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  • Heaps of Trouble?

    - by Paul White NZ
    If you’re not already a regular reader of Brad Schulz’s blog, you’re missing out on some great material.  In his latest entry, he is tasked with optimizing a query run against tables that have no indexes at all.  The problem is, predictably, that performance is not very good.  The catch is that we are not allowed to create any indexes (or even new statistics) as part of our optimization efforts. In this post, I’m going to look at the problem from a slightly different angle, and present an alternative solution to the one Brad found.  Inevitably, there’s going to be some overlap between our entries, and while you don’t necessarily need to read Brad’s post before this one, I do strongly recommend that you read it at some stage; he covers some important points that I won’t cover again here. The Example We’ll use data from the AdventureWorks database, copied to temporary unindexed tables.  A script to create these structures is shown below: CREATE TABLE #Custs ( CustomerID INTEGER NOT NULL, TerritoryID INTEGER NULL, CustomerType NCHAR(1) COLLATE SQL_Latin1_General_CP1_CI_AI NOT NULL, ); GO CREATE TABLE #Prods ( ProductMainID INTEGER NOT NULL, ProductSubID INTEGER NOT NULL, ProductSubSubID INTEGER NOT NULL, Name NVARCHAR(50) COLLATE SQL_Latin1_General_CP1_CI_AI NOT NULL, ); GO CREATE TABLE #OrdHeader ( SalesOrderID INTEGER NOT NULL, OrderDate DATETIME NOT NULL, SalesOrderNumber NVARCHAR(25) COLLATE SQL_Latin1_General_CP1_CI_AI NOT NULL, CustomerID INTEGER NOT NULL, ); GO CREATE TABLE #OrdDetail ( SalesOrderID INTEGER NOT NULL, OrderQty SMALLINT NOT NULL, LineTotal NUMERIC(38,6) NOT NULL, ProductMainID INTEGER NOT NULL, ProductSubID INTEGER NOT NULL, ProductSubSubID INTEGER NOT NULL, ); GO INSERT #Custs ( CustomerID, TerritoryID, CustomerType ) SELECT C.CustomerID, C.TerritoryID, C.CustomerType FROM AdventureWorks.Sales.Customer C WITH (TABLOCK); GO INSERT #Prods ( ProductMainID, ProductSubID, ProductSubSubID, Name ) SELECT P.ProductID, P.ProductID, P.ProductID, P.Name FROM AdventureWorks.Production.Product P WITH (TABLOCK); GO INSERT #OrdHeader ( SalesOrderID, OrderDate, SalesOrderNumber, CustomerID ) SELECT H.SalesOrderID, H.OrderDate, H.SalesOrderNumber, H.CustomerID FROM AdventureWorks.Sales.SalesOrderHeader H WITH (TABLOCK); GO INSERT #OrdDetail ( SalesOrderID, OrderQty, LineTotal, ProductMainID, ProductSubID, ProductSubSubID ) SELECT D.SalesOrderID, D.OrderQty, D.LineTotal, D.ProductID, D.ProductID, D.ProductID FROM AdventureWorks.Sales.SalesOrderDetail D WITH (TABLOCK); The query itself is a simple join of the four tables: SELECT P.ProductMainID AS PID, P.Name, D.OrderQty, H.SalesOrderNumber, H.OrderDate, C.TerritoryID FROM #Prods P JOIN #OrdDetail D ON P.ProductMainID = D.ProductMainID AND P.ProductSubID = D.ProductSubID AND P.ProductSubSubID = D.ProductSubSubID JOIN #OrdHeader H ON D.SalesOrderID = H.SalesOrderID JOIN #Custs C ON H.CustomerID = C.CustomerID ORDER BY P.ProductMainID ASC OPTION (RECOMPILE, MAXDOP 1); Remember that these tables have no indexes at all, and only the single-column sampled statistics SQL Server automatically creates (assuming default settings).  The estimated query plan produced for the test query looks like this (click to enlarge): The Problem The problem here is one of cardinality estimation – the number of rows SQL Server expects to find at each step of the plan.  The lack of indexes and useful statistical information means that SQL Server does not have the information it needs to make a good estimate.  Every join in the plan shown above estimates that it will produce just a single row as output.  Brad covers the factors that lead to the low estimates in his post. In reality, the join between the #Prods and #OrdDetail tables will produce 121,317 rows.  It should not surprise you that this has rather dire consequences for the remainder of the query plan.  In particular, it makes a nonsense of the optimizer’s decision to use Nested Loops to join to the two remaining tables.  Instead of scanning the #OrdHeader and #Custs tables once (as it expected), it has to perform 121,317 full scans of each.  The query takes somewhere in the region of twenty minutes to run to completion on my development machine. A Solution At this point, you may be thinking the same thing I was: if we really are stuck with no indexes, the best we can do is to use hash joins everywhere. We can force the exclusive use of hash joins in several ways, the two most common being join and query hints.  A join hint means writing the query using the INNER HASH JOIN syntax; using a query hint involves adding OPTION (HASH JOIN) at the bottom of the query.  The difference is that using join hints also forces the order of the join, whereas the query hint gives the optimizer freedom to reorder the joins at its discretion. Adding the OPTION (HASH JOIN) hint results in this estimated plan: That produces the correct output in around seven seconds, which is quite an improvement!  As a purely practical matter, and given the rigid rules of the environment we find ourselves in, we might leave things there.  (We can improve the hashing solution a bit – I’ll come back to that later on). Faster Nested Loops It might surprise you to hear that we can beat the performance of the hash join solution shown above using nested loops joins exclusively, and without breaking the rules we have been set. The key to this part is to realize that a condition like (A = B) can be expressed as (A <= B) AND (A >= B).  Armed with this tremendous new insight, we can rewrite the join predicates like so: SELECT P.ProductMainID AS PID, P.Name, D.OrderQty, H.SalesOrderNumber, H.OrderDate, C.TerritoryID FROM #OrdDetail D JOIN #OrdHeader H ON D.SalesOrderID >= H.SalesOrderID AND D.SalesOrderID <= H.SalesOrderID JOIN #Custs C ON H.CustomerID >= C.CustomerID AND H.CustomerID <= C.CustomerID JOIN #Prods P ON P.ProductMainID >= D.ProductMainID AND P.ProductMainID <= D.ProductMainID AND P.ProductSubID = D.ProductSubID AND P.ProductSubSubID = D.ProductSubSubID ORDER BY D.ProductMainID OPTION (RECOMPILE, LOOP JOIN, MAXDOP 1, FORCE ORDER); I’ve also added LOOP JOIN and FORCE ORDER query hints to ensure that only nested loops joins are used, and that the tables are joined in the order they appear.  