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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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  • Is closing/disposing an SqlDataReader needed if you are already closing the sqlconnection?

    - by Brian
    I noticed This question, but my question is a bit more specific. Is there any advantage to using using (SqlConnection conn = new SqlConnection(conStr)) { using (SqlCommand command = new SqlCommand()) { // dostuff } } instead of using (SqlConnection conn = new SqlConnection(conStr)) { SqlCommand command = new SqlCommand(); // dostuff } Obviously it does matter run more than one command with the same connection, since closing an SqlDataReader is more efficient than closing and reopening a connection (calling conn.Close();conn.Open(); will also free up the connection). I see many people insist that failure to close the DataReader means leaving open connection resources around, but doesn't that only apply if you don't close the connection?

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  • max count with joins

    - by trixet
    I have 3 tables: users: Id Login 1 John 2 Bill 3 Jim computers: Id Name 1 Computer1 2 Computer2 3 Computer3 4 Computer4 5 Computer5 sessions: UserId ComputerId Minutes 1 2 47 2 1 32 1 4 15 2 5 5 1 2 7 1 1 40 2 5 31 I would like to display this resulting table: Login Total_sess Total_min Most_freq_computer Sess_on_most_freq Min_on_most_freq John 4 109 Computer2 2 54 Bill 3 68 Computer5 2 36 Jim - - - - - Myself I can only cover first 3 columns with: SELECT Login, COUNT(sessions.UserId), SUM(Minutes) FROM users LEFT JOIN sessions ON users.Id = sessions.UserId GROUP BY users.Id And some kind of other columns with: SELECT main.* FROM (SELECT UserId, ComputerId, COUNT(*) AS cnt ,SUM(Minutes) FROM sessions GROUP BY UserId, ComputerId) AS main INNER JOIN ( SELECT ComputerId, MAX(cnt) AS maxCnt FROM ( SELECT ComputerId, UserId, COUNT(*) AS cnt FROM sessions GROUP BY ComputerId, UserId ) AS Counts GROUP BY ComputerId) AS maxes ON main.ComputerId = maxes.ComputerId AND main.cnt = maxes.maxCnt But I need to get whole resulting table in one query. I feel I'm doing something completely wrong. Need help.

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  • Django: Determining if a user has voted or not

    - by TheLizardKing
    I have a long list of links that I spit out using the below code, total votes, submitted by, the usual stuff but I am not 100% on how to determine if the currently logged in user has voted on a link or not. I know how to do this from within my view but do I need to alter my below view code or can I make use of the way templates work to determine it? I have read http://stackoverflow.com/questions/1528583/django-vote-up-down-method but I don't quite understand what's going on ( and don't need any ofjavascriptery). Models (snippet): class Link(models.Model): category = models.ForeignKey(Category, blank=False, default=1) user = models.ForeignKey(User) created = models.DateTimeField(auto_now_add=True) modified = models.DateTimeField(auto_now=True) url = models.URLField(max_length=1024, unique=True, verify_exists=True) name = models.CharField(max_length=512) def __unicode__(self): return u'%s (%s)' % (self.name, self.url) class Vote(models.Model): link = models.ForeignKey(Link) user = models.ForeignKey(User) created = models.DateTimeField(auto_now_add=True) def __unicode__(self): return u'%s vote for %s' % (self.user, self.link) Views (snippet): links = Link.objects.select_related().annotate(votes=Count('vote')).order_by('-created')

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  • Tentative date casting in tsql

    - by Tewr
    I am looking for something like TRYCAST in TSQL or an equivalent method / hack. In my case I am extracting some date data from an xml column. The following query throws "Arithmetic overflow error converting expression to data type datetime." if the piece of data found in the xml cannot be converted to datetime (in this specific case, the date is "0001-01-01" in some cases). Is there a way to detect this exception before it occurs? select [CustomerInfo].value('(//*:InceptionDate/text())[1]', 'datetime') FROM Customers An example of what I am trying to achieve in pseudocode with an imagined tsql function TRYCAST(expr, totype, defaultvalue): select TRYCAST( [CustomerInfo].value('(//*:InceptionDate/text())[1]', 'nvarchar(100)'), datetime, null) FROM Customers

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  • Returning IEnumerable<T> vs IQueryable<T>

    - by stackoverflowuser
    what is the difference between returning iqueryable vs ienumerable. IQueryable<Customer> custs = from c in db.Customers where c.City == "<City>" select c; IEnumerable<Customer> custs = from c in db.Customers where c.City == "<City>" select c; Will both be deferred execution? When should one be preferred over the other?

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  • linq join query

    - by SamB09
    Hi, im trying to do a join in linq , however for some reason i cant access the primary key of a table. It's the 'h.ProjectId' that doesn't seem to be accepted. The following error is given CW1.SearchWebService.Bid does not contain a definition for 'ProjectId' and no extention method 'ProjectId' accepting a first argument of type 'CW1SearchWebService.Bid' var allProjects = ctxt.Project.ToList() ; var allBids = ctxt.Bid.ToArray();// return all bids var projects = (from project in allProjects join h in allBids on project.ProjectId equals h.ProjectId

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  • Sybase stored procedure - how do I create an index on a #table?

