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  • Most efficient way to LIMIT results in a JOIN?

    - by johnnietheblack
    I have a fairly simple one-to-many type join in a MySQL query. In this case, I'd like to LIMIT my results by the left table. For example, let's say I have an accounts table and a comments table, and I'd like to pull 100 rows from accounts and all the associated comments rows for each. Thy only way I can think to do this is with a sub-select in in the FROM clause instead of simply selecting FROM accounts. Here is my current idea: SELECT a.*, c.* FROM (SELECT * FROM accounts LIMIT 100) a LEFT JOIN `comments` c on c.account_id = a.id ORDER BY a.id However, whenever I need to do a sub-select of some sort, my intermediate level SQL knowledge feels like it's doing something wrong. Is there a more efficient, or faster, way to do this, or is this pretty good? By the way... This might be the absolute simplest way to do this, which I'm okay with as an answer. I'm simply trying to figure out if there IS another way to do this that could potentially compete with the above statement in terms of speed.

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  • Counting context switches per thread

    - by Sarmun
    Is there a way to see how many context switches each thread generates? (both in and out if possible) Either in X/s, or to let it run and give aggregated data after some time. (either on linux or on windows) I have found only tools that give aggregated context-switching number for whole os or per process. My program makes many context switches (50k/s), probably a lot not necessary, but I am not sure where to start optimizing, where do most of those happen.

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  • To create new DB connection or not?

    - by Yeti
    I'm running a cron job (every 15 minutes) which takes about a minute to execute. It makes lots of API calls and stores data to the database. Right now I create a mysql connection at the beginning and use the same connection through out the code. Most of the time is spent making the API calls. Will it be more efficient to create a new database connection only when it's time to store the data (below)? Kill the last connection Wait for API call to complete Create new DB connection Execute query Goto 1

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  • Compile Flex application without debug? Optimisation options for flex compiler?

    - by maoanz
    I have created a simple test application with the following code var i : int; for (i=0; i<3000000; i++){ trace(i); } When I run the application, it's very slow to load, which means the "trace" is running. I check the flash player by right-clicking, the debugger option is not enable. So I wonder if there is an option to put in compiler to exclude the trace. Otherwise, I have to remove manually all the trace in the program. Are there any other options of compiler to optimize the flex application in a maximum way? Thanks

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  • C++ pimpl idiom wastes an instruction vs. C style?

    - by Rob
    (Yes, I know that one machine instruction usually doesn't matter. I'm asking this question because I want to understand the pimpl idiom, and use it in the best possible way; and because sometimes I do care about one machine instruction.) In the sample code below, there are two classes, Thing and OtherThing. Users would include "thing.hh". Thing uses the pimpl idiom to hide it's implementation. OtherThing uses a C style – non-member functions that return and take pointers. This style produces slightly better machine code. I'm wondering: is there a way to use C++ style – ie, make the functions into member functions – and yet still save the machine instruction. I like this style because it doesn't pollute the namespace outside the class. Note: I'm only looking at calling member functions (in this case, calc). I'm not looking at object allocation. Below are the files, commands, and the machine code, on my Mac. thing.hh: class ThingImpl; class Thing { ThingImpl *impl; public: Thing(); int calc(); }; class OtherThing; OtherThing *make_other(); int calc(OtherThing *); thing.cc: #include "thing.hh" struct ThingImpl { int x; }; Thing::Thing() { impl = new ThingImpl; impl->x = 5; } int Thing::calc() { return impl->x + 1; } struct OtherThing { int x; }; OtherThing *make_other() { OtherThing *t = new OtherThing; t->x = 5; } int calc(OtherThing *t) { return t->x + 1; } main.cc (just to test the code actually works...) #include "thing.hh" #include <cstdio> int main() { Thing *t = new Thing; printf("calc: %d\n", t->calc()); OtherThing *t2 = make_other(); printf("calc: %d\n", calc(t2)); } Makefile: all: main thing.o : thing.cc thing.hh g++ -fomit-frame-pointer -O2 -c thing.cc main.o : main.cc thing.hh g++ -fomit-frame-pointer -O2 -c main.cc main: main.o thing.o g++ -O2 -o $@ $^ clean: rm *.o rm main Run make and then look at the machine code. On the mac I use otool -tv thing.o | c++filt. On linux I think it's objdump -d thing.o. Here is the relevant output: Thing::calc(): 0000000000000000 movq (%rdi),%rax 0000000000000003 movl (%rax),%eax 0000000000000005 incl %eax 0000000000000007 ret calc(OtherThing*): 0000000000000010 movl (%rdi),%eax 0000000000000012 incl %eax 0000000000000014 ret Notice the extra instruction because of the pointer indirection. The first function looks up two fields (impl, then x), while the second only needs to get x. What can be done?

