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• Reading from a file not line-by-line

  - by MadH

  Assigning a QTextStream to a QFile and reading it line-by-line is easy and works fine, but I wonder if the performance can be inreased by first storing the file in memory and then processing it line-by-line. Using FileMon from sysinternals, I've encountered that the file is read in chunks of 16KB and since the files I've to process are not that big (~2MB, but many!), loading them into memory would be a nice thing to try. Any ideas how can I do so? QFile is inhereted from QIODevice, which allows me to ReadAll() it into QByteArray, but how to proceed then and divide it into lines?

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• Deleting orphans with JPA

  - by homaxto

  I have a one-to-one relation where I use CascadeType.PERSIST. This has over time build up a huge amount of child records that has not been deleted, to such an extend that it is reflected in the performance. Now I wish to add some code that cleans up the database removing all the child records that are not referenced by a parent. At the moment we are talking 400K+ records, at I need to run the code on all customer installations just to be sure they do not run into the same problem. I think the best solution would be to run a named query (because we support two databases) that deletes the necessary records, and this is where I get into problems, because how should I write it in JPQL? The result I want can be defined like the following sql statement, which unfortunaltely does not run on MySQL. DELETE FROM child c1 WHERE c1.pk NOT IN (SELECT DISTINCT p.pk FROM child c2 JOIN parent p ON p.child = c2.pk);

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• Best data-structure to use for two ended sorted list

  - by fmark

  I need a collection data-structure that can do the following: Be sorted Allow me to quickly pop values off the front and back of the list Remain sorted after I insert a new value Allow a user-specified comparison function, as I will be storing tuples and want to sort on a particular value Thread-safety is not required Optionally allow efficient haskey() lookups (I'm happy to maintain a separate hash-table for this though) My thoughts at this stage are that I need a priority queue and a hash table, although I don't know if I can quickly pop values off both ends of a priority queue. I'm interested in performance for a moderate number of items (I would estimate less than 200,000). Another possibility is simply maintaining an OrderedDictionary and doing an insertion sort it every-time I add more data to it. Furthermore, are there any particular implementations in Python. I would really like to avoid writing this code myself.

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• Pros and cons of sorting data in DB?

  - by Roman

  Let's assume I have a table with field of type VARCHAR. And I need to get data from that table sorted alphabetically by that field. What is the best way (for performance): add sort by field to the SQL-query or sort the data when it's already fetched? I'm using Java (with Hibernate), but I can't tell anything about DB engine. It could be any popular relational database (like MySQL or MS Sql Server or Oracle or HSQL DB or any other). The amount of records in table can vary greatly but let's assume there are 5k records.

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• What to monitor on MSSQL Server

  - by user361434

  Hi all I have been asked to monitor MSSQL Server (2005 & 2008) and am wondering what are good metrics to look at? I can access WMI counters but am slightly lost as to how much depth is going to be useful. Currently I have on my list: user connections logins per second latch waits per second total latch wait time dead locks per second errors per second Log and data file sizes I am looking to be able to monitor values that will indicate a degradation of performance on the machine or a potential serious issue. To this end I am also wondering at what values some of these things would be considered normal vs problematic? As I reckon it would probably be a really good question to have answered for the general community I thought i'd court some of you DBA experts out there (I am certainly not one of them!) Apologies if a rather open ended question. Ry

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• SQL Not Exists in this Query - is it possible

  - by jason barry

  This is my script - it simply looks for the image file associated to a person record. Now the error will display if there is NO .jpg evident when the query runs. Msg 4860, Level 16, State 1, Line 1 Cannot bulk load. The file "C:\Dev\ClientServices\Defence\RAN\Shore\Config\Photos\002054.2009469432270600.001.jpg" does not exist. Is there a way to write this query to 'IF not exists then set id_number = '002054.2009469432270427.001' - so it wil always display this photo for any records without a picture. ALTER procedure [dbo].[as_ngn_sp_REP_PH108_photo] (@PMKEYS nvarchar(50)) AS ---exec [as_ngn_sp_REP_PH108_photo] '8550733' SET NOCOUNT ON DECLARE @PATH AS NVARCHAR(255) DECLARE @ID_NUMBER NVARCHAR(27) DECLARE @SQL AS NVARCHAR(MAX) EXEC DB_GET_DB_SETTING'STAFF PICTURE FILE LOCATION', 0, @PATH OUTPUT IF RIGHT(@PATH,1) <> '\' SET @PATH = @PATH + '\' SELECT @ID_NUMBER = ID_NUMBER FROM aView_person WHERE EXTRA_CODE_1 = @PMKEYS SET @PATH = @PATH + @ID_NUMBER + '.jpg' SET @SQL = 'SELECT ''Picture1'' [Picture], BulkColumn FROM OPENROWSET(Bulk ''' + REPLACE(@PATH,'''','''''') + ''', SINGLE_BLOB) AS RAN' EXEC SP_EXECUTESQL @SQL

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• Does Ruby on Rails "has_many" array provide data on a "need to know" basis?

