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  • MySQL: fetching a null or an empty string

    - by Oden
    Hey, I know whats the difference between a NULL value and an empty string ("") value, but if I want to get a value by using the OR keyword, I get no result for a NULL value The table i want to query looks like this: titles_and_tags +----+----------+------+ | id | title | tag | +----+----------+------+ | 1 | title1 | NULL | | 2 | title2 | tag1 | | 3 | title3 | tag2 | +----+----------+------+ The query i use looks like this: select * from `titles_and_tags` WHERE `title` LIKE "title" AND `tag` = "tag1" OR `tag` IS NULL So i want to get here a rows (id: 1,2), BUT this results 0 rows. What have i done wrong?

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  • mySQL: Joining three tables - how?

    - by Industrial
    Hi everybody, I have the following query in my application. It works well, but I need it to also contain the number of products that are associated with each manufacturer. The current query: SELECT * FROM (`manufacturers`) JOIN `languages` ON `manufacturers`.`lang` = `languages`.`id` ORDER BY `languages`.`id` asc, `id` asc My products table looks like this: id | name | manufacturerid 0 | Product1 | 0

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  • MySQL multiple dependent subqueries, painfully slow

    - by matt80
    I have a working query that retrieves the data that I need, but unfortunately it is painfully slow (runs over 3 minutes). I have indexes in place, but I think the problem is the multiple dependent subqueries. I've been trying to rewrite the query using joins but I can't seem to get it to work. Any help would be greatly appreciated. The tables: Basically, I have 2 tables. The first (prices) holds the prices of items in a store. Each row is the price of an item that day, and new rows are added every day with an updated price. The second table (watches_US) holds the item information (name, description, etc). CREATE TABLE `prices` ( `prices_id` int(11) NOT NULL auto_increment, `prices_locale` enum('CA','DE','FR','JP','UK','US') NOT NULL default 'US', `prices_watches_ID` char(10) NOT NULL, `prices_date` datetime NOT NULL, `prices_am` varchar(10) default NULL, `prices_new` varchar(10) default NULL, `prices_used` varchar(10) default NULL, PRIMARY KEY (`prices_id`), KEY `prices_am` (`prices_am`), KEY `prices_locale` (`prices_locale`), KEY `prices_watches_ID` (`prices_watches_ID`), KEY `prices_date` (`prices_date`) ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=61764 ; CREATE TABLE `watches_US` ( `watches_ID` char(10) NOT NULL, `watches_date_added` datetime NOT NULL, `watches_last_update` datetime default NULL, `watches_title` varchar(255) default NULL, `watches_small_image_height` int(11) default NULL, `watches_small_image_width` int(11) default NULL, `watches_description` text, PRIMARY KEY (`watches_ID`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; The query retrieves the last 10 prices changes over a period of 30 hours, ordered by the size of the price change. So I have subqueries to get the newest price, the oldest price within 30 hours, and then to calculate the price change. Here's the query: SELECT watches_US.*, prices.*, watches_US.watches_ID as current_ID, ( SELECT prices_am FROM prices WHERE prices_watches_ID = current_ID AND prices_locale = 'US' ORDER BY prices_date DESC LIMIT 1 ) as new_price, ( SELECT prices_date FROM prices WHERE prices_watches_ID = current_ID AND prices_locale = 'US' ORDER BY prices_date DESC LIMIT 1 ) as new_price_date, ( SELECT prices_am FROM prices WHERE ( prices_watches_ID = current_ID AND prices_locale = 'US') AND ( prices_date >= DATE_SUB(new_price_date,INTERVAL 30 HOUR) ) ORDER BY prices_date ASC LIMIT 1 ) as old_price, ( SELECT ROUND(((new_price - old_price)/old_price)*100,2) ) as percent_change, ( SELECT (new_price - old_price) ) as absolute_change FROM watches_US LEFT OUTER JOIN prices ON prices.prices_watches_ID = watches_US.watches_ID WHERE ( prices_locale = 'US' ) AND ( prices_am IS NOT NULL ) AND ( prices_am != '' ) HAVING ( old_price IS NOT NULL ) AND ( old_price != 0 ) AND ( old_price != '' ) AND ( absolute_change < 0 ) AND ( prices.prices_date = new_price_date ) ORDER BY absolute_change ASC LIMIT 10 How would I rewrite this to use joins instead, or otherwise optimize this so it doesn't take over 3 minutes to get a result? Any help would be greatly appreciated! Thank you kindly.