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  • How to speed-up a simple method (preferably without changing interfaces or data structures)?

    - by baol
    I have some data structures: all_unordered_m is a big vector containing all the strings I need (all different) ordered_m is a small vector containing the indexes of a subset of the strings (all different) in the former vector position_m maps the indexes of objects from the first vector to their position in the second one. The string_after(index, reverse) method returns the string referenced by ordered_m after all_unordered_m[index]. ordered_m is considered circular, and is explored in natural or reverse order depending on the second parameter. The code is something like the following: struct ordered_subset { // [...] std::vector<std::string>& all_unordered_m; // size = n >> 1 std::vector<size_t> ordered_m; // size << n std::tr1::unordered_map<size_t, size_t> position_m; const std::string& string_after(size_t index, bool reverse) const { size_t pos = position_m.find(index)->second; if(reverse) pos = (pos == 0 ? orderd_m.size() - 1 : pos - 1); else pos = (pos == ordered.size() - 1 ? 0 : pos + 1); return all_unordered_m[ordered_m[pos]]; } }; Given that: I do need all of the data-structures for other purposes; I cannot change them because I need to access the strings: by their id in the all_unordered_m; by their index inside the various ordered_m; I need to know the position of a string (identified by it's position in the first vector) inside ordered_m vector; I cannot change the string_after interface without changing most of the program. How can I speed up the string_after method that is called billions of times and is eating up about 10% of the execution time?

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  • C++ Reserve Memory Space

    - by uray
    is there any way to reserve memory space to be used later by default Windows Memory Manager so that my application won't run out of memory if my program don't use space more than I have reserved at start of my program?

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  • Using Custom Generic Collection faster with objects than List

    - by Kaminari
    I'm iterating through a List<> to find a matching element. The problem is that object has only 2 significant values, Name and Link (both strings), but has some other values which I don't want to compare. I'm thinking about using something like HashSet (which is exactly what I'm searching for -- fast) from .NET 3.5 but target framework has to be 2.0. There is something called Power Collections here: http://powercollections.codeplex.com/, should I use that? But maybe there is other way? If not, can you suggest me a suitable custom collection?

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  • SimpleDB as Denormalized DB

    - by Max
    In an environment where you have a relational database which handles all business transactions is it a good idea to utilise SimpleDB for all data queries to have faster and more lightweight search? So the master data storage would be a relational DB which is "replicated"/"transformed" into SimpleDB to provide very fast read only queries since no JOINS and complicated subselects are needed.

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  • Has anyone ever successfully make index merge work for MySQL?

    - by user198729
    Setup: mysql> create table t(a integer unsigned,b integer unsigned); mysql> insert into t(a,b) values (1,2),(1,3),(2,4); mysql> create index i_t_a on t(a); mysql> create index i_t_b on t(b); mysql> explain select * from t where a=1 or b=4; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t | ALL | i_t_a,i_t_b | NULL | NULL | NULL | 3 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ Is there something I'm missing? Update mysql> explain select * from t where a=1 or b=4; +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ | 1 | SIMPLE | t | ALL | i_t_a,i_t_b | NULL | NULL | NULL | 1863 | Using where | +----+-------------+-------+------+---------------+------+---------+------+------+-------------+ Version: mysql> select version(); +----------------------+ | version() | +----------------------+ | 5.1.36-community-log | +----------------------+ Has anyone ever successfully make index merge work for MySQL? I'll be glad to see successful stories here:)

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  • MySQL, delete and index hint

    - by Manuel Darveau
    I have to delete about 10K rows from a table that has more than 100 million rows based on some criteria. When I execute the query, it takes about 5 minutes. I ran an explain plan (the delete query converted to select * since MySQL does not support explain delete) and found that MySQL uses the wrong index. My question is: is there any way to tell MySQL which index to use during delete? If not, what ca I do? Select to temp table then delete from temp table? Thank you!

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  • Will an optimizing compiler remove calls to a method whose result will be multiplied by zero?

