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• Avoiding dog-piling or thundering herd in a memcached expiration scenario

  - by Quintin Par

  I have the result of a query that is very expensive. It is the join of several tables and a map reduce job. This is cached in memcached for 15 minutes. Once the cache expires the queries are obviously run and the cache warmed again. But at the point of expiration the thundering herd problem issue can happen. One way to fix this problem, that I do right now is to run a scheduled task that kicks in the 14th minute. But somehow this looks very sub optimal to me. Another approach I like is nginx’s proxy_cache_use_stale updating; mechanism. The webserver/machine continues to deliver stale cache while a thread kicks in the moment expiration happens and updates the cache. Has someone applied this to memcached scenario though I understand this is a client side strategy? If it benefits, I use Django.

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• MySQL query for initial filling of order column

  - by Sejanus

  Sorry for vague question title. I've got a table containing huge list of, say, products, belonging to different categories. There's a foreign key column indicating which category that particular product belongs to. I.e. in "bananas" row category might be 3 which indicates "fruits". Now I added additional column "order" which is for display order within that particular category. I need to do initial ordering. Since the list is big, I dont wanna change every row by hand. Is it possible to do with one or two queries? I dont care what initial order is as long as it starts with 1 and goes up. I cant do something like SET order = id because id counts from 1 up regardless of product category and order must start anew from 1 up for every different category.

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• MongoDB map/reduce counts

  - by ibz

  The output from MongoDB's map/reduce includes something like 'counts': {'input': I, 'emit': E, 'output': O}. I thought I clearly understand what those mean, until I hit a weird case which I can't explain. According to my understanding, counts.input is the number of rows that match the condition (as specified in query). If so, how is it possible that the following two queries have different results? db.mycollection.find({MY_CONDITION}).count() db.mycollection.mapReduce(SOME_MAP, SOME_REDUCE, {'query': {MY_CONDITION}}).counts.input I thought the two should always give the same result, independent of the map and reduce functions, as long as the same condition is used.

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• Making OR/M loosely coupled and abstracted away from other layers.

  - by Genuine

  Hi all. In an n-tier architecture, the best place to put an object-relational mapping (OR/M) code is in the data access layer. For example, database queries and updates can be delegated to a tool like NHibernate. Yet, I'd like to keep all references to NHibernate within the data access layer and abstract dependencies away from the layers below or above it. That way, I can swap or plug in another OR/M tool (e.g. Entity Framework) or some approach (e.g. plain vanilla stored procedure calls, mock objects) without causing compile-time errors or a major overhaul of the entire application. Testability is an added bonus. Could someone please suggest a wrapper (i.e. an interface or base class) or approach that would keep OR/M loosely coupled and contained in 1 layer? Or point me to resources that would help? Thanks.

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• Getting the final value to this MySQL query...

  - by Jack W-H

  I've got my database set up with three tables - code, tags, and code_tags for tagging posts. This will be the SQL query processed when a post is submitted. Each tag is sliced up by PHP and individually inserted using these queries. INSERT IGNORE INTO tags (tag) VALUES ('$tags[1]'); SELECT tags.id FROM tags WHERE tag = '$tags[1]' ORDER BY id DESC LIMIT 1; INSERT INTO code_tags (code_id, tag_id) VALUES ($codeid, WHAT_GOES_HERE?) The WHAT_GOES_HERE? value at the end is what I need to know. It needs to be the ID of the tag that the second query fetched. How can I put that ID into the third query? I hope I explained that correctly. I'll rephrase if necessary.

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• Is this a valid benefit of using embedded SQL over stored procedures?

  - by George

  Here's an argument for SPs that I haven't heard. Flamers, be gentle with the down tick, Since there is overhead associated with each trip to the database server, I would suggest that a POSSIBLE reason for placing your SQL in SPs over embedded code is that you are more insulated to change without taking a performance hit. For example. Let's say you need to perform Query A that returns a scalar integer. Then, later, the requirements change and you decide that it the results of the scalar is x that then, and only then, you need to perform another query. If you performed the first query in a SP, you could easily check the result of the first query and conditionally execute the 2nd SQL in the same SP. How would you do this efficiently in embedded SQL w/o perform a separate query or an unnecessary query? Here's an example: --This SP may return 1 or two queries. SELECT @CustCount = COUNT(*) FROM CUSTOMER IF @CustCount 10 SELECT * FROM PRODUCT Can this/what is the best way to do this in embedded SQL?