The new estimated execution plan is: This new query runs in under 2 seconds. Why Is It Faster? The main reason for the improvement is the appearance of the eager Index Spools, which are also known as index-on-the-fly spools.  If you read my Inside The Optimiser series you might be interested to know that the rule responsible is called JoinToIndexOnTheFly. An eager index spool consumes all rows from the table it sits above, and builds a index suitable for the join to seek on.  Taking the index spool above the #Custs table as an example, it reads all the CustomerID and TerritoryID values with a single scan of the table, and builds an index keyed on CustomerID.  The term ‘eager’ means that the spool consumes all of its input rows when it starts up.  The index is built in a work table in tempdb, has no associated statistics, and only exists until the query finishes executing. The result is that each unindexed table is only scanned once, and just for the columns necessary to build the temporary index.  From that point on, every execution of the inner side of the join is answered by a seek on the temporary index – not the base table. A second optimization is that the sort on ProductMainID (required by the ORDER BY clause) is performed early, on just the rows coming from the #OrdDetail table.  The optimizer has a good estimate for the number of rows it needs to sort at that stage – it is just the cardinality of the table itself.  The accuracy of the estimate there is important because it helps determine the memory grant given to the sort operation.  Nested loops join preserves the order of rows on its outer input, so sorting early is safe.  (Hash joins do not preserve order in this way, of course). The extra lazy spool on the #Prods branch is a further optimization that avoids executing the seek on the temporary index if the value being joined (the ‘outer reference’) hasn’t changed from the last row received on the outer input.  It takes advantage of the fact that rows are still sorted on ProductMainID, so if duplicates exist, they will arrive at the join operator one after the other. The optimizer is quite conservative about introducing index spools into a plan, because creating and dropping a temporary index is a relatively expensive operation.  It’s presence in a plan is often an indication that a useful index is missing. I want to stress that I rewrote the query in this way primarily as an educational exercise – I can’t imagine having to do something so horrible to a production system. Improving the Hash Join I promised I would return to the solution that uses hash joins.  You might be puzzled that SQL Server can create three new indexes (and perform all those nested loops iterations) faster than it can perform three hash joins.  The answer, again, is down to the poor information available to the optimizer.  Let’s look at the hash join plan again: Two of the hash joins have single-row estimates on their build inputs.  SQL Server fixes the amount of memory available for the hash table based on this cardinality estimate, so at run time the hash join very quickly runs out of memory. This results in the join spilling hash buckets to disk, and any rows from the probe input that hash to the spilled buckets also get written to disk.  The join process then continues, and may again run out of memory.  This is a recursive process, which may eventually result in SQL Server resorting to a bailout join algorithm, which is guaranteed to complete eventually, but may be very slow.  The data sizes in the example tables are not large enough to force a hash bailout, but it does result in multiple levels of hash recursion.  You can see this for yourself by tracing the Hash Warning event using the Profiler tool. The final sort in the plan also suffers from a similar problem: it receives very little memory and has to perform multiple sort passes, saving intermediate runs to disk (the Sort Warnings Profiler event can be used to confirm this).  Notice also that because hash joins don’t preserve sort order, the sort cannot be pushed down the plan toward the #OrdDetail table, as in the nested loops plan. Ok, so now we understand the problems, what can we do to fix it?  We can address the hash spilling by forcing a different order for the joins: SELECT P.ProductMainID AS PID, P.Name, D.OrderQty, H.SalesOrderNumber, H.OrderDate, C.TerritoryID FROM #Prods P JOIN #Custs C JOIN #OrdHeader H ON H.CustomerID = C.CustomerID JOIN #OrdDetail D ON D.SalesOrderID = H.SalesOrderID ON P.ProductMainID = D.ProductMainID AND P.ProductSubID = D.ProductSubID AND P.ProductSubSubID = D.ProductSubSubID ORDER BY D.ProductMainID OPTION (MAXDOP 1, HASH JOIN, FORCE ORDER); With this plan, each of the inputs to the hash joins has a good estimate, and no hash recursion occurs.  The final sort still suffers from the one-row estimate problem, and we get a single-pass sort warning as it writes rows to disk.  Even so, the query runs to completion in three or four seconds.  That’s around half the time of the previous hashing solution, but still not as fast as the nested loops trickery. Final Thoughts SQL Server’s optimizer makes cost-based decisions, so it is vital to provide it with accurate information.  We can’t really blame the performance problems highlighted here on anything other than the decision to use completely unindexed tables, and not to allow the creation of additional statistics. I should probably stress that the nested loops solution shown above is not one I would normally contemplate in the real world.  It’s there primarily for its educational and entertainment value.  I might perhaps use it to demonstrate to the sceptical that SQL Server itself is crying out for an index. Be sure to read Brad’s original post for more details.  My grateful thanks to him for granting permission to reuse some of his material. Paul White Email: [email protected] Twitter: @PaulWhiteNZ

