query-optimization

From Joel Spolsky's article on leaky abstractions: [C]ertain SQL queries are thousands of times slower than other logically equivalent queries. A famous example of this is that some SQL servers are dramatically faster if you specify "where a=b and b=c and a=c" than if you only specify "where a=b and b=c" even though the result set is the same. Does anyone know the details of this? Obviously, a = b and b = c => a = c - this is related to transitive closure. The point Joel was making is that some SQL servers are poor at optimizing queries, so some of the SQL queries might be written with "extra"

I am using MySQL version 5.5.14 to run the following query, QUERY 1 , from a table of 5 Million rows: SELECT P.ID, P.Type, P.Name, P.cty , X(P.latlng) as 'lat', Y(P.latlng) as 'lng' , P.cur, P.ak, P.tn, P.St, P.Tm, P.flA, P.ldA, P.flN , P.lv, P.bd, P.bt, P.nb , P.ak * E.usD as 'usP' FROM PIG P INNER JOIN EEL E ON E.cur = P.cur WHERE act='1' AND flA >= '1615' AND ldA >= '0' AND yr >= (YEAR(NOW()) - 100) AND lv >= '0' AND bd >= '3' AND bt >= '2' AND nb <= '5' AND cDate >= NOW() AND MBRContains(LineString( Point(39.9097, -2.1973) , Point(65.5130, 41.7480) ), latlng) AND Type = 'g' AND tn = 'l'

using PHP and MySQL i have grabbed an array of facebook user ids from facebook. Now i want to find the corresponding username in my application for this array. Clearly in my application the user table contains unique username and unique fb_uid values. my rudimentary understanding oof programming led me to 2 ways: 1) use a loop and run through the array of fb_uid and find the username one by one. OR 2) create a monster query like select distinct(username) from users where fb_uid = value1 OR fb_uid = value2 ... so is there a better way out? Thank you. Use SQL's IN operator instead: select

I have this SQL that works fine. Want the my filter to return the LATEST unique SessionGuids with the highest UserSessionSequenceID. Problem is performance sucks - even though I have good indexes. How can I rewrite this - to omit the ROW_NUMBER line? SELECT TOP(@resultCount) * FROM ( SELECT [UserSessionSequenceID] ,[SessionGuid] ,[IP] ,[Url] ,[UrlTitle] ,[SiteID] ,[BrowserWidth] ,[BrowserHeight] ,[Browser] ,[BrowserVersion] ,[Referer] ,[Timestamp] ,ROW_NUMBER() over (PARTITION BY [SessionGuid] ORDER BY UserSessionSequenceID DESC) AS sort FROM [tblSequence] ) AS t WHERE ([Timestamp] > DATEADD

I have an XML hierarchy like this in an XML type column of a table with 10,000 records- <Root> <Elem1> <Parent1> <Separator> <Child1/> </Separator> </Parent1> </Elem1> </Root> I have a query like this - DECLARE @Root VARCHAR(50) DECLARE @Entity VARCHAR(50) DECLARE @ParentNode VARCHAR(50) DECLARE @Separator VARCHAR(50) DECLARE @ChildNode VARCHAR(50) SET @Root = 'Root' SET @Entity = 'Elem1' SET @ParentNode = 'Parent1' SET @Separator = 'separator' SET @ChildNode = 'Child1' select Parent.P.value('.', 'varchar(max)') as MyValue, T.uniqueId, T.XMLCol from [XMLTable] as T cross apply (SELECT XMLTable

Could anyone clarify this point from the official MySQL documentation Indexes are used ... To find the MIN() or MAX() value for a specific indexed column key_col. This is optimized by a preprocessor that checks whether you are using WHERE key_part_N = constant on all key parts that occur before key_col in the index. In this case, MySQL does a single key lookup for each MIN() or MAX() expression and replaces it with a constant. If all expressions are replaced with constants, the query returns at once. For example: SELECT MIN(key_part2),MAX(key_part2) FROM tbl_name WHERE key_part1=10; So in

I'm using Rails and MySQL, and have an efficiency question based on row counting. I have a Project model that has_many :donations . I want to count the number of unique donors for a project. Is having a field in the projects table called num_donors , and incrementing it when a new donor is created a good idea? Or is something like @num_donors = Donor.count(:select => 'DISTINCT user_id') going to be similar or the same in terms of efficiency thanks to database optimization? Will this require me to create indexes for user_id and any other fields I want to count? Does the same answer hold for