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  • How does MySQL's ORDER BY RAND() work?

    - by Eugene
    Hi, I've been doing some research and testing on how to do fast random selection in MySQL. In the process I've faced some unexpected results and now I am not fully sure I know how ORDER BY RAND() really works. I always thought that when you do ORDER BY RAND() on the table, MySQL adds a new column to the table which is filled with random values, then it sorts data by that column and then e.g. you take the above value which got there randomly. I've done lots of googling and testing and finally found that the query Jay offers in his blog is indeed the fastest solution: SELECT * FROM Table T JOIN (SELECT CEIL(MAX(ID)*RAND()) AS ID FROM Table) AS x ON T.ID >= x.ID LIMIT 1; While common ORDER BY RAND() takes 30-40 seconds on my test table, his query does the work in 0.1 seconds. He explains how this functions in the blog so I'll just skip this and finally move to the odd thing. My table is a common table with a PRIMARY KEY id and other non-indexed stuff like username, age, etc. Here's the thing I am struggling to explain SELECT * FROM table ORDER BY RAND() LIMIT 1; /*30-40 seconds*/ SELECT id FROM table ORDER BY RAND() LIMIT 1; /*0.25 seconds*/ SELECT id, username FROM table ORDER BY RAND() LIMIT 1; /*90 seconds*/ I was sort of expecting to see approximately the same time for all three queries since I am always sorting on a single column. But for some reason this didn't happen. Please let me know if you any ideas about this. I have a project where I need to do fast ORDER BY RAND() and personally I would prefer to use SELECT id FROM table ORDER BY RAND() LIMIT 1; SELECT * FROM table WHERE id=ID_FROM_PREVIOUS_QUERY LIMIT 1; which, yes, is slower than Jay's method, however it is smaller and easier to understand. My queries are rather big ones with several JOINs and with WHERE clause and while Jay's method still works, the query grows really big and complex because I need to use all the JOINs and WHERE in the JOINed (called x in his query) sub request. Thanks for your time!

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  • Can I write for .Net 4 using VS2008?

    - by chillitom
    Can I write code against .Net 4 and its libraries using VS2008? If so how should I configure it? I realise that VS2008 will probably struggle with language changes such as the dynamic keyword but can I install the version 4 and target it anyway? My only real interest is in being able to use some of the new libraries, in particular I'm interested in the TPL (http://en.wikipedia.org/wiki/Parallel_Extensions)

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  • Programicaly measure size and way-order of L1 and L2 caches

    - by osgx
    Hello How can I measure programicaly (not query the OS, but measure) the size and order of associativity of L1 and L2 caches (data caches)? Assumtions about system: It has L1 and L2 cache (may be L3 too, may be cache sharing), It may have a hardware prefetch unit (just like P4+), it has a stable clocksource (tickcounter or good HPET for gettimeofday). There are no assumtions about OS (it can be Linux, Windows, smth non-standart), and we can't use posix queries. Language is C. And Compiler optimizations may be disabled.

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  • Listing like appstore in firefox

    - by jenso
    A little question about the listing in iphone os and some other applications, and possibly in android market. When listing the apps, it fills the list in an automatic process, when reaching or is near to reach the end. My question is: How is that written in iphone applications. But the main question is: Is that possible to make for firefox users with javascript, ajax or possibly any other language? out

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  • Django grappelli

    - by dzen
    Does anyone here use django-grappelli here ? I would like to read some experience of developers or users, if there are common mistake to avoid or why you use or do not use grappelli. Thanks for sharing

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  • Why can't I inherit static classes?

    - by User
    I have several classes that do not really need any state. From the organizational point of view, I would like to put them into hierarchy. But it seems I can't declare inheritance for static classes. Something like that: public static class Base { } public static class Inherited : Base { } will not work. Why have the designers of the language closed that possibility?

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  • Is Programming Right for me?

    - by L1th1um
    I'm interested in programming, but it seems to me that I can't get into it. Every time I've tried to learn a language and stuff by looking through tutorials or books I'd never get past the part where I use the syntax to make something. And by interest, I mean that I read stack overflow a lot, coding horror, and stuff but the actual coding part is hard for me to get into. Did anybody start this way? How did you get past this block?

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  • Looking for a free xsd scheme editor

    - by Klaim
    I'm looking for a free alternative to all the XML/XSD editors around here (that are relatively expensive--at least for me). I totally fail to find one. I need it to allow me to edit xsd files to help in writting an xml-based language specification. I need it to be visual to help with the design, making it clear. Other features are less interesting for me. Any suggestion?

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  • Using Castle DynamicProxy is it possible to change the invocation target on class proxy?