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  • get-wmiobject sql join in powershell - trying to find physical memory vs. virtual memory of remote s

    - by Willy
    get-wmiobject -query "Select TotalPhysicalMemory from Win32_LogicalMemoryConfiguration" -computer COMPUTERNAME output.csv get-wmiobject -query "Select TotalPageFileSpace from Win32_LogicalMemoryConfiguration" -computer COMPUTERNAME output.csv I am trying to complete this script with an output as such: Computer Physical Memory Virtual Memory server1 4096mb 8000mb server2 2048mb 4000mb

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  • How can this SQL be wrong? What am I not seeing?

    - by ropstah
    Can anybody please spot my error, this should be a legal query in SQL shouldn't it?? Unknown column u.usr_auto_key in the ON clause This is the database schema: User: (usr_auto_key, name, etc...) Setting: (set_auto_key, name etc..) User_Setting: (usr_auto_key, set_auto_key, value) And this is the query... SELECT `u`.`usr_auto_key` AS `u__usr_auto_key`, `s`.`set_auto_key` AS `s__set_auto_key`, `u2`.`usr_auto_key` AS `u2__usr_auto_key`, `u2`.`set_auto_key` AS `u2__set_auto_key`, `u2`.`value` AS `u2__value` FROM `User` `u`, `Setting` `s` LEFT JOIN `User_Setting` `u2` ON `u`.`usr_auto_key` = `u2`.`usr_auto_key` WHERE (`s`.`sct_auto_key` = 1 AND `u`.`usr_auto_key` = 1 AND admin_property is null)

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  • Mysql - GROUP BY Avoid using tempoary

    - by jwzk
    The goal of this query is to get a total of unique records (by IP) per ref ID. SELECT COUNT(DISTINCT ip), GROUP_CONCAT(ref.id) FROM `sess` sess JOIN `ref` USING(row_id) WHERE sess.time BETWEEN '2010-04-21 00:00:00' AND '2010-04-21 23:59:59' GROUP BY ref.id ORDER BY sess.time DESC The query works fine, but its using a temporary table. Any ideas?

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  • Data from two tables without repeating data from the first?

    - by Aran
    I have two tables. Users table and Users Meta Table I am looking for a way to get all the information out of both tables with one query. But without repeating the information from Users table. This is all information relating to the users id number as well. So for example user_id = 1. Is there a way to query the database and collect all the information I from both tables without repeating the information from the first?

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  • NUL symbol gets inserted anywhere in the XML

    - by Racs
    I am using Oracle 11g and when I query data from database manually, it looks ok. But when I view it using Notepad++, there is a special character that gets inserted anywhere in the xml produced. Please see image below: I tried everything and deleted some parts of the xml, updated the db and query again but the invalid character just differ in location. Any idea is much appreciated, thanks! Best regards, Racs

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  • Configuring varnish and django (apache/modwsgi)