  - by Jian Lin

  On Ruby on Rails, say, if the Actor model object is Tom Hanks, and the "has_many" fans is 20,000 Fan objects, then actor.fans gives an Array with 20,000 elements. Probably, the elements are not pre-populated with values? Otherwise, getting each Actor object from the DB can be extremely time consuming. So it is on a "need to know" basis? So does it pull data when I access actor.fans[500], and pull data when I access actor.fans[0]? If it jumps from each record to record, then it won't be able to optimize performance by doing sequential read, which can be faster on the hard disk because those records could be in the nearby sector / platter layer -- for example, if the program touches 2 random elements, then it will be faster just to read those 2 records, but what if it touches all elements in random order, then it may be faster just to read all records in a sequential way, and then process the random elements. But how will RoR know whether I am doing only a few random elements or all elements in random?

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• query in sql server for retrieving rows

  - by Arash khangaldi

  I have a table that contains the following 4 columns: id name lastname phone I want to write a stored procedure that gets an id as parameter, and then gets the name of that id and then use that name to get all the rows that their name is equal to the names that i found in last step! here it is my query, i know it's wrong but i'm new to sql commands: ALTER PROCEDURE dbo.GetAllNames @id int AS select name as Name from Users where id = @id -- i don't how to retrieve the names that are equal to Name select * from Users where name = Name can you correct my query and help me? Thanks.

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• DB4o Linq query - How to check for null strings

  - by Dave

  Hey there - simple query: var q = (from SomeObject o in container where o.SomeInt > 8 && o.SomeString != null //Null Ref here select o; I always get a null reference exception. If I use String.IsNullOrEmpty(o.SomeString) the query takes about 100 times as long, as if I use && o.SomeString != "" (which is way faster, but obviously not correct). I'm guessing because DB4o needs to activate the objects, in order to pass them in to the IsNullOrEmpty call, and can't use the indexes. My question is, what's a better way to check for nulls in this situation? Is there something like: mystring != Db4o.DBNull.Value, or something? Cheers, Dave

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• MongoDB efficient dealing with embedded documents

  - by Sebastian Nowak

  I have serious trouble finding anything useful in Mongo documentation about dealing with embedded documents. Let's say I have a following schema: { _id: ObjectId, ... data: [ { _childId: ObjectId // let's use custom name so we can distinguish them ... } ] } What's the most efficient way to remove everything inside data for particular _id? What's the most efficient way to remove embedded document with particular _childId inside given _id? What's the performance here, can _childId be indexed in order to achieve logarithmic (or similar) complexity instead of linear lookup? If so, how? What's the most efficient way to insert a lot of (let's say a 1000) documents into data for given _id? And like above, can we get O(n log n) or similar complexity with proper indexing? What's the most efficient way to get the count of documents inside data for given _id?

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• Is str.replace(..).replace(..) ad nauseam a standard idiom in Python?

  - by meeselet

  For instance, say I wanted a function to escape a string for use in HTML (as in Django's escape filter): def escape(string): """ Returns the given string with ampersands, quotes and angle brackets encoded. """ return string.replace('&', '&amp;').replace('<', '&lt;').replace('>', '&gt;').replace("'", '&#39;').replace('"', '&quot;') This works, but it gets ugly quickly and appears to have poor algorithmic performance (in this example, the string is repeatedly traversed 5 times). What would be better is something like this: def escape(string): """ Returns the given string with ampersands, quotes and angle brackets encoded. """ # Note that ampersands must be escaped first; the rest can be escaped in # any order. return replace_multi(string.replace('&', '&amp;'), {'<': '&lt;', '>': '&gt;', "'": '&#39;', '"': '&quot;'}) Does such a function exist, or is the standard Python idiom to use what I wrote before?

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• Per query relevance elevation for solr?