    - by Gareth D
    Hi Using Castle DynamicProxy v2, I'd like to change the target of an invocation for a class proxy. The new target is simply a different instance of the same type as the original target. The target types do not implement a common interface so I cannot use the IProxyTargetAccessor as detailed in Krzysztof's post on the subject - I cannot cast from a class proxy invocator to a IProxyTargetAccessor. Is there a way to do this?

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  • Can' get couchdb external http handlers to work.

    - by fuzzy lollipop
    following the instructions here http://wiki.apache.org/couchdb/ExternalProcesses this is what I get { * error: "{{badarg,[{erlang,port_command, [#Port<0.2056>, [123, [34,<<"info">>,34], 58, [123, [34,"db_name",34], 58, [34,<<"transfer_central">>,34], 44, [34,"doc_count",34], 58,"39441",44, [34,"doc_del_count",34], 58,"0",44, [34,"update_seq",34], 58,"56508",44, [34,"purge_seq",34], 58,"0",44, [34,"compact_running",34], 58,<<"false">>,44, [34,"disk_size",34], 58,"43593828",44, [34,"instance_start_time",34], 58, [34,<<"1272560477320483">>,34], 44, [34,"disk_format_version",34], 58,"5",125], 44, [34,<<"id">>,34], 58,<<"null">>,44, [34,<<"method">>,34], 58, [34,"GET",34], 44, [34,<<"path">>,34], 58, [91, [34,<<"transfer_central">>,34], 44, [34,<<"_test">>,34], 93], 44, [34,<<"query">>,34], 58,<<"{}">>,44, [34,<<"headers">>,34], 58, [123, [34,<<"Accept">>,34], 58, [34, <<"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8,application/json">>, 34], 44, [34,<<"Accept-Charset">>,34], 58, [34,<<"ISO-8859-1,utf-8;q=0.7,*;q=0.7">>,34], 44, [34,<<"Accept-Encoding">>,34], 58, [34,<<"gzip,deflate">>,34], 44, [34,<<"Accept-Language">>,34], 58, [34,<<"en-us,en;q=0.5">>,34], 44, [34,<<"Connection">>,34], 58, [34,<<"keep-alive">>,34], 44, [34,<<"Host">>,34], 58, [34,<<"127.0.0.1:5984">>,34], 44, [34,<<"Keep-Alive">>,34], 58, [34,<<"115">>,34], 44, [34,<<"User-Agent">>,34], 58, [34, <<"Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3">>, 34], 125], 44, [34,<<"body">>,34], 58, [34,"undefined",34], 44, [34,<<"peer">>,34], 58, [34,<<"127.0.0.1">>,34], 44, [34,<<"form">>,34], 58,<<"{}">>,44, [34,<<"cookie">>,34], 58,<<"{}">>,44, [34,<<"userCtx">>,34], 58, [123, [34,<<"db">>,34], 58, [34,<<"transfer_central">>,34], 44, [34,<<"name">>,34], 58,<<"null">>,44, [34,<<"roles">>,34], 58,<<"[]">>,125], 125,10]]}, {couch_os_process,writeline,2}, {couch_os_process,writejson,2}, {couch_os_process,handle_call,3}, {gen_server,handle_msg,5}, {proc_lib,init_p_do_apply,3}]}, {gen_server,call, [<0.110.0>, {prompt,{[{<<"info">>, {[{db_name,<<"transfer_central">>}, {doc_count,39441}, {doc_del_count,0}, {update_seq,56508}, {purge_seq,0}, {compact_running,false}, {disk_size,43593828}, {instance_start_time,<<"1272560477320483">>}, {disk_format_version,5}]}}, {<<"id">>,null}, {<<"method">>,'GET'}, {<<"path">>,[<<"transfer_central">>,<<"_test">>]}, {<<"query">>,{[]}}, {<<"headers">>, {[{<<"Accept">>, <<"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8,application/json">>}, {<<"Accept-Charset">>, <<"ISO-8859-1,utf-8;q=0.7,*;q=0.7">>}, {<<"Accept-Encoding">>,<<"gzip,deflate">>}, {<<"Accept-Language">>,<<"en-us,en;q=0.5">>}, {<<"Connection">>,<<"keep-alive">>}, {<<"Host">>,<<"127.0.0.1:5984">>}, {<<"Keep-Alive">>,<<"115">>}, {<<"User-Agent">>, <<"Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3">>}]}}, {<<"body">>,undefined}, {<<"peer">>,<<"127.0.0.1">>}, {<<"form">>,{[]}}, {<<"cookie">>,{[]}}, {<<"userCtx">>, {[{<<"db">>,<<"transfer_central">>}, {<<"name">>,null}, {<<"roles">>,[]}]}}]}}, infinity]}}" * reason: "{gen_server,call, [<0.109.0>, {execute,{[{<<"info">>, {[{db_name,<<"transfer_central">>}, {doc_count,39441}, {doc_del_count,0}, {update_seq,56508}, {purge_seq,0}, {compact_running,false}, {disk_size,43593828}, {instance_start_time,<<"1272560477320483">>}, {disk_format_version,5}]}}, {<<"id">>,null}, {<<"method">>,'GET'}, {<<"path">>,[<<"transfer_central">>,<<"_test">>]}, {<<"query">>,{[]}}, {<<"headers">>, {[{<<"Accept">>, <<"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8,application/json">>}, {<<"Accept-Charset">>, <<"ISO-8859-1,utf-8;q=0.7,*;q=0.7">>}, {<<"Accept-Encoding">>,<<"gzip,deflate">>}, {<<"Accept-Language">>,<<"en-us,en;q=0.5">>}, {<<"Connection">>,<<"keep-alive">>}, {<<"Host">>,<<"127.0.0.1:5984">>}, {<<"Keep-Alive">>,<<"115">>}, {<<"User-Agent">>, <<"Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3">>}]}}, {<<"body">>,undefined}, {<<"peer">>,<<"127.0.0.1">>}, {<<"form">>,{[]}}, {<<"cookie">>,{[]}}, {<<"userCtx">>, {[{<<"db">>,<<"transfer_central">>}, {<<"name">>,null}, {<<"roles">>,[]}]}}]}}, infinity]}" }