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  • SQL Spatial: Getting “nearest” calculations working properly

    - by Rob Farley
    If you’ve ever done spatial work with SQL Server, I hope you’ve come across the ‘nearest’ problem. You have five thousand stores around the world, and you want to identify the one that’s closest to a particular place. Maybe you want the store closest to the LobsterPot office in Adelaide, at -34.925806, 138.605073. Or our new US office, at 42.524929, -87.858244. Or maybe both! You know how to do this. You don’t want to use an aggregate MIN or MAX, because you want the whole row, telling you which store it is. You want to use TOP, and if you want to find the closest store for multiple locations, you use APPLY. Let’s do this (but I’m going to use addresses in AdventureWorks2012, as I don’t have a list of stores). Oh, and before I do, let’s make sure we have a spatial index in place. I’m going to use the default options. CREATE SPATIAL INDEX spin_Address ON Person.Address(SpatialLocation); And my actual query: WITH MyLocations AS (SELECT * FROM (VALUES ('LobsterPot Adelaide', geography::Point(-34.925806, 138.605073, 4326)),                        ('LobsterPot USA', geography::Point(42.524929, -87.858244, 4326))                ) t (Name, Geo)) SELECT l.Name, a.AddressLine1, a.City, s.Name AS [State], c.Name AS Country FROM MyLocations AS l CROSS APPLY (     SELECT TOP (1) *     FROM Person.Address AS ad     ORDER BY l.Geo.STDistance(ad.SpatialLocation)     ) AS a JOIN Person.StateProvince AS s     ON s.StateProvinceID = a.StateProvinceID JOIN Person.CountryRegion AS c     ON c.CountryRegionCode = s.CountryRegionCode ; Great! This is definitely working. I know both those City locations, even if the AddressLine1s don’t quite ring a bell. I’m sure I’ll be able to find them next time I’m in the area. But of course what I’m concerned about from a querying perspective is what’s happened behind the scenes – the execution plan. This isn’t pretty. It’s not using my index. It’s sucking every row out of the Address table TWICE (which sucks), and then it’s sorting them by the distance to find the smallest one. It’s not pretty, and it takes a while. Mind you, I do like the fact that it saw an indexed view it could use for the State and Country details – that’s pretty neat. But yeah – users of my nifty website aren’t going to like how long that query takes. The frustrating thing is that I know that I can use the index to find locations that are within a particular distance of my locations quite easily, and Microsoft recommends this for solving the ‘nearest’ problem, as described at http://msdn.microsoft.com/en-au/library/ff929109.aspx. Now, in the first example on this page, it says that the query there will use the spatial index. But when I run it on my machine, it does nothing of the sort. I’m not particularly impressed. But what we see here is that parallelism has kicked in. In my scenario, it’s split the data up into 4 threads, but it’s still slow, and not using my index. It’s disappointing. But I can persuade it with hints! If I tell it to FORCESEEK, or use my index, or even turn off the parallelism with MAXDOP 1, then I get the index being used, and it’s a thing of beauty! Part of the plan is here: It’s massive, and it’s ugly, and it uses a TVF… but it’s quick. The way it works is to hook into the GeodeticTessellation function, which is essentially finds where the point is, and works out through the spatial index cells that surround it. This then provides a framework to be able to see into the spatial index for the items we want. You can read more about it at http://msdn.microsoft.com/en-us/library/bb895265.aspx#tessellation – including a bunch of pretty diagrams. One of those times when we have a much more complex-looking plan, but just because of the good that’s going on. This tessellation stuff was introduced in SQL Server 2012. But my query isn’t using it. When I try to use the FORCESEEK hint on the Person.Address table, I get the friendly error: Msg 8622, Level 16, State 1, Line 1 Query processor could not produce a query plan because of the hints defined in this query. Resubmit the query without specifying any hints and without using SET FORCEPLAN. And I’m almost tempted to just give up and move back to the old method of checking increasingly large circles around my location. After all, I can even leverage multiple OUTER APPLY clauses just like I did in my recent Lookup post. WITH MyLocations AS (SELECT * FROM (VALUES ('LobsterPot Adelaide', geography::Point(-34.925806, 138.605073, 4326)),                        ('LobsterPot USA', geography::Point(42.524929, -87.858244, 4326))                ) t (Name, Geo)) SELECT     l.Name,     COALESCE(a1.AddressLine1,a2.AddressLine1,a3.AddressLine1),     COALESCE(a1.City,a2.City,a3.City),     s.Name AS [State],     c.Name AS Country FROM MyLocations AS l OUTER APPLY (     SELECT TOP (1) *     FROM Person.Address AS ad     WHERE l.Geo.STDistance(ad.SpatialLocation) < 1000     ORDER BY l.Geo.STDistance(ad.SpatialLocation)     ) AS a1 OUTER APPLY (     SELECT TOP (1) *     FROM Person.Address AS ad     WHERE l.Geo.STDistance(ad.SpatialLocation) < 5000     AND a1.AddressID IS NULL     ORDER BY l.Geo.STDistance(ad.SpatialLocation)     ) AS a2 OUTER APPLY (     SELECT TOP (1) *     FROM Person.Address AS ad     WHERE l.Geo.STDistance(ad.SpatialLocation) < 20000     AND a2.AddressID IS NULL     ORDER BY l.Geo.STDistance(ad.SpatialLocation)     ) AS a3 JOIN Person.StateProvince AS s     ON s.StateProvinceID = COALESCE(a1.StateProvinceID,a2.StateProvinceID,a3.StateProvinceID) JOIN Person.CountryRegion AS c     ON c.CountryRegionCode = s.CountryRegionCode ; But this isn’t friendly-looking at all, and I’d use the method recommended by Isaac Kunen, who uses a table of numbers for the expanding circles. It feels old-school though, when I’m dealing with SQL 2012 (and later) versions. So why isn’t my query doing what it’s supposed to? Remember the query... WITH MyLocations AS (SELECT * FROM (VALUES ('LobsterPot Adelaide', geography::Point(-34.925806, 138.605073, 4326)),                        ('LobsterPot USA', geography::Point(42.524929, -87.858244, 4326))                ) t (Name, Geo)) SELECT l.Name, a.AddressLine1, a.City, s.Name AS [State], c.Name AS Country FROM MyLocations AS l CROSS APPLY (     SELECT TOP (1) *     FROM Person.Address AS ad     ORDER BY l.Geo.STDistance(ad.SpatialLocation)     ) AS a JOIN Person.StateProvince AS s     ON s.StateProvinceID = a.StateProvinceID JOIN Person.CountryRegion AS c     ON c.CountryRegionCode = s.CountryRegionCode ; Well, I just wasn’t reading http://msdn.microsoft.com/en-us/library/ff929109.aspx properly. The following requirements must be met for a Nearest Neighbor query to use a spatial index: A spatial index must be present on one of the spatial columns and the STDistance() method must use that column in the WHERE and ORDER BY clauses. The TOP clause cannot contain a PERCENT statement. The WHERE clause must contain a STDistance() method. If there are multiple predicates in the WHERE clause then the predicate containing STDistance() method must be connected by an AND conjunction to the other predicates. The STDistance() method cannot be in an optional part of the WHERE clause. The first expression in the ORDER BY clause must use the STDistance() method. Sort order for the first STDistance() expression in the ORDER BY clause must be ASC. All the rows for which STDistance returns NULL must be filtered out. Let’s start from the top. 1. Needs a spatial index on one of the columns that’s in the STDistance call. Yup, got the index. 2. No ‘PERCENT’. Yeah, I don’t have that. 3. The WHERE clause needs to use STDistance(). Ok, but I’m not filtering, so that should be fine. 4. Yeah, I don’t have multiple predicates. 5. The first expression in the ORDER BY is my distance, that’s fine. 6. Sort order is ASC, because otherwise we’d be starting with the ones that are furthest away, and that’s tricky. 7. All the rows for which STDistance returns NULL must be filtered out. But I don’t have any NULL values, so that shouldn’t affect me either. ...but something’s wrong. I do actually need to satisfy #3. And I do need to make sure #7 is being handled properly, because there are some situations (eg, differing SRIDs) where STDistance can return NULL. It says so at http://msdn.microsoft.com/en-us/library/bb933808.aspx – “STDistance() always returns null if the spatial reference IDs (SRIDs) of the geography instances do not match.” So if I simply make sure that I’m filtering out the rows that return NULL… …then it’s blindingly fast, I get the right results, and I’ve got the complex-but-brilliant plan that I wanted. It just wasn’t overly intuitive, despite being documented. @rob_farley