    - by Hedde
    I am trying to work out why my application keeps hitting the database while I have setup varnish infront of apache. I think I am missing some vital configuration, any tips are welcome This is my curl result: HTTP/1.1 200 OK Server: Apache/2.2.16 (Debian) Content-Language: en-us Vary: Accept,Accept-Encoding,Accept-Language,Cookie Cache-Control: s-maxage=60, no-transform, max-age=60 Content-Type: application/json; charset=utf-8 Date: Sat, 15 Sep 2012 08:19:17 GMT Connection: keep-alive My varnishlog: 13 BackendClose - apache 13 BackendOpen b apache 127.0.0.1 47665 127.0.0.1 8000 13 TxRequest b GET 13 TxURL b /api/v1/events/?format=json 13 TxProtocol b HTTP/1.1 13 TxHeader b User-Agent: curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8r zlib/1.2.3 13 TxHeader b Host: foobar.com 13 TxHeader b Accept: */* 13 TxHeader b X-Forwarded-For: 92.64.200.145 13 TxHeader b X-Varnish: 979305817 13 TxHeader b Accept-Encoding: gzip 13 RxProtocol b HTTP/1.1 13 RxStatus b 200 13 RxResponse b OK 13 RxHeader b Date: Sat, 15 Sep 2012 08:21:28 GMT 13 RxHeader b Server: Apache/2.2.16 (Debian) 13 RxHeader b Content-Language: en-us 13 RxHeader b Content-Encoding: gzip 13 RxHeader b Vary: Accept,Accept-Encoding,Accept-Language,Cookie 13 RxHeader b Cache-Control: s-maxage=60, no-transform, max-age=60 13 RxHeader b Content-Length: 6399 13 RxHeader b Content-Type: application/json; charset=utf-8 13 Fetch_Body b 4(length) cls 0 mklen 1 13 Length b 6399 13 BackendReuse b apache 11 SessionOpen c 92.64.200.145 53236 :80 11 ReqStart c 92.64.200.145 53236 979305817 11 RxRequest c HEAD 11 RxURL c /api/v1/events/?format=json 11 RxProtocol c HTTP/1.1 11 RxHeader c User-Agent: curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8r zlib/1.2.3 11 RxHeader c Host: foobar.com 11 RxHeader c Accept: */* 11 VCL_call c recv lookup 11 VCL_call c hash 11 Hash c /api/v1/events/?format=json 11 Hash c foobar.com 11 VCL_return c hash 11 VCL_call c miss fetch 11 Backend c 13 apache apache 11 TTL c 979305817 RFC 60 -1 -1 1347697289 0 1347697288 0 60 11 VCL_call c fetch deliver 11 ObjProtocol c HTTP/1.1 11 ObjResponse c OK 11 ObjHeader c Date: Sat, 15 Sep 2012 08:21:28 GMT 11 ObjHeader c Server: Apache/2.2.16 (Debian) 11 ObjHeader c Content-Language: en-us 11 ObjHeader c Content-Encoding: gzip 11 ObjHeader c Vary: Accept,Accept-Encoding,Accept-Language,Cookie 11 ObjHeader c Cache-Control: s-maxage=60, no-transform, max-age=60 11 ObjHeader c Content-Type: application/json; charset=utf-8 11 Gzip c u F - 6399 69865 80 80 51128 11 VCL_call c deliver deliver 11 TxProtocol c HTTP/1.1 11 TxStatus c 200 11 TxResponse c OK 11 TxHeader c Server: Apache/2.2.16 (Debian) 11 TxHeader c Content-Language: en-us 11 TxHeader c Vary: Accept,Accept-Encoding,Accept-Language,Cookie 11 TxHeader c Cache-Control: s-maxage=60, no-transform, max-age=60 11 TxHeader c Content-Type: application/json; charset=utf-8 11 TxHeader c Date: Sat, 15 Sep 2012 08:21:29 GMT 11 TxHeader c Connection: keep-alive 11 Length c 0 11 ReqEnd c 979305817 1347697288.292612076 1347697289.456128597 0.000086784 1.163468122 0.000048399

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  • Better way to design a database

    - by cMinor
    I have a conceptual problem and I would like to get your ideas on how I'll be able to do what I am aiming. My goal is to create a database with information of persons who work at a place depending on their profession and skills,and keep control of salary and projects (how much would cost summing all the hours of work) I have 3 categories which can have subcategories: Outsourcing Technician welder turner assistant Administrative supervisor manager So each person has its information and the projects they are working on, also one person may do several jobs... I was thinking about having 5 tables (EMPLOYEE, SKILLS, PROYECTS, SALARY, PROFESSION) but I guess there is a better way of doing this. create table Employee ( PRIMARY KEY [Person_ID] int(10), [Name] varchar(30), [sex] varchar(10), [address] varchar(10), [profession] varchar(10), [Skills_ID] int(10), [Proyect_ID] int(10), [Salary_ID] int(10), [Salary] float ) create table Skills ( PRIMARY KEY [Skills_ID] int(10), FOREIGN KEY [Skills_name] varchar(10) REFERENCES Employee(Person_ID), [Skills_pay] float(10), [Comments] varchar(50) ) create table Proyects ( PRIMARY KEY [Proyect_ID] int(10), FOREIGN KEY [Skills_name] varchar(10) REFERENCES Employee(Person_ID) [Proyect_name] varchar(10), [working_Hours] float(10), [Comments] varchar(50) ) create table Salary ( PRIMARY KEY [Salary_ID] int(10), FOREIGN KEY [Skills_name] varchar(10) REFERENCES Employee(Person_ID) [Proyect_name] varchar(10), [working_Hours] float(10), [Comments] varchar(50) ) So to get the total amount of the cost of a project I would just sum the working hours of each employee envolved and sum some extra costs in an aggregate query. Is there a way to do this in a more efficient way? What to add or delete of this small model? I guess I am missing something in the salary - maybe I need another table for that?