    - by Tim R.
    Suppose you have a computationally expensive method, Compute(p), which returns some float, and another method, Falloff(p), which returns another float from zero to one. If you compute Falloff(p) * Compute(p), will Compute(p) still run when Falloff(p) returns zero? Or would you need to write a special case to prevent Compute(p) from running unnecessarily? Theoretically, an optimizing compiler could determine that omitting Compute when Falloff returns zero would have no effect on the program. However, this is kind of hard to test, since if you have Compute output some debug data to determine whether it is running, the compiler would know not to omit it because of that debug info, resulting in sort of a Schrodinger's cat situation. I know the safe solution to this problem is just to add the special case, but I'm just curious.

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  • iPhone App takes up too much memory

    - by Stephen Furlani
    Ok, so here's my problem. My iPhone app is 1.2MB on disk. Granted I have a bunch of Images for the GUI buttons and backgrounds, etc. In-memory, my app takes up a whopping 15MB! That means if I then take a picture with the camera, 8MB default, it gives a memory warning (several) even before the picker calls its delegate! How can I tell what is grabbing so much memory, and how to remove it? I've removed all of my debugging symbols and added [-Os], but it still takes up a huge amount of memory! Also, (how) can I change the default resolution of the camera?

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  • Best way to update/insert into a table based on a remote table.

    - by martilyo
    I have two very large enterprise tables in an Oracle 10g database. One table keeps the historical information of the other table. The problem is, I'm getting to the point where the records are just too many that my insert update is taking too long and my session is getting killed by the governor. Here's a pseudocode of my update process: sqlsel := 'SELECT col1, col2, col3, sysdate FROM table2@remote_location dpi WHERE (col1, col2, col3) IN ( SELECT col1, col2, col3 FROM table2@remote_location MINUS SELECT DISTINCT col1, col2, col3 FROM table1 mpc WHERE facility = '''||load_facility||''' )'; EXECUTE IMMEDIATE sqlsel BULK COLLECT INTO table1; I've tried the MERGE statement: MERGE INTO table1 t1 USING ( SELECT col1, col2, col3 FROM table2@remote_location ) t2 ON ( t1.col1 = t2.col1 AND t1.col2 = t2.col2 AND t1.col3 = t2.col3 ) WHEN NOT MATCHED THEN INSERT (t1.col1, t1.col2, t1.col3, t1.update_dttm ) VALUES (t2.col1, t2.col2, t2.col3, sysdate ) But there seems to be a confirmed bug on versions prior to Oracle 10.2.0.4 on the merge statement when doing a merge using a remote database. The chance of getting an enterprise upgrade is slim so is there a way to further optimize my first query or write it in another way to have it run best performance wise? Thanks.

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  • C++ performance, optimizing compiler, empty function in .cpp

    - by Dodo
    I've a very basic class, name it Basic, used in nearly all other files in a bigger project. In some cases, there needs to be debug output, but in release mode, this should not be enabled and be a NOOP. Currently there is a define in the header, which switches a makro on or off, depending on the setting. So this is definetely a NOOP, when switched off. I'm wondering, if I have the following code, if a compiler (MSVS / gcc) is able to optimize out the function call, so that it is again a NOOP. (By doing that, the switch could be in the .cpp and switching will be much faster, compile/link time wise). --Header-- void printDebug(const Basic* p); class Basic { Basic() { simpleSetupCode; // this should be a NOOP in release, // but constructor could be inlined printDebug(this); } }; --Source-- // PRINT_DEBUG defined somewhere else or here #if PRINT_DEBUG void printDebug(const Basic* p) { // Lengthy debug print } #else void printDebug(const Basic* p) {} #endif

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  • How to make if-elif-else statement in python more space-saving?

    - by Neverland
    I have a lot of if-elif-else statements in my code if message == '0' or message == '3' or message == '5' or message == '7': ... elif message == '1' or message == '2' or message == '4' or message == '6' or message == '8': ... else: ... Is it possible to format this in a more space-saving way? I tried it this way: if message == '0' or '3' or '5' or '7': ... elif message == '1' or '2' or '4' or '6' or '8': ... else: ... But without success.