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  • Spooling in SQL execution plans

    - by Rob Farley
    Sewing has never been my thing. I barely even know the terminology, and when discussing this with American friends, I even found out that half the words that Americans use are different to the words that English and Australian people use. That said – let’s talk about spools! In particular, the Spool operators that you find in some SQL execution plans. This post is for T-SQL Tuesday, hosted this month by me! I’ve chosen to write about spools because they seem to get a bad rap (even in my song I used the line “There’s spooling from a CTE, they’ve got recursion needlessly”). I figured it was worth covering some of what spools are about, and hopefully explain why they are remarkably necessary, and generally very useful. If you have a look at the Books Online page about Plan Operators, at http://msdn.microsoft.com/en-us/library/ms191158.aspx, and do a search for the word ‘spool’, you’ll notice it says there are 46 matches. 46! Yeah, that’s what I thought too... Spooling is mentioned in several operators: Eager Spool, Lazy Spool, Index Spool (sometimes called a Nonclustered Index Spool), Row Count Spool, Spool, Table Spool, and Window Spool (oh, and Cache, which is a special kind of spool for a single row, but as it isn’t used in SQL 2012, I won’t describe it any further here). Spool, Table Spool, Index Spool, Window Spool and Row Count Spool are all physical operators, whereas Eager Spool and Lazy Spool are logical operators, describing the way that the other spools work. For example, you might see a Table Spool which is either Eager or Lazy. A Window Spool can actually act as both, as I’ll mention in a moment. In sewing, cotton is put onto a spool to make it more useful. You might buy it in bulk on a cone, but if you’re going to be using a sewing machine, then you quite probably want to have it on a spool or bobbin, which allows it to be used in a more effective way. This is the picture that I want you to think about in relation to your data. I’m sure you use spools every time you use your sewing machine. I know I do. I can’t think of a time when I’ve got out my sewing machine to do some sewing and haven’t used a spool. However, I often run SQL queries that don’t use spools. You see, the data that is consumed by my query is typically in a useful state without a spool. It’s like I can just sew with my cotton despite it not being on a spool! Many of my favourite features in T-SQL do like to use spools though. This looks like a very similar query to before, but includes an OVER clause to return a column telling me the number of rows in my data set. I’ll describe what’s going on in a few paragraphs’ time. So what does a Spool operator actually do? The spool operator consumes a set of data, and stores it in a temporary structure, in the tempdb database. This structure is typically either a Table (ie, a heap), or an Index (ie, a b-tree). If no data is actually needed from it, then it could also be a Row Count spool, which only stores the number of rows that the spool operator consumes. A Window Spool is another option if the data being consumed is tightly linked to windows of data, such as when the ROWS/RANGE clause of the OVER clause is being used. You could maybe think about the type of spool being like whether the cotton is going onto a small bobbin to fit in the base of the sewing machine, or whether it’s a larger spool for the top. A Table or Index Spool is either Eager or Lazy in nature. Eager and Lazy are Logical operators, which talk more about the behaviour, rather than the physical operation. If I’m sewing, I can either be all enthusiastic and get all my cotton onto the spool before I start, or I can do it as I need it. “Lazy” might not the be the best word to describe a person – in the SQL world it describes the idea of either fetching all the rows to build up the whole spool when the operator is called (Eager), or populating the spool only as it’s needed (Lazy). Window Spools are both physical and logical. They’re eager on a per-window basis, but lazy between windows. And when is it needed? The way I see it, spools are needed for two reasons. 1 – When data is going to be needed AGAIN. 2 – When data needs to be kept away from the original source. If you’re someone that writes long stored procedures, you are probably quite aware of the second scenario. I see plenty of stored procedures being written this way – where the query writer populates a temporary table, so that they can make updates to it without risking the original table. SQL does this too. Imagine I’m updating my contact list, and some of my changes move data to later in the book. If I’m not careful, I might update the same row a second time (or even enter an infinite loop, updating it over and over). A spool can make sure that I don’t, by using a copy of the data. This problem is known as the Halloween Effect (not because it’s spooky, but because it was discovered in late October one year). As I’m sure you can imagine, the kind of spool you’d need to protect against the Halloween Effect would be eager, because if you’re only handling one row at a time, then you’re not providing the protection... An eager spool will block the flow of data, waiting until it has fetched all the data before serving it up to the operator that called it. In the query below I’m forcing the Query Optimizer to use an index which would be upset if the Name column values got changed, and we see that before any data is fetched, a spool is created to load the data into. This doesn’t stop the index being maintained, but it does mean that the index is protected from the changes that are being done. There are plenty of times, though, when you need data repeatedly. Consider the query I put above. A simple join, but then counting