tried is keeping the window.addEventListener('load', init, false); line and using window.setTimeout(init, 1000); inside init() in case the canvasframe is not found. So please do lemme know what would be the best way to make this script cross-browser compatible. Oh and im all ears for making this script better/efficient code wise (which is sure there is)

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• Reversing permutation of an array in Java efficiently

  - by HansDampf

  Okay, here is my problem: Im implementing an algorithm in Java and part of it will be following: The Question is to how to do what I will explain now in an efficient way. given: array a of length n integer array perm, which is a permutation of [1..n] now I want to shuffle the array a, using the order determined by array perm, i.e. a=[a,b,c,d], perm=[2,3,4,1] ------ shuffledA[b,c,d,a], I figured out I can do that by iterating over the array with: shuffledA[i]=a[perm[i-1]], (-1 because the permutation indexes in perm start with 1 not 0) Now I want to do some operations on shuffledA... And now I want to do the reverse the shuffle operation. This is where I am not sure how to do it. Note that a can hold an item more than once, i.e. a=[a,a,a,a] If that was not the case, I could iterate perm, and find the corresponding indexes to the values. Now I thought that using a Hashmap instead of the the perm array will help. But I am not sure if this is the best way to do.

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• SSIS - Bulk Update at Database Field Level

  - by Adam

  Hello, Here's our mission: Receive files from clients. Each file contains anywhere from 1 to 1,000,000 records. Records are loaded to a staging area and business-rule validation is applied. Valid records are then pumped into an OLTP database in a batch fashion, with the following rules: If record does not exist (we have a key, so this isn't an issue), create it. If record exists, optionally update each database field. The decision is made based on one of 3 factors...I don't believe it's important what those factors are. Our main problem is finding an efficient method of optionally updating the data at a field level. This is applicable across ~12 different database tables, with anywhere from 10 to 150 fields in each table (original DB design leaves much to be desired, but it is what it is). Our first attempt has been to introduce a table that mirrors the staging environment (1 field in staging for each system field) and contains a masking flag. The value of the masking flag represents the 3 factors. We've then put an UPDATE similar to... UPDATE OLTPTable1 SET Field1 = CASE WHEN Mask.Field1 = 0 THEN Staging.Field1 WHEN Mask.Field1 = 1 THEN COALESCE( Staging.Field1 , OLTPTable1.Field1 ) WHEN Mask.Field1 = 2 THEN COALESCE( OLTPTable1.Field1 , Staging.Field1 ) ... As you can imagine, the performance is rather horrendous. Has anyone tackled a similar requirement? We're a MS shop using a Windows Service to launch SSIS packages that handle the data processing. Unfortunately, we're pretty much novices at this stuff.

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• Comparing all values within a List against each other

  - by Kave

  I am a bit stuck here and can't think further. public struct CandidateDetail { public int CellX { get; set; } public int CellY { get; set; } public int CellId { get; set; } } var dic = new Dictionary<int, List<CandidateDetail>>(); How can I compare each CandidateDetail item against other CandidateDetail items within the same dictionary in the most efficient way? Example: There are three keys for the dictionary: 5, 6 and 1. Therefore we have three entries. now each of these key entries would have a List associated with. In this case let say each of these three numbers has exactly two CandidateDetails items within the list associated to each key. This means in other words we have two 5, two 6 and two 1 in different or in the same cells. I would like to know: if[5].1stItem.CellId == [6].1stItem.CellId = we got a hit. That means we have a 5 and a 6 within the same Cell if[5].2ndItem.CellId == [6].2ndItem.CellId = perfect. We found out that the other 5 and 6 are together within a different cell. if[1].1stItem.CellId == ... Now I need to check the 1 also against the other 5 and 6 to see if the one exists within the previous same two cells or not. Could a Linq expression help perhaps? I am quite stuck here... I don't know...Maybe I am taking the wrong approach. I am trying to solve the "Hidden pair" of the game Sudoku. :) http://www.sudokusolver.eu/ExplainSolveMethodD.aspx Many Thanks, Kave