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  • Defined variables and arrays vs functions in php

    - by Frank Presencia Fandos
    Introduction I have some sort of values that I might want to access several times each page is loaded. I can take two different approaches for accessing them but I'm not sure which one is 'better'. Three already implemented examples are several options for the Language, URI and displaying text that I describe here: Language Right now it is configured in this way: lang() is a function that returns different values depending on the argument. Example: lang("full") returns the current language, "English", while lang() returns the abbreviation of the current language, "en". There are many more options, like lang("select"), lang("selectact"), etc that return different things. The code is too long and irrelevant for the case so if anyone wants it just ask for it. Url The $Url array also returns different values depending on the request. The whole array is fully defined in the beginning of the page and used to get shorter but accurate links of the current page. Example: $Url['full'] would return "http://mypage.org/path/to/file.php?page=1" and $Url['file'] would return "file.php". It's useful for action="" within the forms and many other things. There are more values for $Url['folder'], $Url['file'], etc. Same thing about the code, if wanted, just request it. Text [You can skip this section] There's another array called $Text that is defined in the same way than $Url. The whole array is defined at the beginning, making a mysql call and defining all $Text[$i] for current page with a while loop. I'm not sure if this is more efficient than multiple calls for a single mysql cell. Example: $Text['54'] returns "This is just a test array!" which this could perfectly be implemented with a function like text(54). Question With the 3 examples you can see that I use different methods to do almost the same function (no pun intended), but I'm not sure which one should become the standard one for my code. I could create a function called url() and other called text() to output what I want. I think that working with functions in those cases is better, but I'm not sure why. So I'd really appreciate your opinions and advice. Should I mix arrays and functions in the way I described or should I just use funcions? Please, base your answer in this: The source needs to be readable and reusable by other developers Resource consumption (processing, time and memory). The shorter the code the better. The more you explain the reasons the better. Thank you PS, now I know the differences between $Url and $Uri.

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  • How do I select a random record efficiently in MySQL?

    - by user198729
    mysql> EXPLAIN SELECT * FROM urls ORDER BY RAND() LIMIT 1; +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+ | 1 | SIMPLE | urls | ALL | NULL | NULL | NULL | NULL | 62228 | Using temporary; Using filesort | +----+-------------+-------+------+---------------+------+---------+------+-------+---------------------------------+ The above doesn't qualify as efficient,how should I do it properly?

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  • How to speed-up a simple method? (possibily without changing interfaces or data structures)

    - by baol
    Hello. I have some data structures: all_unordered_mordered_m is a big vector containing all the strings I need (all different) ordered_m is a small vector containing the indexes of a subset of the strings (all different) in the former vector position_m maps the indexes of objects from the first vector to their position in the second one. The string_after(index, reverse) method returns the string referenced by ordered_m after all_unordered_m[index]. ordered_m is considered circular, and is explored in natural or reverse order depending on the second parameter. The code is something like the following: struct ordered_subset { // [...] std::vector<std::string>& all_unordered_m; // size = n >> 1 std::vector<size_t> ordered_m; // size << n std::map<size_t, size_t> position_m; // positions of strings in ordered_m const std::string& string_after(size_t index, bool reverse) const { size_t pos = position_m.find(index)->second; if(reverse) pos = (pos == 0 ? orderd_m.size() - 1 : pos - 1); else pos = (pos == ordered.size() - 1 ? 0 : pos + 1); return all_unordered_m[ordered_m[pos]]; } }; Given that: I do need all of the data-structures for other purposes; I cannot change them because I need to access the strings: by their id in the all_unordered_m; by their index inside the various ordered_m; I need to know the position of a string (identified by it's position in the first vector) inside ordered_m vector; I cannot change the string_after interface without changing most of the program. How can I speed up the string_after method that is called billions of times and is eating up about 10% of the execution time?

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  • Thin down jQuery

    - by Taylor Satula
    Hi, I have been optimizing my website but the one problem that stands in my way is all the jQuery functions that I do not use. The only ones that I use are for a smooth page scroller. It just seems like such a waste of download time. My question is: Is there any script or program that will remove the jQuery code that I do not need and leave the 1 or 2 functions that I do need.

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  • Reduce Processing Time of accessing databse

    - by medma
    hello all, I m making an app which requires remote databse connection. I want the values in picker from database but when I click on button to invoke picker it takes some time to fetch the values and displaying. Is there any way to do it fast? and also is there any way to reduce the time of transition between 2 views? Thanx

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  • How can i optimize this recursive method