the number of rows that came through. The way that this has executed (be it ideal or not), is to ask that a Table Spool be populated. That’s the Table Spool operator on the top row. That spool can produce the same set of rows repeatedly. This is the behaviour that we see in the bottom half of the plan. In the bottom half of the plan, we see that the a join is being done between the rows that are being sourced from the spool – one being aggregated and one not – producing the columns that we need for the query. Table v Index When considering whether to use a Table Spool or an Index Spool, the question that the Query Optimizer needs to answer is whether there is sufficient benefit to storing the data in a b-tree. The idea of having data in indexes is great, but of course there is a cost to maintaining them. Here we’re creating a temporary structure for data, and there is a cost associated with populating each row into its correct position according to a b-tree, as opposed to simply adding it to the end of the list of rows in a heap. Using a b-tree could even result in page-splits as the b-tree is populated, so there had better be a reason to use that kind of structure. That all depends on how the data is going to be used in other parts of the plan. If you’ve ever thought that you could use a temporary index for a particular query, well this is it – and the Query Optimizer can do that if it thinks it’s worthwhile. It’s worth noting that just because a Spool is populated using an Index Spool, it can still be fetched using a Table Spool. The details about whether or not a Spool used as a source shows as a Table Spool or an Index Spool is more about whether a Seek predicate is used, rather than on the underlying structure. Recursive CTE I’ve already shown you an example of spooling when the OVER clause is used. You might see them being used whenever you have data that is needed multiple times, and CTEs are quite common here. With the definition of a set of data described in a CTE, if the query writer is leveraging this by referring to the CTE multiple times, and there’s no simplification to be leveraged, a spool could theoretically be used to avoid reapplying the CTE’s logic. Annoyingly, this doesn’t happen. Consider this query, which really looks like it’s using the same data twice. I’m creating a set of data (which is completely deterministic, by the way), and then joining it back to itself. There seems to be no reason why it shouldn’t use a spool for the set described by the CTE, but it doesn’t. On the other hand, if we don’t pull as many columns back, we might see a very different plan. You see, CTEs, like all sub-queries, are simplified out to figure out the best way of executing the whole query. My example is somewhat contrived, and although there are plenty of cases when it’s nice to give the Query Optimizer hints about how to execute queries, it usually doesn’t do a bad job, even without spooling (and you can always use a temporary table). When recursion is used, though, spooling should be expected. Consider what we’re asking for in a recursive CTE. We’re telling the system to construct a set of data using an initial query, and then use set as a source for another query, piping this back into the same set and back around. It’s very much a spool. The analogy of cotton is long gone here, as the idea of having a continual loop of cotton feeding onto a spool and off again doesn’t quite fit, but that’s what we have here. Data is being fed onto the spool, and getting pulled out a second time when the spool is used as a source. (This query is running on AdventureWorks, which has a ManagerID column in HumanResources.Employee, not AdventureWorks2012) The Index Spool operator is sucking rows into it – lazily. It has to be lazy, because at the start, there’s only one row to be had. However, as rows get populated onto the spool, the Table Spool operator on the right can return rows when asked, ending up with more rows (potentially) getting back onto the spool, ready for the next round. (The Assert operator is merely checking to see if we’ve reached the MAXRECURSION point – it vanishes if you use OPTION (MAXRECURSION 0), which you can try yourself if you like). Spools are useful. Don’t lose sight of that. Every time you use temporary tables or table variables in a stored procedure, you’re essentially doing the same – don’t get upset at the Query Optimizer for doing so, even if you think the spool looks like an expensive part of the query. I hope you’re enjoying this T-SQL Tuesday. Why not head over to my post that is hosting it this month to read about some other plan operators? At some point I’ll write a summary post – once I have you should find a comment below pointing at it. @rob_farley

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  • Changing Palette for Day/Light Mode using GIMP

    - by J.C.
    Hello, Suppose I've a picture, which want to achieve day/light mode by changing 8bpp color palette. If I want the pixel index of my picture is always fixed for both day mode and night mode. For example, the 1st pixel index is 100. Which I can look up index 100 in day mode palette and night mode palette. How can I use GIMP to do so? My goal is to not update my pixel index of my picture. Also, as you see in two palette, they are not one one mapping. That is index 1 of the day mode palette and index 1 of the night mode palette may not used in the same pixel of the picture, how can I tackle this problem? Actually, my use case is as follow I want to use one 8bpp picture to achieve day/night mode by update only the color palette (without updating the pixel index). The advantage is I only have to prepare 2 256 byte palette rather than saving 2 big pictures in my limited data ram. Thanks a lot

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  • Can .htaccess slow down a site?