  - by plusplus

  I want to tune the relevance of solr search results on a per user basis - based on the number of times the user has clicked through a result before. Frequently hit items FOR THAT USER should rise to the top of their search results. Is there a way to provide custom boost/elevation for particular document ids on the query? I'm thinking in the order of ~100s of particular documents to elevate. The elevation should have no effect if the rest of the query doesn't find those documents. Alternatively, if this isn't possible, what is a sane way for setting up an alternative indexing approach that would make this possible? Could I add a field per user in the index to store their scores? I'm thinking in the order of 1000 users. The major drawback of that approach is the number of times a document would need to be reindexed (i.e. each time it was used by the user).

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• How do I guarante node order for an Xpath 'OR' query

  - by Tom Carter

  I have a snippet of XML that looks like <body> Some text.... <nodeA>....</nodeA> more text <someOtherNode> <nodeA>.......</nodeA> </someOtherNode> <nodeB>.......</nodeB> etc..... </body> I'm selecting all nodeA and NodeB nodes inside <body> using an XPATH query similar to "//nodeA|//nodeB" As I understand it, .NET supports XPATH 1.0 which does not guarantee node order. How can I guarantee selected nodes are returned in document order in my OR query : that's to say : nodeA, nodeA, nodeB

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• How to keep Hibernate mapping use under control as requirements grow

  - by David Plumpton

  I've worked on a number of Java web apps where persistence is via Hibernate, and we start off with some central class (e.g. an insurance application) without any time being spent considering how to break things up into manageable chunks. Over time as features are added we add more mappings (rates, clients, addresses, etc.) and then amount of time spent saving and loading an insurance object and everything it connects to grows. In particular you get close to a go-live date and performance testing with larger amounts of data in each table is starting to demonstrate that it's all too slow. Obviously there are a number of ways that we could attempt to partition things up, e.g. map only the client classes for the client CRUD screens, etc., which would have been better to get in place earlier rather than trying to work it in at the end of the dev cycle. I'm just wondering if there are recommendations about ways to handle/mitigate this.

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• How can I optimize MVC and IIS pipeline to obtain higher speed?

  - by Andy

  Hi, I am doing performance tweaking of a simple app that uses MVC on IIS 7.5. I have a StopWatch starting up in Application_BeginRequest and I take a snapshot at Controller.OnActionExecuting. So I measure the time spend in the entire IIS pipeline: from request receipt to the moment execution finally gets to my controller. I obtain 700 microseconds on my 3GHz quad-core (project compiled Release x64), and I wonder where the bottleneck is, especially hearing some people say that one can get up to 8000 page loads per second with MVC. How can I optimize MVC and IIS pipeline to obtain higher speed?

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• Formatting a query to enumerate through 2 different datatables

  - by boiler1974

  I have 2 datatables sendTable and recvTable They both have identical column names and numbers of columns "NODE" "DSP Name" "BUS" "IDENT" "STATION" "REF1" "REF2" "REF3" "REF4" "REF5" "REF6" "REF7" "REF8" I need to compare these 2 tables and separate out the mismatches I only need to check Columns 3-11 and Ignore col 1 and 2 I tried at first removing the 2 columns and then loop thru row by row and return matches and mismatches but the problem with this approach is that I no longer have the "NODE" and "DSP Name" associated with the row when I finalize my results So I need help with a query Here is my attempt var samerecordQuery = from r1 in sendTable.AsEnumerable() where r1.Field<int>("BUS").Equals(from r2 in recvTable.AsEnumerable() where r2.Field<int>("BUS")) this obviously doesn't work so how do I format the query to say from r1 cols[3-11] equals r2 cols [3-11] and once I have this I can use the except to pull out the mismatches

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• Django check for any exists for a query

  - by Hulk

  In django how to check whether any entry exists for a query sc=scorm.objects.filter(Header__id=qp.id) This was how it was done in php if(mysql_num_rows($resultn)) { } else { }

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• COMET chat application - IIS7 slows down over time

  - by Yaron

  I have built a chat application which uses this code to push messages to clients (web pages) and to monitor online users and their information. Basically, the code creates and manages a custom thread pool for maintaining the list of connected users & their state. The application was hosted on a shared hosting account (IIS6), and worked fine. After moving the site (ASP.Net App) to a dedicated virtual server it seems I have a problem where IIS7 gets slower and slower as time passes, and my only "solution" is to restart IIS. I am trying to look at the performance counters and have do idea on which one to look.