    - by DVK
    I have a stored procedure which creates and works with a temporary #table Some of the queries would be tremendously optimized if that temporary #table would have an index created on it. However, creating an index within the stored procedure fails: create procedure test1 as SELECT f1, f2, f3 INTO #table1 FROM main_table WHERE 1 = 2 -- insert rows into #table1 create index my_idx on #table1 (f1) SELECT f1, f2, f3 FROM #table1 (index my_idx) WHERE f1 = 11 -- "QUERY X" When I call the above, the query plan for "QUERY X" shows a table scan. If I simply run the code above outside the stored procedure, the messages show the following warning: Index 'my_idx' specified as optimizer hint in the FROM clause of table '#table1' does not exist. Optimizer will choose another index instead. This can be resolved when running ad-hoc (outside the stored procedure) by splitting the code above in two batches by addding "go" after index creation: create index my_idx on #table1 (f1) go Now, "QUERY X" query plan shows the use of index "my_idx". QUESTION: How do I mimique running the "create index" in a separate batch when it's inside the stored procedure? I can't insert a "go" there like I do with the ad-hoc copy above. P.S. If it matters, this is on Sybase 12.

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  • Systems design question: DB connection management in load-balanced n-tier

    - by aoven
    I'm wondering about the best approach to designing a DB connection manager for a load-balanced n-tier system. Classic n-tier looks like this: Client -> BusinessServer -> DBServer A load-balancing solution as I see it would then look like this: +--> ... +--+ +--> BusinessServer +--+--> SessionServer --+ Client -> Gateway --+--> BusinessServer +--| +--> DBServer +--> BusinessServer +--+--------------------+ +--> ... +--+ As pictured, the business server component is being load-balanced via multiple instances, and a hardware gateway is distributing the load among them. Session server probably needs to be situated outside the load-balancing array, because it manages state, which mustn't be duplicated. Barring any major errors in design so far, what is the best way to implement DB connection management? I've come up with a couple of options, but there may be others I'm not aware of: Introduce a new Broker component between the DBServer and the other components and let it handle the DB connections. The upside is that all the connections can be managed from a single point, which is very convenient. The downside is that now there is an additional "single point of failure" in the system. Other components must go through it for every request that involves DB in some way, which also makes this a bottleneck. Move the DB connection management into BusinessServer and SessionServer components and let each handle its own DB connections. The upside is that there is no additional "single point of failure" or bottleneck components. The downside is that there is also no control over possible conflicts and deadlocks apart from what DBServer itself can provide. What else can be done? FWIW: Technology is .NET, but none of the vendor-specific stacks are used (e.g. no WCF, MSMQ or the like).

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  • LINQ Query returns nothing.

    - by gtas
    Why is this query returns 0 lines? There is a record matching the arguments. Deafkaw.Where(p => (p.ImerominiaKataxorisis >= aDate && p.ImerominiaKataxorisis <= DateTime.Now) && (p.Year == etos && p.IsYpodeigma == false) ).ToList(); Am i missing something?

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  • Avoiding Nested Queries

    - by Midhat
    How Important is it to avoid nested queries. I have always learnt to avoid them like a plague. But they are the most natural thing to me. When I am designing a query, the first thing I write is a nested query. Then I convert it to joins, which sometimes takes a lot of time to get right. And rarely gives a big performance improvement (sometimes it does) So are they really so bad. Is there a way to use nested queries without temp tables and filesort

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  • How can I update a record using a correlated subquery?

    - by froadie
    I have a function that accepts one parameter and returns a table/resultset. I want to set a field in a table to the first result of that recordset, passing in one of the table's other fields as the parameter. If that's too complicated in words, the query looks something like this: UPDATE myTable SET myField = (SELECT TOP 1 myFunctionField FROM fn_doSomething(myOtherField) WHERE someCondition = 'something') WHERE someOtherCondition = 'somethingElse' In this example, myField and myOtherField are fields in myTable, and myFunctionField is a field return by fn_doSomething. This seems logical to me, but I'm getting the following strange error: 'myOtherField' is not a recognized OPTIMIZER LOCK HINTS option. Any idea what I'm doing wrong, and how I can accomplish this? *UPDATE: * Based on Anil Soman's answer, I realized that the function is expecting a string parameter and the field being passed is an integer. I'm not sure if this should be a problem as an explicit call to the function using an integer value works - e.g. fn_doSomething(12345) seems to automatically cast the number to an string. However, I tried to do an explicit cast: UPDATE myTable SET myField = (SELECT TOP 1 myFunctionField FROM fn_doSomething(CAST(myOtherField AS varchar(1000))) WHERE someCondition = 'something') WHERE someOtherCondition = 'somethingElse' Now I'm getting the following error: Line 5: Incorrect syntax near '('.