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• ASP.NET Thread Safety in aspx.cs code behind file

  - by Tim Michalski

  I am thinking of adding a DataContext as a member variable to my aspx.cs code-behind class for executing LinqToSql queries. Is this thread safe? I am not sure if a new instance of this code-behind class is created for each HTTP request, or if the instance is shared amongst all request threads? My fear is that I will get 10 simultaneous concurrent http requests that will be using the same database session. public partial class MyPage : System.Web.UI.Page { private DataContext myDB = new DataContext(); protected void MyAction_Click(object sender, EventArgs e) { myDB.DoWork(); } }

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• Has anyone used an object database with a large amount of data?

  - by Jon Kruger

  Object databases like MongoDB and db4o are getting lots of pub lately. Everyone that plays with them seems to love it. I'm guessing that they are dealing with about 640K of data in their sample apps. Has anyone tried to use an object database with a large amount of data (say, 50GB or more)? Are you able to still execute complex queries against it (like from a search screen)? How does it compare to your usual relational database of choice? I'm just curious. I want to take the object database plunge, but I need to know if it'll work on something more than a sample app.

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• Proper abstraction of the database tier in a 3 tier system?

  - by Earlz

  Hello, I am creating a 3 tier application. Basically it goes Client - (through optional server to be a thin-client) - Business Logic - Database Layer And basically making it so that there is never any skipping around. As such, I want for all of the SQL queries and such to be in the Database Layer. Well, now I'm a bit confused. I made a few static classes to start off the database tier but what should I do for the database connections? Should I just create a new database connection anytime I enter the Database Layer or would that be wasteful? Does Connection.Open() take time whenever you have a ConnectionPool? To me, it just feels wrong for the Business tier to have to pass in a IdbConnection object to the Database tier. It seems like the Database tier should handle all of that DB-specific code. What do you think? How can I do it the proper way while staying practical?

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• Servicestack CorsFeature Global Options Handler Not Firing on Certain Routes;

  - by gizmoboy

  I've got a service setup using the CorsFeature, and am using the approach that mythz suggested in other answers, collected in a function used in the appHost file: private void ConfigureCors(Funq.Container container) { Plugins.Add(new CorsFeature(allowedOrigins: "*", allowedMethods: "GET, POST, PUT, DELETE, OPTIONS", allowedHeaders: "Content-Type, Authorization, Accept", allowCredentials: true)); PreRequestFilters.Add((httpReq, httpRes) => { //Handles Request and closes Responses after emitting global HTTP Headers if (httpReq.HttpMethod == "OPTIONS") { httpRes.EndRequest(); } }); } However, the pre-request filter is only firing on some of the service requests. One of the base entities we have in the service is a question entity, and there are custom routes defined as follows: [Route("/question")] [Route("/question/{ReviewQuestionId}", "GET,DELETE")] [Route("/question/{ReviewQuestionId}/{ReviewSectionId}", "GET")] Using POSTMAN to fire test queries (all using the OPTIONS verb), we can see that this will fire the pre-request filter: http://localhost/myservice/api/question/ But this will not: http://localhost/myservice/api/question/66 Presumably, this is because the second and third routes explicitly defined the verbs they accept, and OPTIONS isn't one of them. Is it really necessary to spell out OPTIONS in every defined route that restricts the verbs supported?

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• Help needed in AdventureWorks in a sql query.

  - by vaibhav

  I was just playing with adventureworks database in sqlserver. I got stuck in a query. I wanted to Select all titles from HumanResources.Employee which are either 'Male' or 'Female' but not both. i.e if title Accountant is Male and Female both I want to leave that title. I need only those titles where Gender is either Male or Female. I have done this till yet. select distinct(title) from humanresources.employee where gender='M' select distinct(title) from humanresources.employee where gender='F' Probably a join between these two queries, would work. But If you have any other solution, please let me know. It is not a homework. :) Thanks in advance.

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• which is better, creating a materialized view or a new table?

  - by Carson

  I have some demanding mysql queries that are needed to grap same up-to-date datasets from 5-7 mysql tables. I am thinking of creating a table or materialized view to gather all demanding columns from other tables, so as to increase performance. If I create that table, I may need to do extra insert / update / delete operation each time other tables updated. if I create materialized view, I am worrying if the performance can be greatly improved. Because data from other tables are changing very frequently. Most likely, the view may need to be created first everytime before selecting it. Any ideas? e.g. how to cache? other extra measures I can do?