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  • Trying to Set Up SSH Tunneling To MySQL Server for MySQL Query Browser

    - by Teno
    I'm trying to set up SSH tunneling on a remote web server to another MySQL server so that the database can be browsed easily with MySQL Query Browser. I'm following this page but cannot connect to the MySQL server. http://www.howtogeek.com/howto/ubuntu/access-your-mysql-server-remotely-over-ssh/ What I've done: logged in to the web server with Putty via SSH. typed ssh -L 33060:[database]:3306 [myusername]@[webserver_address] where [...]s are altered by the actual information. I was asked a password and typed it and got the following message. So it seems login was successful. socket: Protocol not supported Last login: .... 2012 from .... Copyright (c) 1980, 1983, 1986, 1988, 1990, 1991, 1993, 1994 The Regents of the University of California. All rights reserved. FreeBSD 7.1-RELEASE.... Welcome to FreeBSD! Opened MySQL Query Browser in Windows and entered Server Host: localhost Port: 33060 UserName: myusername PassWord: mypassword And it says, Could not connect to the specified instance. MySQL Error Number 2003 Can't connect to MySQL Server on 'localhost' (10061) Sorry if this is too basic. Thanks for your information.

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  • DNS something is wrong?

    - by Nickolas R.
    Hello I am configuring bind9 on a server with two network interfaces, one is connected to the LAN and the other is connected to the Internet through NAT so bind is not faced directly to the Internet. Everything seems to work fine, clients can do both forward and reverse lookups but somethings seems strange. On the server if i try to ping www.google.com one time, a great amount of network activity is genereated, alot more that one would expect so i decided to sniff the traffic with tcpdump. When loading the dump into Wireshark i can see about 250 entries with "Standard query A" and "Standard query response" Here a some of the entries from the dump DNS Standard query A www.google.com DNS Standard query A blackhole-1.iana.org DNS Standard query A blackhole-2.iana.org DNS Standard query response DNS Standard query A ns2.isc-sns.com DNS Standard query A ns1.isc-sns.net DNS Standard query A ns3.isc-sns.info DNS Standard query response PTR b.iana-servers.net RRSIG DNS Standard query A auth2.dns.cogentco.com DNS Standard query A ns1.crsnic.net DNS Standard query A ns2.nsiregistry.net DNS Standard query A ns3.verisign-grs.net DNS Standard query A ns4.verisign-grs.net DNS Standard query PTR 79.52.19.199.in-addr.arpa I do not have too much experince with DNS yet, but i am pretty sure that something is wrong. Anybody that have an idea of whats is going on?

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  • Mysql server high trafic makes websites really slow or unable to load

    - by Holapress
    Lately we have been having a lot of problems with our mysql server, from websites being really slow or even unable to load them at all. The server is a dedicated server that only runs our mysql database. i have been running some test using a profiler (JetProfiler) and tool to stress test (loadUI). If I use loadUI to connect with 50 simultaneous connections to one of our websites that runs a resently big query it will already make the website be unable to load. One of the things that makes me worried is that when I look at Jetprofile it always shows a Treads_connected of 1.00 and it seems that when it hits around 2.00 that I'm unable to connect. The 3 big peaks are when I run a test with loadUI, first one was 15 simultaneous connections wich made it still able for me to load the website but just really slow, the second one was 40 simultaneous connections which already made it impossible to load and the third one was with 100 connection which also didn't make it load anymore. Another thing that worries me is that in JetProfiler it says all the queries that get used are full table scans, could this maybe be the problem? The website I run as a test runs 3 queries, one for a menu that outputs around 1000 rows, one for the adds that has around 560 rows and a big one to get posts that has around 7000 rows (see screenshot bellow) I also have monitored the cpu of the server and there seems to be no problem there, even when I make a lot of connections with loadui the cpu stays low. I can't seem to figure out what is the main cause of the websites being unable to load when there is a high amount of traffic, if anyone has other suggestions for testing or something that might cause the problem please let me know.