    - by Cody Sharp
    I'm working with a client on an e-commerce website. I implemented clean URLs using .htaccess. I also used .htaccess to solve canonical issues such as redirecting www to non-www and removing index.php from the URL. The website recently began to slow down dramatically, sometimes not even loading. The site is hosted on GoDaddy, and when the client called GoDaddy they told him it was the .htaccess file slowing down the website. I find this highly unlikely because of my past experiences, but I'm not 100% sure. My thinking is that the client's website is most likely on a shared server with a busy neighborhood, thus slowing down the site. It's not always slow, but rather sporadic throughout the day, loading fast at some points and slow at other points in time. Can the .htaccess file slow down a website to a crawl? If so, are there better ways to solve these problems with different rewrite rules and such? Here is what the actual .htaccess file looks like: Options +FollowSymlinks RewriteEngine On RewriteBase / RewriteCond %{HTTP_HOST} ^www.example.net [NC] RewriteRule ^(.*)$ http://example.net/$1 [L,R=301] RewriteRule ^products/([0-9a-zA-Z\_\-]*)\.htm([l]?)$ index.php p=product&product_code=$1 [L] RewriteRule ^catalog/([0-9a-zA-Z\_\-]*)\.htm([l]?)$ index.php p=catalog&catalog_code=$1 [L] RewriteRule ^pages/([0-9a-zA-Z\_\-]*)\.htm([l]?)$ index.php?p=page&page_id=$1 [L] RewriteRule ^index\.htm([l]?)$ index.php?p=home [L] RewriteRule ^site_map\.htm([l]?)$ index.php?p=site_map [L] RewriteCond %{QUERY_STRING} ^p=home$ RewriteRule (.*) ? [R=permanent] I'm a .htaccess and regex novice, so any pointed out mistakes would also help. Thank you.

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  • Collision detection code style

    - by Marian Ivanov
    Not only there are two useful broad-phase algorithms and a lot of useful narrowphase algorithms, there are also multiple code styles. Arrays vs. calling Make an array of broadphase checks, then filter them with narrowphase checks, then resolve them. function resolveCollisions(thingyStructure * a,thingyStructure * b,int index){ possibleCollisions = getPossibleCollisions(b,a->get(index)); for(i=0; i<possibleCollitionsNumber; i++){ if(narrowphase(possibleCollisions[i],a[index])) { collisions->push(possibleCollisions[i]); }; }; for(i=0; i<collitionsNumber; i++){ //CODE FOR RESOLUTION }; }; Make the broadphase call the narrowphase, and the narrowphase call the resolution function resolveCollisions(thingyStructure * a,thingyStructure * b,int index){ broadphase(b,a->get(index)); }; function broadphase(thingy * with, thingy * what){ while(blah){ //blahcode narrowphase(what,collidingThing); }; }; Events vs. in-the-loop Fire an event. This abstracts the check away, but it's trickier to make an equal interaction. a[index] -> collisionEvent(eventdata); //much later int collisionEvent(eventdata){ //resolution gets here } Resolve the collision inside the loop. This glues narrowphase and resolution into one layer. if(narrowphase(possibleCollisions[i],a[index])) { //CODE GOES HERE }; The questions are: Which of the first two is better, and how am I supposed to make a zero-sum Newtonian interaction under B1.

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  • SQL SERVER – Curious Case of Disappearing Rows – ON UPDATE CASCADE and ON DELETE CASCADE – T-SQL Example – Part 2 of 2

    - by pinaldave
    Yesterday I wrote a real world story of how a friend who thought they have an issue with intrusion or virus whereas the issue was really in the code. I strongly suggest you read my earlier blog post Curious Case of Disappearing Rows – ON UPDATE CASCADE and ON DELETE CASCADE – Part 1 of 2 before continuing this blog post as this is second part of the first blog post. Let me reproduce the simple scenario in T-SQL. Building Sample Data USE [TestDB] GO -- Creating Table Products CREATE TABLE [dbo].[Products]( [ProductID] [int] NOT NULL, [ProductDesc] [varchar](50) NOT NULL, CONSTRAINT [PK_Products] PRIMARY KEY CLUSTERED ( [ProductID] ASC )) ON [PRIMARY] GO -- Creating Table ProductDetails CREATE TABLE [dbo].[ProductDetails]( [ProductDetailID] [int] NOT NULL, [ProductID] [int] NOT NULL, [Total] [int] NOT NULL, CONSTRAINT [PK_ProductDetails] PRIMARY KEY CLUSTERED ( [ProductDetailID] ASC )) ON [PRIMARY] GO ALTER TABLE [dbo].[ProductDetails] WITH CHECK ADD CONSTRAINT [FK_ProductDetails_Products] FOREIGN KEY([ProductID]) REFERENCES [dbo].[Products] ([ProductID]) ON UPDATE CASCADE ON DELETE CASCADE GO -- Insert Data into Table USE TestDB GO INSERT INTO Products (ProductID, ProductDesc) SELECT 1, 'Bike' UNION ALL SELECT 2, 'Car' UNION ALL SELECT 3, 'Books' GO INSERT INTO ProductDetails ([ProductDetailID],[ProductID],[Total]) SELECT 1, 1, 200 UNION ALL SELECT 2, 1, 100 UNION ALL SELECT 3, 1, 111 UNION ALL SELECT 4, 2, 200 UNION ALL SELECT 5, 3, 100 UNION ALL SELECT 6, 3, 100 UNION ALL SELECT 7, 3, 200 GO Select Data from Tables -- Selecting Data SELECT * FROM Products SELECT * FROM ProductDetails GO Delete Data from Products Table -- Deleting Data DELETE FROM Products WHERE ProductID = 1 GO Select Data from Tables Again -- Selecting Data SELECT * FROM Products SELECT * FROM ProductDetails GO Clean up Data -- Clean up DROP TABLE ProductDetails DROP TABLE Products GO My friend was confused as there was no delete was firing over ProductsDetails Table still there was a delete happening. The reason was because there is a foreign key created between Products and ProductsDetails Table with the keywords ON DELETE CASCADE. Due to ON DELETE CASCADE whenever is specified when the data from Table A is deleted and if it is referenced in another table using foreign key it will be deleted as well. Workaround 1: Design Changes – 3 Tables Change the design to have more than two tables. Create One Product Mater Table with all the products. It should historically store all the products list in it. No products should be ever removed from it. Add another table called Current Product and it should contain only the table which should be visible in the product catalogue. Another table should be called as ProductHistory table. There should be no use of CASCADE keyword among them. Workaround 2: Design Changes - Column IsVisible You can keep the same two tables. 1) Products and 2) ProductsDetails. Add a column with BIT datatype to it and name it as a IsVisible. Now change your application code to display the catalogue based on this column. There should be no need to delete anything. Workaround 3: Bad Advices (Bad advises begins here) The reason I have said bad advices because these are going to be bad advices for sure. You should make necessary design changes and not use poor workarounds which can damage the system and database integrity further. Here are the examples 1) Do not delete the data – well, this is not a real solution but can give time to implement design changes. 2) Do not have ON CASCADE DELETE – in this case, you will have entry in productsdetails which will have no corresponding product id and later on there will be lots of confusion. 3) Duplicate Data – you can have all the data of the product table move to the product details table and repeat them at each row. Now remove CASCADE code. This will let you delete the product table rows without any issue. There are so many things wrong this suggestion, that I will not even start here. (Bad advises ends here)  Well, did I miss anything? Please help me with your suggestions. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology