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• Getting user data from Active Directory using PL/SQL

  - by David Neale

  I had a discussion today regarding an Oracle procedure I wrote some time ago. I wanted to get 7500 user email addresses from Active Directory using PL/SQL. AD will return a maximum of 1000 rows and the LDAP provider used by Oracle will not support paging. Therefore, my solution was to filter on the last two characters of the sAMAccountName (*00,*01,*02...etc.). This results in 126 queries (100 for account names ending in digits, 26 for those ending in a letter...this was sufficient for my AD setup). The person I was speaking to (it was a job interview by the way) said he could have done it a better way, but he would not tell me what that method was. Could anybody hazard a guess at what this method was?

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• Overriding unique indexed values

  - by Yeti

  This is what I'm doing right now (name is UNIQUE): SELECT * FROM fruits WHERE name='apple'; Check if the query returned any result. If yes, don't do anything. If no, a new value has to be inserted: INSERT INTO fruits (name) VALUES ('apple'); Instead of the above is it ok to insert the value into the table without checking if it already exists? If the name already exists in the table, an error will be thrown and if it doesn't, a new record will be inserted. Right now I am having to insert 500 records in a for loop, which results in 1000 queries. Will it be ok to skip the "already-exists" check?

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• Help with an SQL query on a single (comments) table (screenshot included)

  - by citrus

  Please see screenshot Goal: id like to have comments nested 1 level deep The comments would be arranged so that rating of the parent is in descending order the rating of the children comments is irrelevant The left hand side of the screenshot shows the output that Id like. The RHS shows the table data. All of the comments are held in 1 table. Im a beginner with SQL queries, the best I can do is: SELECT * FROM [Comments] WHERE ([ArticleId] = @ArticleId) ORDER BY [ThreadId] DESC, [DateMade] This somewhat does the job, but it obviously neglects the rating. So the above statement would show output where Bobs Comment and all of the children comments are before Amy's and her childrens comments. How can I run this query correctly?

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• Post High Score and Retrieve Position

  - by majman

  I'm not so savvy with MYSQL, so my apologies in advance is this is a dumb question. I've created a super basic PHP High Scores table. Upon inserting a new score into the DB Table, I'd like to retrieve the position of that score so that I can get 10 results with the persons score falling within that range. My INSERT Query looks something like: $stmt = $mysqli->prepare("INSERT INTO highscores (name, time, score) VALUES (?, ?, ?)"); $stmt->bind_param('sdi', $name, $time, $score); UPDATE - I'm looking for a way to do this with as few queries as possible. I recall reading something about getting an INSERT ID when making an insert, but I would then still have to make a second query to get those results.

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• Temporary intermediate table

  - by user289429

  In our project to generate massive reports in oracle we use some permanent table to hold intermediate results. For example to generate one report we run few queries and populate the table, at the final step we join the intermediate table with huge application tables. These intermediate tables are cleared for next report run. We have few concerns in performance areas. These intermediate tables are transactional and don't have statistics. Is it good idea to join these with application tables which are partitioned and have up to date statistics. We need these results stored in the intermediate tables to be available across requests from UI hence we are not in a position to use oracle provided temporary tables. Any thoughts on what could be done would be appreciated.

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• Performing complex query with Dynamics CRM 4.0

  - by dub

  Hi, I have two custom entites, Product and ProductType, linked together in many-to-one relationship. Product has a lookup field to ProductType. I'm trying to write a query to fetch Type1 products with a price over 100, and Type2 products with a price lower than 100. Here's how I would do it in SQL : select * from Product P inner join ProductType T on T.Id = P.TypeId where (T.Code = 'Type1' and P.Price >= 100) or (T.Code = 'Type2' and P.Price < 100) I can't figure out a way to build a QueryExpression to do exactly that. I know I could do it with two queries, but I'd like to minimize roundtrips to the server. Is there a way to perform that query in only one operation ? Thanks!