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• StringBuilder/StringBuffer vs. "+" Operator

  - by matt.seil

  I'm reading "Better, Faster, Lighter Java" (by Bruce Tate and Justin Gehtland) and am familiar with the readability requirements in agile type teams, such as what Robert Martin discusses in his clean coding books. On the team I'm on now, I've been told explicitly not to use the "+" operator because it creates extra (and unnecessary) string objects during runtime. But this article: http://www.ibm.com/developerworks/java/library/j-jtp01274.html Written back in '04 talks about how object allocation is about 10 machine instructions. (essentially free) It also talks about how the GC also helps to reduce costs in this environment. What is the actual performance tradeoffs between using "+," "StringBuilder," or "StringBuffer?" (In my case it is StringBuffer only as we are limited to Java 1.4.2.) StringBuffer to me results in ugly, less readable code, as a couple of examples in Tate's book demonstrates. And StringBuffer is thread-synchronized which seems to have its own costs that outweigh the "danger" in using the "+" operator. Thoughts/Opinions?

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• Query joining in sql server 2005

  - by Domnic

  I have two queries like: SELECT PC_COMP_CODE, PC_SL_LDGR_CODE, PC_SL_ACNO ACCOUNT, COUNT(PC_CHEQUE_NO) CHQS, SUM(CONVERT(FLOAT, PC_AMOUNT)) CHQ_AMT FROM GLAS_PDC_CHEQUES WHERE PC_COMP_CODE = '1' AND PC_DISCD IS NULL GROUP BY PC_SL_LDGR_CODE, PC_SL_ACNO ,PC_COMP_CODE ORDER BY PC_SL_ACNO -------------------------------------------------- SELECT COAD_PTY_FULL_NAME,PC_COMP_CODE, PC_SL_LDGR_CODE, PC_SL_ACNO, PC_DEPT_NO, PC_DOC_TYPE, PC_CHEQUE_NO, PC_BANK_AC_NO FROM GLAS_PTY_ADDRESS,GLAS_SBLGR_MASTERS,GLAS_PDC_CHEQUES WHERE COAD_COMP_CODE = '1' AND SLMA_COMP_CODE = COAD_COMP_CODE AND SLMA_ADDR_ID = COAD_ADDR_ID AND SLMA_LDGRCTL_CODE = PC_SL_LDGR_CODE AND PC_COMP_CODE=SLMA_COMP_CODE AND SLMA_ACNO = PC_SL_ACNO AND SLMA_LDGRCTL_YEAR = DBO.GLAS_VALIDATIONS_GET_OPEN_YEAR(PC_COMP_CODE) If I execute first query alone I get 5 records... If I join the above two query like: SELECT PC_COMP_CODE, PC_SL_LDGR_CODE, PC_SL_ACNO ACCOUNT, COUNT(PC_CHEQUE_NO) CHQS, SUM(CONVERT(FLOAT, PC_AMOUNT)) CHQ_AMT, COAD_PTY_FULL_NAME FROM GLAS_PDC_CHEQUES LEFT OUTER JOIN GLAS_SBLGR_MASTERS ON( SLMA_COMP_CODE=PC_COMP_CODE AND SLMA_LDGRCTL_CODE = PC_SL_LDGR_CODE AND SLMA_ACNO = PC_SL_ACNO ) LEFT OUTER JOIN GLAS_PTY_ADDRESS ON( SLMA_COMP_CODE = COAD_COMP_CODE AND SLMA_ADDR_ID = COAD_ADDR_ID) WHERE PC_COMP_CODE = '1' AND PC_DISCD IS NULL AND SLMA_LDGRCTL_YEAR = DBO.GLAS_VALIDATIONS_GET_OPEN_YEAR(PC_COMP_CODE) GROUP BY PC_SL_LDGR_CODE, PC_SL_ACNO ,PC_COMP_CODE,COAD_PTY_FULL_NAME ORDER BY PC_SL_ACNO then I just get 2 records.... I need that 5 records to display after join..... How can I do it?

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• How to calculate a operation's time in micro second precision

  - by Sanjeet Daga

  I want to calculate performance of a function in micro second precision on Windows platform. Now Windows itself has milisecond granuality, so how can I achieve this. I tried following sample, but not getting correct results. LARGE_INTEGER ticksPerSecond = {0}; LARGE_INTEGER tick_1 = {0}; LARGE_INTEGER tick_2 = {0}; double uSec = 1000000; // Get the frequency QueryPerformanceFrequency(&ticksPerSecond); //Calculate per uSec freq double uFreq = ticksPerSecond.QuadPart/uSec; // Get counter b4 start of op QueryPerformanceCounter(&tick_1); // The ope itself Sleep(10); // Get counter after opfinished QueryPerformanceCounter(&tick_2); // And now the op time in uSec double diff = (tick_2.QuadPart/uFreq) - (tick_1.QuadPart/uFreq);

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• Whats wrong with my SQL query?