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  • How to create a WHM/cPanel account, without creating a new sub-domain?

    - by Cyclops
    I have a basic VPS (full root access), with WHM/cPanel, and am learning the ropes. I'm trying to create a new account for an existing domain (mysite.com), and so far WHM won't let me - it either wants a sub-domain or fake domain, but won't allow two accounts for one domain. In the beginning, there was only the root account, and it wouldn't let me login to cPanel - a quick chat with tech support, and I am informed that I need to create a second account, which I did. So now I have an account, call it ns1me, for domain mysite.com. Now I want to create a django account. I go through the same process, but WHM won't allow me to use mysite.com as the domain for django. The docs recommend a sub-domain, so I fill the box in with django.mysite.com. I then realize that has actually created a sub-domain - going to django.mysite.com shows me its home directory, along with helpful information about what version of Apache, Python, and other mods its running (thanks, Apache). I really don't want a sub-domain, so that's out. Another chat with tech support, and they recommend a fake domain name, as it won't create anything. Sure enough, using a domain of djangomysite.com works, and WHM allows me to create a django account. But of course, I can't send email to [email protected] (where I could to [email protected]). What I want, is to be able to create a second account, associated with mysite.com (so I can run cPanel logged in as django, send email to [email protected], etc) - without creating a whole new sub-domain, or fake domain.

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  • Managing records of bugs and notes

    - by Jim
    Hi. I want to create a knowledgebase for a piece of software. I'd also like to be able to track bugs and common points of failure in that application. Linking knowledgebase articles to bug records would be a real boon, as would the ability to do complex queries for particular articles and bugs on the basis of tags or metadata. I've never done anything like this before, and like to install as little as possible. I've been looking at creating a wiki with Wiki On A Stick, and it seems to offer a lot. But I can't make complex queries. I can create pages that list all 'articles' with a particular single tag, but I can't specify multiple tags or filters. Is there any software that can help? I don't want to spend money until I've tried something out thoroughly, and I'd ideally like something that demands little-to-no installation. Are there any tools that can help me? If something could easily export its data, or stored data in XML, that would be a real plus too. Otherwise, are there any simple apps that allow me to set up forms for bugs, store data as XML then query and process that XML on demand? Thanks in advance.

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  • Passing integer lists in a sql query, best practices