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• Aggregate functions in ANSI SQL

  - by morpheous

  I want to use multiple aggregate functions in a query. All the examples i have seem on aggregate functions however, are trivial. Typically, they are of the form: SELECT field1,agg_func1, agg_func2 GROUP BY SOME_COLUMNS HAVING agg_func1 OP SOME_SCALAR Where: OP: is a boolean operator (e.g. <, = etc) SOME_SCALAR: is a scalar (i.e. a constant number) What I want to know is if it is possible to write (IN ANSI SQL) queries like: SELECT field1,agg_func1, agg_func2, agg_func3 GROUP BY SOME_COLUMNS HAVING (agg_func1 OP1 agg_func2) OP2 (agg_func2 OP3 agg_func3) Where: OP[N] are boolean operators or ANSI SQL clause operators like 'BETWEEN', 'LIKE', 'IN' etc. Also, assuming this is possible (I have not seen any documentation saying otherwise) are there any efficiency/performance considerations (i.e. penalties) when the HAVING clause consists of a boolean expression combining the output of the aggregate functions - instead of the normal comparison of the output of the aggregate with a constant number (e.g. min('salary') 100 ) - which is often used in the most banal examples involving aggregate functions?

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• Mysql - multiple values and between

  - by realshadow

  Hey, I need to select 10 products and display them. Each product has 3 different prices. The select to get the prices looks like this: SELECT * FROM products_loans WHERE CODE IN('10X15/12', '10X15/Q10-10', '10X15/Q20-10') AND 550 BETWEEN PRICE_FROM AND PRICE_TO; Where 550 is the base price. Now this select returns 3 rows, but I want to modify it so it will return 30 results. I dont like the idea to execute 10 queries at once. I know I can easily achieve that with "OR", but I would like to ask if there is some other more elegant way to this. The select "should" look like this: SELECT * FROM products_loans WHERE KOD IN('10X15/12', '10X15/Q10-10', '10X15/Q20-10') AND (550, 325, 780) BETWEEN CENA_OD AND CENA_DO; Note that there is no "price" column or anything in the table which I could use to do a JOIN and I cant modify the table.

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• Adding a MongoDB collection to Netbeans

  - by Saif Bechan

  In Netbeans I have an option to add my mysql databases to netbeans. This way I can easily browse and so small queries. Now I am working on a MongoDB project, and I want to know if it is possible to use the same functionality. I see that on the website of mongo there is a list of drivers, and I see that you can add drivers in netbeans. I do not know if the same thing, or if this can be used. I have tried google, but no luck. Anyone have an idea?

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• mongoDB many to many with one query?

  - by PowderKeg

  in mysql i use JOIN and one query is no problem. what about mongo? imagine categories and products. products may have more categories. categories may have more product. (many to many structure) and administrator may edit categories in administration (categories must be separated) its possible write product with categories names in one query? i used this structure categories { name:"categoryName", product_id:["4b5783300334000000000aa9","5783300334000000000aa943","6c6793300334001000000006"] } products { name:"productName", category_id:["4b5783300334000000000bb9","5783300334000000000bb943","6c6793300334001000000116"] } now i can simply get all product categories, and product in some category and categories alone for editation. but if i want write product with categories names i need two queries - one to get product categories id and second to get categories names from categories by that ids. is this the right way? or this structure is unsuitable? i would like to have only one query but i dont know if its possible.

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• "lock request time out period exceeded" Error When Trying to See DB Hierarchies

  - by Lloyd Banks

  I have a DB that I can run basic queries (albeit much slower than normal) off of. When I try to see the hierarchy trees for tables, views, or procedures in SSMS Object Explorer, I get the "lock request time out period exceeded". My Report Server reports that run off of objects in this DB are no longer completing. Jobs associated with procedures stored on this DB also do not run. I tried using sp_who2 to find and kill all connections on the DB. This has not solved the problem. What is going on here? How can I resolve this?

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• How to limit data to users who own it without limiting admin users in CakePHP?

  - by cdburgess

  Currently I am writing an application where I have multiple users. They have data that should only be visible to them and not the other authenticated users in the system. I also have administrators who manage the system and have access to all of the information. What is the best way to limit users to their data without limiting admin users? Currently I am using a callback to limit the queries by user, but the admin will get the same limits. So I need to know a better way to do it. More importantly, the right way to do it. For example, I want the standard user to be able to see their user information only and be limited to CRUD operations on their information only. The admin, however, should be able to see ALL users and CRUD ALL user data. Any ideas?

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• Custom keys for Google App Engine models (Python)

  - by Cameron

  First off, I'm relatively new to Google App Engine, so I'm probably doing something silly. Say I've got a model Foo: class Foo(db.Model): name = db.StringProperty() I want to use name as a unique key for every Foo object. How is this done? When I want to get a specific Foo object, I currently query the datastore for all Foo objects with the target unique name, but queries are slow (plus it's a pain to ensure that name is unique when each new Foo is created). There's got to be a better way to do this! Thanks.

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