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  • Existing laravel 4 project gives 404 in browser

    - by Richard A
    I'm trying to set up a development environment on a virtual machine running Ubuntu 14.04 LTS using Nginx and HHVM. To do this, I followed the tutorial here. This goes well with a new installation of Laravel. But when I import an existing Laravel 4 project and try to open that on my actual machine (which will serve as the client running Windows 7), I'm getting a 404 File Not Found error on the screen while connecting to http://sav.savrichard.dev. I did add this to the hosts file with the correct IP Address. The virtual machine is receiving the request and responds with a 404 error. How do I solve this error? I'm pretty new to Ubuntu so I'm not exactly sure what's wrong. The project is located at /var/www/sav.savrichard.net The server configuration is as follow: server { listen 80 default_server; root /var/www/sav.savrichard.net/public; index index.html index.htm index.php; server_name sav.savrichard.dev; access_log /var/log/nginx/localhost.sav.savrichard.dev-access.log; error_log /var/log/nginx/localhost.sav.savrichard.dev-error.log error; charset utf-8; location / { try_files \$uri \$uri/ /index.php?\$query_string; } location = /favicon.ico { log_not_found off; access_log off; } location = /robots.txt { log_not_found off; access_log off; } error_page 404 /index.php; include hhvm.conf; # Deny .htaccess file access location ~ /\.ht { deny all; } } And the hhvm.conf file is: location ~ \.(hh|php)$ { fastcgi_keep_conn on; fastcgi_pass 127.0.0.1:9000; fastcgi_index index.php; fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name; include fastcgi_params; }

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  • Installing MOSS 2007 on Windows 2008 R2

    - by Manesh Karunakaran
    When you try to install MOSS 2007 on Windows 2008 R2, if you are using an installation media that is older than SP2, you would get the following error, saying that “This program is blocked due to compatibility issues”    All is not lost though, all you need to do is to slip stream the SP2 updates to the MOSS 2007 Setup. Here’s a nice how to on how to do that. http://blogs.technet.com/seanearp/archive/2009/05/20/slipstreaming-sp2-into-sharepoint-server-2007.aspx Once you slipstream the SP2 updates, you would be able to continue with the installation with out the above error. HTH.   You may already read from blogs about April Cumulative Update for separate components in SharePoint. Now, the server-packages (also known as “Uber” packages) of April Cumulative Update for Microsoft Office SharePoint Server 2007 and Windows SharePoint Services 3.0 are ready for download. Download Information Windows SharePoint Services 3.0 April cumulative update package http://support.microsoft.com/hotfix/KBHotfix.aspx?kbnum=968850 Office SharePoint Server 2007 April cumulative update package http://support.microsoft.com/hotfix/KBHotfix.aspx?kbnum=968851 Detail Description Description of the Windows SharePoint Services 3.0 April cumulative update package http://support.microsoft.com/kb/968850 Description of the Office SharePoint Server 2007 April cumulative update package http://support.microsoft.com/kb/968851 Installation Recommendation for a fresh SharePoint Server To keep all files in a SharePoint installation up-to-date, the following sequence is recommended. Service Pack 2 for Windows SharePoint Services 3.0 Service Pack 2 for Office SharePoint Server 2007 April Cumulative Update package for Windows SharePoint Services 3.0 April Cumulative Update package for Office SharePoint Server 2007 Please note: Start from April Cumulative Update, the packages will no longer install on a farm without a service pack installed. You must have installed either Service Pack 1 (SP1) or SP2 prior to the installation of the cumulative updates. After applying the preceding updates, run the SharePoint Products and Technologies Configuration Wizard or “psconfig –cmd upgrade –inplace b2b -wait” in command line. This needs to be done on every server in the farm with SharePoint installed.  The version of content databases should be 12.0.6504.5000 after successfully applying these updates. For more in-depth guidance for the update process, we recommend that customers refer to the following articles. These articles provide a correct way to deploy updates, identify known issues (and resolutions), and provide information about creating slipstream builds. Deploy software updates for Windows SharePoint Services 3.0 http://technet.microsoft.com/en-us/library/cc288269.aspx Deploy software updates for Office SharePoint Server 2007 http://technet.microsoft.com/en-us/library/cc263467.aspx Create an installation source that includes software updates (Windows SharePoint Services 3.0) http://technet.microsoft.com/en-us/library/cc287882.aspx Create an installation source that includes software updates (Office SharePoint Server 2007) http://technet.microsoft.com/en-us/library/cc261890.aspx

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  • Comments show up in database, but only show up on my index page after a refresh.