    - by Artiom Chilaru
    I'm currently looking at ways to pass lists of integers in a SQL query, and try to decide which of them is best in which situation, what are the benefots of each, and what are the pitfalls, what should be avoided :) Right now I know of 3 ways that we currently use in our application. 1) Table valued parameter: Create a new Table Valued Parameter in sql server: CREATE TYPE [dbo].[TVP_INT] AS TABLE( [ID] [int] NOT NULL ) Then run the query against it: using (var conn = new SqlConnection(DataContext.GetDefaultConnectionString)) { var comm = conn.CreateCommand(); comm.CommandType = CommandType.Text; comm.CommandText = @" UPDATE DA SET [tsLastImportAttempt] = CURRENT_TIMESTAMP FROM [Account] DA JOIN @values IDs ON DA.ID = IDs.ID"; comm.Parameters.Add(new SqlParameter("values", downloadResults.Select(d => d.ID).ToDataTable()) { TypeName = "TVP_INT" }); conn.Open(); comm.ExecuteScalar(); } The major disadvantages of this method is the fact that Linq doesn't support table valued params (if you create an SP with a TVP param, linq won't be able to run it) :( 2) Convert the list to Binary and use it in Linq! This is a bit better.. Create an SP, and you can run it within linq :) To do this, the SP will have an IMAGE parameter, and we'll be using a user defined function (udf) to convert this to a table.. We currently have implementations of this function written in C++ and in assembly, both have pretty much the same performance :) Basically, each integer is represented by 4 bytes, and passed to the SP. In .NET we have an extension method that convers an IEnumerable to a byte array The extension method: public static Byte[] ToBinary(this IEnumerable intList) { return ToBinaryEnum(intList).ToArray(); } private static IEnumerable<Byte> ToBinaryEnum(IEnumerable<Int32> intList) { IEnumerator<Int32> marker = intList.GetEnumerator(); while (marker.MoveNext()) { Byte[] result = BitConverter.GetBytes(marker.Current); Array.Reverse(result); foreach (byte b in result) yield return b; } } The SP: CREATE PROCEDURE [Accounts-UpdateImportAttempts] @values IMAGE AS BEGIN UPDATE DA SET [tsLastImportAttempt] = CURRENT_TIMESTAMP FROM [Account] DA JOIN dbo.udfIntegerArray(@values, 4) IDs ON DA.ID = IDs.Value4 END And we can use it by running the SP directly, or in any linq query we need using (var db = new DataContext()) { db.Accounts_UpdateImportAttempts(downloadResults.Select(d => d.ID).ToBinary()); // or var accounts = db.Accounts .Where(a => db.udfIntegerArray(downloadResults.Select(d => d.ID).ToBinary(), 4) .Select(i => i.Value4) .Contains(a.ID)); } This method has the benefit of using compiled queries in linq (which will have the same sql definition, and query plan, so will also be cached), and can be used in SPs as well. Both these methods are theoretically unlimited, so you can pass millions of ints at a time :) 3) The simple linq .Contains() It's a more simple approach, and is perfect in simple scenarios. But is of course limited by this. using (var db = new DataContext()) { var accounts = db.Accounts .Where(a => downloadResults.Select(d => d.ID).Contains(a.ID)); } The biggest drawback of this method is that each integer in the downloadResults variable will be passed as a separate int.. In this case, the query is limited by sql (max allowed parameters in a sql query, which is a couple of thousand, if I remember right). So I'd like to ask.. What do you think is the best of these, and what other methods and approaches have I missed?

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  • GeoDjango: Exceptions for basic geographic queries

    - by omat
    I am having problems with geographic queries with GeoDjango running on SpatiaLite on my development environment. from django.contrib.gis.db import models class Test(models.Model): poly = models.PolygonField() point = models.PointField() geom = models.GeometryField() objects = models.GeoManager() Testing via shell: >>> from geotest.models import Test >>> from django.contrib.gis.geos import GEOSGeometry >>> >>> point = GEOSGeometry("POINT(0 0)") >>> point <Point object at 0x105743490> >>> poly = GEOSGeometry("POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))") >>> poly <Polygon object at 0x105743370> >>> With these definitions, lets try some basic geographic queries. First contains: >>> Test.objects.filter(point__within=poly) Assertion failed: (0), function appendGeometryTaggedText, file WKTWriter.cpp, line 228. Abort trap And the django shell dies. And with within: >>> Test.objects.filter(poly__contains=point) GEOS_ERROR: Geometry must be a Point or LineString Traceback (most recent call last): ... GEOSException: Error encountered checking Coordinate Sequence returned from GEOS C function "GEOSGeom_getCoordSeq_r". >>> Other combinations also cause a variety of exceptions. I must be missing something obvious as these are very basic. Any ideas?

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  • Query doesn't use a covering-index when applicable