  - by William

  I'm trying to set up a query that shows the first post of each thread and is ordered by the date of the last post in each thread. I got the first part down with this query: SELECT * FROM ( SELECT Min( ID ) AS MinID FROM test_posts GROUP BY Thread )tmin JOIN test_posts ON test_posts.ID = tmin.MinID Now I need to figure out how to call the last post of each thread into a table, than use that table to order the first tables results. So far I got this, but it doesn't work. SELECT * FROM ( SELECT Min( ID ) AS MinID FROM test_posts GROUP BY Thread )tmin JOIN test_posts ON test_posts.ID = tmin.MinID ORDER BY (SELECT MAX( ID ) AS MaxID, Thread, MAX( Date ) FROM test_posts GROUP BY Thread )tmax tmax.Date

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• why is there extra using where in execution plan of query

  - by user366534

  I see plan of query: EXPLAIN SELECT * FROM `subscribers` WHERE state =4 AND date_added < '2010-12-23 11:47:45' It shows: id select_type table type possible_keys key key_len ref rows Extra 1 SIMPLE subscribers range state_date_added state_date_added 9 NULL 8 Using where Here is indexes of table: Table Non_unique Key_name Seq_in_index Column_name Collation Cardinality Sub_part Packed Null Index_type Comment subscribers 0 PRIMARY 1 subscriber_id A 382039 NULL NULL BTREE subscribers 0 email_list_id 1 email_address A 191019 NULL NULL BTREE subscribers 0 email_list_id 2 list_id A 382039 NULL NULL BTREE subscribers 1 FK_list_id 1 list_id A 10 NULL NULL BTREE subscribers 1 state_date_added 1 state A 12 NULL NULL BTREE subscribers 1 state_date_added 2 date_added A 8128 NULL NULL BTREE The last two lines describes index what is supposed for the query. Why is there in extra column using where? Even If I fetch only state and date_added column, it has in extra column: Using where; Using index. I understand why it has using index, but I don't understand Using where here.

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• MongoDB query to return only embedded document

  - by Matt

  assume that i have a BlogPost model with zero-to-many embedded Comment documents. can i query for and have MongoDB return only Comment objects matching my query spec? eg, db.blog_posts.find({"comment.submitter": "some_name"}) returns only a list of comments. edit: an example: import pymongo connection = pymongo.Connection() db = connection['dvds'] db['dvds'].insert({'title': "The Hitchhikers Guide to the Galaxy", 'episodes': [{'title': "Episode 1", 'desc': "..."}, {'title': "Episode 2", 'desc': "..."}, {'title': "Episode 3", 'desc': "..."}, {'title': "Episode 4", 'desc': "..."}, {'title': "Episode 5", 'desc': "..."}, {'title': "Episode 6", 'desc': "..."}]}) episode = db['dvds'].find_one({'episodes.title': "Episode 1"}, fields=['episodes']) in this example, episode is: {u'_id': ObjectId('...'), u'episodes': [{u'desc': u'...', u'title': u'Episode 1'}, {u'desc': u'...', u'title': u'Episode 2'}, {u'desc': u'...', u'title': u'Episode 3'}, {u'desc': u'...', u'title': u'Episode 4'}, {u'desc': u'...', u'title': u'Episode 5'}, {u'desc': u'...', u'title': u'Episode 6'}]} but i just want: {u'desc': u'...', u'title': u'Episode 1'}

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• C sharp: read last "n" lines of log file

  - by frictionlesspulley

  need a snippet of code which would read out last "n lines" of a log file. I came up with the following code from the net.I am kinda new to C sharp. Since the log file might be quite large, I want to avoid overhead of reading the entire file.Can someone suggest any performance enhancement. I do not really want to read each character and change position. var reader = new StreamReader(filePath, Encoding.ASCII); reader.BaseStream.Seek(0, SeekOrigin.End); var count = 0; while (count <= tailCount) { if (reader.BaseStream.Position <= 0) break; reader.BaseStream.Position--; int c = reader.Read(); if (reader.BaseStream.Position <= 0) break; reader.BaseStream.Position--; if (c == '\n') { ++count; } } var str = reader.ReadToEnd();

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