    - by Tirdyr
    Hi there. I'm trying to make a word puzzle game, and for that i'm using a recursive method to find all possible words in the given letters. The letters is in a 4x4 board. Like this: ABCD EFGH HIJK LMNO The recursive method is called inside this loop: for (int y = 0; y < width; y++) { for (int x = 0; x < height; x++) { myScabble.Search(letters, y, x, width, height, "", covered, t); } } letters is a 2D array of chars. y & x is ints that shows where in the board width & height is also int, that tells the dimensions of the board "" is the string we are trying to make (the word) covered is an array of bools, to check if we allready used that square. t is a List (wich contains all the words to check against). The recursive method that need optimizing: public void Search(char[,] letters, int y, int x, int width, int height, string build, bool[,] covered, List<aWord> tt) { // Dont get outside the bounds if (y >= width || y < 0 || x >= height || x < 0) { return; } // Dont deal with allrady covered squares if (covered[x, y]) { return; } // Get Letter char letter = letters[x, y]; // Append string pass = build + letter; // check if its a possibel word //List<aWord> t = myWords.aWord.Where(w => w.word.StartsWith(pass)).ToList(); List<aWord> t = tt.Where(w => w.word.StartsWith(pass)).ToList(); // check if the list is emphty if (t.Count < 10 && t.Count != 0) { //stop point } if (t.Count == 0) { return; } // Check if its a complete word. if (t[0].word == pass) { //check if its allrdy present in the _found dictinary if (!_found.ContainsKey(pass)) { //if not add the word to the dictionary _found.Add(pass, true); } } // Check to see if there is more than 1 more that matches string pass // ie. are there more words to find. if (t.Count > 1) { // make a copy of the covered array bool[,] cov = new bool[height, width]; for (int i = 0; i < width; i++) { for (int a = 0; a < height; a++) { cov[a, i] = covered[a, i]; } } // Set the current square as covered. cov[x, y] = true; // Continue in all 8 directions. Search(letters, y + 1, x, width, height, pass, cov, t); Search(letters, y, x + 1, width, height, pass, cov, t); Search(letters, y + 1, x + 1, width, height, pass, cov, t); Search(letters, y - 1, x, width, height, pass, cov, t); Search(letters, y, x - 1, width, height, pass, cov, t); Search(letters, y - 1, x - 1, width, height, pass, cov, t); Search(letters, y - 1, x + 1, width, height, pass, cov, t); Search(letters, y + 1, x - 1, width, height, pass, cov, t); } } The code works as i expected it to do, however it is very slow.. it takes about 2 mins to find the words. EDIT: i clarified that the letters array is 2D

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  • negative values in integer programming model

    - by Lucia
    I'm new at using the glpk tool, and after writing a model for certain integer problem and running the solver (glpsol) i get negative values in some constraint that shouldn't be negative at all: No.Row name Activity Lower bound Upper bound 8 act[1] 0 -0 9 act[2] -3 -0 10 act[2] -2 -0 That constraint is defined like this: act{j in J}: sum{i in I} d[i,j] <= y[j]*m; where the sets and variables used are like this: param m, integer, 0; param n, integer, 0; set I := 1..m; set J := 1..n; var y{j in J}, binary; As the upper bound is negative, i think the problem may be in the y[j]*m parte, of the right side of the inequality.. perhaps something with the multiplication of binarys? or that the j in that side of the constrait is undefined? i dont know... i would be greatly grateful if someone can help me with this! :) and excuse for my bad english thanks in advance!

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  • Whether to put method code in a VB.Net data storage class, or put it in a separate class?

    - by Alan K
    TLDR summary: (a) Should I include (lengthy) method code in classes which may spawn multiple objects at runtime, (b) does doing so cause memory usage bloat, (c) if so should I "outsource" the code to a class that is loaded only once and have the class methods call that, or alternatively (d) does the code get loaded only once with the object definition anyway and I'm worrying about nothing? ........ I don't know whether there's a good answer to this but if there is I haven't found it yet by searching in the usual places. In my VB.Net (2010 if it matters) WinForms project I have about a dozen or so class objects in an object model. Some of these are pretty simple and do little more than act as data storage repositories. The ones further up the object model, however, have an increasing number of methods. There can be a significant number of higher level objects in use though the exact number will be runtime dependent so I can't be more precise than that. As I was writing the method code for one of the top level ones I noticed that it was starting to get quite lengthy. Memory optimisation is something of a lost art given how much memory the average PC has these days but I don't want to make my application a resource hog. So my questions for anyone who knows .Net way better than I do (of which there will be many) are: Is the code loaded into memory with each instance of the class that's created? Alternatively is it loaded only once with the definition of the class, and all derived objects just refer to that definition? (I'm not really sure how that could be possible given that, for example, event handlers can be assigned dynamically, but no harm asking.) If the answer to the first one is yes, would it be more efficient to write the code in a "utility" object which is loaded only once and called from the real class' methods? Any thoughts appreciated.

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