    - by Dor
    I've downloaded the employees database and executed some queries for benchmarking purposes. Then I noticed that one query didn't use a covering index, although there was a corresponding index that I created earlier. Only when I added a FORCE INDEX clause to the query, it used a covering index. I've uploaded two files, one is the executed SQL queries and the other is the results. Can you tell why the query uses a covering-index only when a FORCE INDEX clause is added? The EXPLAIN shows that in both cases, the index dept_no_from_date_idx is being used anyway. To adapt myself to the standards of SO, I'm also writing the content of the two files here: The SQL queries: USE employees; /* Creating an index for an index-covered query */ CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date); /* Show `dept_emp` table structure, indexes and generic data */ SHOW TABLE STATUS LIKE "dept_emp"; DESCRIBE dept_emp; SHOW KEYS IN dept_emp; /* The EXPLAIN shows that the subquery doesn't use a covering-index */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery should use a covering index, but isn't */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`); /* The EXPLAIN shows that the subquery DOES use a covering-index, thanks to the FORCE INDEX clause */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery use a covering index */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`); The results: -------------- /* Creating an index for an index-covered query */ CREATE INDEX dept_no_from_date_idx ON dept_emp (dept_no, from_date) -------------- Query OK, 331603 rows affected (33.95 sec) Records: 331603 Duplicates: 0 Warnings: 0 -------------- /* Show `dept_emp` table structure, indexes and generic data */ SHOW TABLE STATUS LIKE "dept_emp" -------------- +----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+ | Name | Engine | Version | Row_format | Rows | Avg_row_length | Data_length | Max_data_length | Index_length | Data_free | Auto_increment | Create_time | Update_time | Check_time | Collation | Checksum | Create_options | Comment | +----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+ | dept_emp | InnoDB | 10 | Compact | 331883 | 36 | 12075008 | 0 | 21544960 | 29360128 | NULL | 2010-05-04 13:07:49 | NULL | NULL | utf8_general_ci | NULL | | | +----------+--------+---------+------------+--------+----------------+-------------+-----------------+--------------+-----------+----------------+---------------------+-------------+------------+-----------------+----------+----------------+---------+ 1 row in set (0.47 sec) -------------- DESCRIBE dept_emp -------------- +-----------+---------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +-----------+---------+------+-----+---------+-------+ | emp_no | int(11) | NO | PRI | NULL | | | dept_no | char(4) | NO | PRI | NULL | | | from_date | date | NO | | NULL | | | to_date | date | NO | | NULL | | +-----------+---------+------+-----+---------+-------+ 4 rows in set (0.05 sec) -------------- SHOW KEYS IN dept_emp -------------- +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | dept_emp | 0 | PRIMARY | 1 | emp_no | A | 331883 | NULL | NULL | | BTREE | | | dept_emp | 0 | PRIMARY | 2 | dept_no | A | 331883 | NULL | NULL | | BTREE | | | dept_emp | 1 | emp_no | 1 | emp_no | A | 331883 | NULL | NULL | | BTREE | | | dept_emp | 1 | dept_no | 1 | dept_no | A | 7 | NULL | NULL | | BTREE | | | dept_emp | 1 | dept_no_from_date_idx | 1 | dept_no | A | 13 | NULL | NULL | | BTREE | | | dept_emp | 1 | dept_no_from_date_idx | 2 | from_date | A | 165941 | NULL | NULL | | BTREE | | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 6 rows in set (0.23 sec) -------------- /* The EXPLAIN shows that the subquery doesn't use a covering-index */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery should use a covering index, but isn't */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`) -------------- +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+ | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 50 | | | 1 | PRIMARY | dept_emp | eq_ref | PRIMARY,emp_no,dept_no,dept_no_from_date_idx | PRIMARY | 16 | der.emp_no,der.dept_no | 1 | | | 2 | DERIVED | dept_emp | ref | dept_no,dept_no_from_date_idx | dept_no_from_date_idx | 12 | | 21402 | Using where | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+-------------+ 3 rows in set (0.09 sec) -------------- /* The EXPLAIN shows that the subquery DOES use a covering-index, thanks to the FORCE INDEX clause */ EXPLAIN SELECT SQL_NO_CACHE * FROM dept_emp INNER JOIN ( /* The subquery use a covering index */ SELECT SQL_NO_CACHE emp_no, dept_no FROM dept_emp FORCE INDEX(dept_no_from_date_idx) WHERE dept_no="d001" ORDER BY from_date DESC LIMIT 20000,50 ) AS `der` USING (`emp_no`, `dept_no`) -------------- +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+ | 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 50 | | | 1 | PRIMARY | dept_emp | eq_ref | PRIMARY,emp_no,dept_no,dept_no_from_date_idx | PRIMARY | 16 | der.emp_no,der.dept_no | 1 | | | 2 | DERIVED | dept_emp | ref | dept_no_from_date_idx | dept_no_from_date_idx | 12 | | 37468 | Using where; Using index | +----+-------------+------------+--------+----------------------------------------------+-----------------------+---------+------------------------+-------+--------------------------+ 3 rows in set (0.05 sec) Bye

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