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  • Passing in a lambda to a Where statement

    - by sonicblis
    I noticed today that if I do this: var items = context.items.Where(i => i.Property < 2); items = items.Where(i => i.Property > 4); Once I access the items var, it executes only the first line as the data call and then does the second call in memory. However, if I do this: var items = context.items.Where(i => i.Property < 2).Where(i => i.Property > 4); I get only one expression executed against the context that includes both where statements. I have a host of variables that I want to use to build the expression for the linq lambda, but their presence or absence changes the expression such that I'd have to have a rediculous number of conditionals to satisfy all cases. I thought I could just add the Where() statements as in my first example above, but that doesn't end up in a single expression that contains all of the criteria. Therefore, I'm trying to create just the lambda itself as such: //bogus syntax if (var1 == "something") var expression = Expression<Func<item, bool>>(i => i.Property == "Something); if (var2 == "somethingElse") expression = expression.Where(i => i.Property2 == "SomethingElse"); And then pass that in to the where of my context.Items to evaluate. A) is this right, and B) if so, how do you do it?

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  • MySQL Query order by numer of rows?

    - by Clemens
    hi, i have a mysql table for votes. there's am id, a project_id and a vote field (which is 1 if a specific project is voted). now i want to generate a ranking from those entries. is there a way to get the number of votes for each project_id and automatically sort the entries by the number of TRUE votes of a project with a single mysql query? Or do you know a php way? e.g. ID - Project ID - Vote 1 - 2 - 1 2 - 2 - 1 3 - 1 - 1 == Project Nr. 2 has 2 Votes Project Nr. 1 has 1 Vote Thanks in advance!

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  • How to show unread subforums?

    - by bilygates
    I have written a simple forum in PHP using PostgreSQL. The forum consists of a number of subforums (or categories, if you like) that contain topics. I have a table that stores when was the last time a user visited a topic. It's something like this: user_id, topic_id, timestamp. I can easily determine what topics should be marked as unread by comparing the timestamp of the last topic reply with the timestamp of the last user visit. My question is: how do I efficiently determine what subforums (categories) should be marked as unread? All I've come up with is this: every time a user visits a topic, update the visit timestamp and check if all the topics from the current subforum are read or unread. If they are all read, mark the subforum as read for the user. Else, mark it as unread. But I think there must be another way. Thank you in advance.

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  • Adding values from different tables

    - by damdeok
    Friends, I have these tables: Contestant Table: Winner Peter Group Table: Id Name Score Union 1 Bryan 3 77 2 Mary 1 20 3 Peter 5 77 4 Joseph 2 25 5 John 6 77 I want to give additional score of 5 to Peter on Group Table. So, I came up with this query. UPDATE Group SET Score = Score+5 FROM Contestant, Group WHERE Contestant.Winner = Group.Name Now, I want also to give additional score of 5 to the same Union as Peter which is 77. How can I integrate it as one query to my existing query?

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  • transforming from 'Y' or 'N' to bit

    - by rap-uvic
    Hello, I have a table which has a column called Direct of type char(1). It's values are either 'Y' or 'N' or NULL. I am creating a view and I want the value to be transformed to either 0 or 1 of type bit. Right now it's of type INT. How do I go about doing this? Following is the code: CASE WHEN Direct = 'Y' THEN (SELECT 1) WHEN Direct <> 'Y' THEN (SELECT 0) END AS DirectDebit

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  • Pick Return Values of Stored Procedure

    - by Juergen
    Hi, I have a stored procedure that returns a result with 250!!! columns. But I only need 3 out of the 250. I want to call the SP and put only the 3 column values in a temp table. I don't want to define a temp table with 250 columns! This is how I would like to do it, but this doesn't work of course: create #myTempTable (id int, value1 int, value2 int) insert into #myTempTable exec myBigFatStoredProc Can it be done anyhow? Bye Juergen

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  • Using CASE Statements in LEFT OUTER JOIN in SQL

    - by s khan
    I've got a scenario where I want to switch on two different tables in an outer join. It goes something like this:- select mytable.id, yourtable.id from mytable left outer join (case when mytable.id = 2 then table2 yourtable on table1.id = table2.id else table3 yourtable on table1.id = table3.id end) ...but it doesn't work. Any suggestions?

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  • MS Access 2003 - Failure to create MDE file: error VBA is corrupt?

    - by Justin
    Ok so this is a brand new snag I have run into. I am trying to launch a new MDE from my source MDB file, and it is locking up Access. So in my mdb, I am first compacting and repairing, and then selecting create a new mde (just as I have done many times before). It looks like it is starting the process, but never gets to where it compacts when it is done, and access is not responding. So after I force close the app, I look in the folder where I am trying to create the MDE to and I see there is a new access db1 file there. If I try to open that it gives me an error that says file not found, and then it says the Visual Basic for Applications is corrupt. The thing is, I just made a very simple adjustment to the code since last launching an mde, and after this I double and triple checked it...its not that because its just a simple open this form and close this one addition. I did however have my source mdb file on a disc that I copied to my laptop, and then tried to re link the tables to the network drive (had them linked to other tables on my local drive so that I could develop offline)?? PLEASE HELP!!!

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