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  • What is the best way to format a date in JSON for Mongo DB storage

    - by Poul
    I have a date with a time. I'm using ruby, but the language shouldn't matter. d = "2010-04-01 13:00:00" What is the best way to format this date for Mongo DB? By 'best' I mean, is there a certain format I could use where Mongo would recognize it as a date and might give me more-advanced filtering optons? ie: If formatted correctly, could I ask Mongo to return all records whose month is '04'? Thanks!

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  • Select users in couchdb

    - by 2x2p1p
    Hi guys I have an HTML form for authentication of users, with SQL language is easier to extract data select name, password from users where name='nameField' and password='passwordField' In couchdb I cant use local views just temp views: curl -X POST -H 'Content-Type: application/json' -d '{"map": "function (doc) {if (doc.name === "nameField" && doc.password === "passwordField") {emit (doc.name, doc.passWord)}}"}' http://localhost:5984/somedb/_temp_view But it isnt recommended (Click here), what should I do ? :( Thanks

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  • How to copy subversion repository as a new directory to existing repository?

    - by Juha Syrjälä
    I have two existing subversion repositories on different hosts (host-a and host-b) and I'd like to copy one directory from repo A to repo B. Basically https://host-a/repo/some/path/moduleA should be copied to https://host-b/repo/some/other/path/moduleA. All the history should be preserved and existing data in host-b should be preserved. The two repositories do not have any conflicting directory hierarchies. The repositories do not share common ancestry.

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  • Downsides to using FakeWeb compared to writing mocks for testing

    - by ajmurmann
    I never liked writing mocks and a while ago someone here recommended to use FakeWeb. I immediately fell completely in love with FakeWeb. However, I have to wonder if there is a downside to using FakeWeb. It seems like mocks are still much more common, so I wonder what I am missing that's wrong with using FakeWeb instead. Is there a certain kind of error you can't cover with Fakeweb or is it something about the TDD or BDD process?

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  • Automatic bookkeeping for exception retries

    - by pilcrow
    Do any languages that support retry constructs in exception handling track and expose the number of times their catch/rescue (and/or try/begin) blocks have been executed in a particular run? I find myself counting (and limiting) the number of times a code block is re-executed after an exception often enough that this would be a handy language built-in.

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  • Will these optimizations to my Ruby implementation of diff improve performance in a Rails app?