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• How to avoid multiple, unused has_many associations when using multiple models for the same entity (

  - by mikep

  Hello, I'm looking for a nice, Ruby/Rails-esque solution for something. I'm trying to split up some data using multiple tables, rather than just using one gigantic table. My reasoning is pretty much to try and avoid the performance drop that would come with having a big table. So, rather than have one table called books, I have multiple tables: books1, books2, books3, etc. (I know that I could use a partition, but, for now, I've decided to go the 'multiple tables' route.) Each user has their books placed into a specific table. The actual book table is chosen when the user is created, and all of their books go into the same table. The goal is to try and keep each table pretty much even -- but that's a different issue. One thing I don't particularly want to have is a bunch of unused associations in the User class. Right now, it looks like I'd have to do the following: class User < ActiveRecord::Base has_many :books1, :books2, :books3, :books4, :books5 end class Books1 < ActiveRecord::Base belongs_to :user end class Books2 < ActiveRecord::Base belongs_to :user end First off, for each specific user, only one of the book tables would be usable/applicable, since all of a user's books are stored in the same table. So, only one of the associations would be in use at any time and any other has_many :bookX association that was loaded would be a waste. I don't really know Ruby/Rails does internally with all of those has_many associations though, so maybe it's not so bad. But right now I'm thinking that it's really wasteful, and that there may just be a better, more efficient way of doing this. Is there's some sort of special Ruby/Rails methodology that could be applied here to avoid having to have all of those has_many associations? Also, does anyone have any advice on how to abstract the fact that there's multiple book tables behind a single books model/class?

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• Reading text files line by line, with exact offset/position reporting

  - by Benjamin Podszun

  Hi. My simple requirement: Reading a huge ( a million) line test file (For this example assume it's a CSV of some sorts) and keeping a reference to the beginning of that line for faster lookup in the future (read a line, starting at X). I tried the naive and easy way first, using a StreamWriter and accessing the underlying BaseStream.Position. Unfortunately that doesn't work as I intended: Given a file containing the following Foo Bar Baz Bla Fasel and this very simple code using (var sr = new StreamReader(@"C:\Temp\LineTest.txt")) { string line; long pos = sr.BaseStream.Position; while ((line = sr.ReadLine()) != null) { Console.Write("{0:d3} ", pos); Console.WriteLine(line); pos = sr.BaseStream.Position; } } the output is: 000 Foo 025 Bar 025 Baz 025 Bla 025 Fasel I can imagine that the stream is trying to be helpful/efficient and probably reads in (big) chunks whenever new data is necessary. For me this is bad.. The question, finally: Any way to get the (byte, char) offset while reading a file line by line without using a basic Stream and messing with \r \n \r\n and string encoding etc. manually? Not a big deal, really, I just don't like to build things that might exist already..

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• Best way to randomly select columns from random rows of SQL results.

  - by LesterDove

  A search of SO yields many results describing how to select random rows of data from a database table. My requirement is a bit different, though, in that I'd like to select individual columns from across random rows in the most efficient/random/interesting way possible. To better illustrate: I have a large Customers table, and from that I'd like to generate a bunch of fictitious demo Customer records that aren't real people. I'm thinking of just querying randomly from the Customers table, and then randomly pairing FirstNames with LastNames, Address, City, State, etc. So if this is my real Customer data (simplified): FirstName LastName State ========================== Sally Simpson SD Will Warren WI Mike Malone MN Kelly Kline KS Then I'd generate several records that look like this: FirstName LastName State ========================== Sally Warren MN Kelly Malone SD Etc. My initial approach works, but it lacks the elegance that I'm hoping the final answer will provide. (I'm particularly unhappy with the repetitiveness of the subqueries, and the fact that this solution requires a known/fixed number of fields and therefore isn't reusable.) SELECT FirstName = (SELECT TOP 1 FirstName FROM Customer ORDER BY newid()), LastName= (SELECT TOP 1 LastNameFROM Customer ORDER BY newid()), State = (SELECT TOP 1 State FROM Customer ORDER BY newid()) Thanks!

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