    - by grg-n-sox
    <tl;dr> In source version control diff patch generation, would it be worth it to use the optimizations listed at the very bottom of this writing (see <optimizations>) in my Ruby implementation of diff for making diff patches? </tl;dr> <introduction> I am programming something I have never done before and there might already be tools out there to do the exact thing I am programming but at this point I am having too much fun to care so I am still going to do it from scratch, even if there is a tool for this. So anyways, I am working on a Ruby on Rails app and need a certain feature. Basically I want each entry in a table of mine, let's say for example a table of video games, to have a stored chunk of text that represents a review or something of the sort for that table entry. However, I want this text to be both editable by any registered user and also keep track of different submissions in a version control system. The simplest solution I could think of is just implement a solution that keeps track of the text body and the diff patch history of different versions of the text body as objects in Ruby and then serialize it, preferably in human readable form (so I'll most likely use YAML for this) for editing if needed due to corruption by a software bug or a mistake is made by an admin doing some version editing. So at first I just tried to dive in head first into this feature to find that the problem of generating a diff patch is more difficult that I thought to do efficiently. So I did some research and came across some ideas. Some I have implemented already and some I have not. However, it all pretty much revolves around the longest common subsequence problem, as you would already know if you have already done anything with diff or diff-like features, and optimization the function that solves it. Currently I have it so it truncates the compared versions of the text body from the beginning and end until non-matching lines are found. Then it solves the problem using a comparison matrix, but instead of incrementing the value stored in a cell when it finds a matching line like in most longest common subsequence algorithms I have seen examples of, I increment when I have a non-matching line so as to calculate edit distance instead of longest common subsequence. Although as far as I can tell between the two approaches, they are essentially two sides of the same coin so either could be used to derive an answer. It then back-traces through the comparison matrix and notes when there was an incrementation and in which adjacent cell (West, Northwest, or North) to determine that line's diff entry and assumes all other lines to be unchanged. Normally I would leave it at that, but since this is going into a Rails environment and not just some stand-alone Ruby script, I started getting worried about needing to optimize at least enough so if a spammer that somehow knew how I implemented the version control system and knew my worst case scenario entry still wouldn't be able to hit the server that bad. After some searching and reading of research papers and articles through the internet, I've come across several that seem decent but all seem to have pros and cons and I am having a hard time deciding how well in this situation that the pros and cons balance out. So are the ones listed here worth it? I have listed them with known pros and cons. </introduction> <optimizations> Chop the compared sequences into multiple chucks of subsequences by splitting where lines are unchanged, and then truncating each section of unchanged lines at the beginning and end of each section. Then solve the edit distance of each subsequence. Pro: Changes the time increase as the changed area gets bigger from a quadratic increase to something more similar to a linear increase. Con: Figuring out where to split already seems like you have to solve edit distance except now you don't care how it is changed. Would be fine if this was solvable by a process closer to solving hamming distance but a single insertion would throw this off. Use a cryptographic hash function to both convert all sequence elements into integers and ensure uniqueness. Then solve the edit distance comparing the hash integers instead of the sequence elements themselves. Pro: The operation of comparing two integers is faster than the operation of comparing two strings, so a slight performance gain is received after every comparison, which can be a lot overall. Con: Using a cryptographic hash function takes time to convert all the sequence elements and may end up costing more time to do the conversion that you gain back from the integer comparisons. You could use the built in hash function for a string but that will not guarantee uniqueness. Use lazy evaluation to only calculate the three center-most diagonals of the comparison matrix and then only calculate additional diagonals as needed. And then also use this approach to possibly remove the need on some comparisons to compare all three adjacent cells as desribed here. Pro: Can turn an algorithm that always takes O(n * m) time and make it so only worst case scenario is that time, best case becomes practically linear, and average case is somewhere between the two. Con: It is an algorithm I've only seen implemented in functional programming languages and I am having a difficult time comprehending how to convert this into Ruby based on how it is described at the site linked to above. Make a C module and do the hard work at the native level in C and just make a Ruby wrapper for it so Ruby can make all the calls to it that it needs. Pro: I have to imagine that evaluating something like this in could be a LOT faster. Con: I have no idea how Rails handles apps with ruby code that has C extensions and it hurts the portability of the app. This is an optimization for after the solving of edit distance, but idea is to store additional combined diffs with the ones produced by each version to make a delta-tree data structure with the most recently made diff as the root node of the tree so getting to any version takes worst case time of O(log n) instead of O(n). Pro: Would make going back to an old version a lot faster. Con: It would mean every new commit, the delta-tree would get a new root node that will cost time to reorganize the delta-tree for an operation that will be carried out a lot more often than going back a version, not to mention the unlikelihood it will be an old version. </optimizations> So are these things worth the effort?

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  • What is the best service/tool to put short audio clips on a website so users can click and listen im

    - by Edward Tanguay
    I'm making a foreign language flashcard website in which I want to have 100s of short 3-10 second audio files available for users to click and listen. So I am looking for a tool/service such as YouTube or Screenr.com but for audio which e.g.: allows me to easily upload multiple kinds of audio files: mp3, wav, etc. easy to manage them online (delete, replace) has a simple, small player (e.g. flash) that integrates nicely into any site

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