    - by Truong
    Hi, I have AJAX, PHP, jquery, and mySQL in this very simple website I'm trying to make. All there is is a text area that sends data to the database and uses ajax\jquery to display that data onto the index page. For some reason though, I press submit and the data goes to the database, but I have to refresh the page myself to see that data on the page. I'm assuming that the problem has to do with my AJAX JQuery or even some mistake in the index. Also, when I type the text into the text area and press submit, the text remains in the textarea until I refresh the page. Haha, sorry if this is such a noob question.. I'm trying to learn. Thanks so much Here is the AJAX: <script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.0/jquery.min.js"></script> <script type="text/javascript"> $(function() { $(".submit").click(function() { var comment = $("#comment").val(); var post_id = $("#post").val(); var dataString = '&comment=' + comment if(comment=='') { alert('Fill something in please!'); } else { $("#flash").show(); $("#flash").fadeIn(400).html('<img src="noworries.jpg" /> '); $.ajax({ type: "POST", url: "commentajax.php", data: dataString, cache: false, success: function(html){ $("ol#update").append(html); $("ol#update li:last").fadeIn("slow"); $("#flash").hide(); } }); }return false; }); }); </script> Here is the index\form area: <body> <div id="container"><img src="banner.jpg" width="890" height="150" alt="title" /></div> <id="update" class="timeline"> <div id="flash"></div> <div id="container"> <form action="#" method="post"> <textarea name="comment" id="comment" cols="35" rows="4"></textarea><br /> <input name="submit" type="submit" class="submit" id="submit" value=" Submit Comment " /><br /> </form> </div> <id="update" class="timeline"> <?php include('config.php'); //$post_id value comes from the POSTS table $prefix="I'm happy when"; $sql=mysql_query("select * from comments order by com_id desc"); while($row=mysql_fetch_array($sql)) { $comment=$row['com_dis']; ?> <!--Displaying comments--> <div id="container"> <class="box"> <?php echo "$prefix $comment"; ?> </div> <?php } ?> Here is my commentajax.php <?php include('config.php'); if($_POST) { $comment=$_POST['comment']; $comment=mysql_real_escape_string($comment); mysql_query("INSERT INTO comments(com_id,com_dis) VALUES ('NULL', '$comment')"); } ?> <li class="box"><br /> <?php echo $comment; ?> </li> I'm sorry for so much code but I just started learning this four days ago and this is probably one of the last bugs until the website is functional.

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  • How to Use Windows’ Advanced Search Features: Everything You Need to Know

    - by Chris Hoffman
    You should never have to hunt down a lost file on modern versions of Windows — just perform a quick search. You don’t even have to wait for a cartoon dog to find your files, like on Windows XP. The Windows search indexer is constantly running in the background to make quick local searches possible. This enables the kind of powerful search features you’d use on Google or Bing — but for your local files. Controlling the Indexer By default, the Windows search indexer watches everything under your user folder — that’s C:\Users\NAME. It reads all these files, creating an index of their names, contents, and other metadata. Whenever they change, it notices and updates its index. The index allows you to quickly find a file based on the data in the index. For example, if you want to find files that contain the word “beluga,” you can perform a search for “beluga” and you’ll get a very quick response as Windows looks up the word in its search index. If Windows didn’t use an index, you’d have to sit and wait as Windows opened every file on your hard drive, looked to see if the file contained the word “beluga,” and moved on. Most people shouldn’t have to modify this indexing behavior. However, if you store your important files in other folders — maybe you store your important data a separate partition or drive, such as at D:\Data — you may want to add these folders to your index. You can also choose which types of files you want to index, force Windows to rebuild the index entirely, pause the indexing process so it won’t use any system resources, or move the index to another location to save space on your system drive. To open the Indexing Options window, tap the Windows key on your keyboard, type “index”, and click the Indexing Options shortcut that appears. Use the Modify button to control the folders that Windows indexes or the Advanced button to control other options. To prevent Windows from indexing entirely, click the Modify button and uncheck all the included locations. You could also disable the search indexer entirely from the Programs and Features window. Searching for Files You can search for files right from your Start menu on Windows 7 or Start screen on Windows 8. Just tap the Windows key and perform a search. If you wanted to find files related to Windows, you could perform a search for “Windows.” Windows would show you files that are named Windows or contain the word Windows. From here, you can just click a file to open it. On Windows 7, files are mixed with other types of search results. On Windows 8 or 8.1, you can choose to search only for files. If you want to perform a search without leaving the desktop in Windows 8.1, press Windows Key + S to open a search sidebar. You can also initiate searches directly from Windows Explorer — that’s File Explorer on Windows 8. Just use the search box at the top-right of the window. Windows will search the location you’ve browsed to. For example, if you’re looking for a file related to Windows and know it’s somewhere in your Documents library, open the Documents library and search for Windows. Using Advanced Search Operators On Windows 7, you’ll notice that you can add “search filters” form the search box, allowing you to search by size, date modified, file type, authors, and other metadata. On Windows 8, these options are available from the Search Tools tab on the ribbon. These filters allow you to narrow your search results. If you’re a geek, you can use Windows’ Advanced Query Syntax to perform advanced searches from anywhere, including the Start menu or Start screen. Want to search for “windows,” but only bring up documents that don’t mention Microsoft? Search for “windows -microsoft”. Want to search for all pictures of penguins on your computer, whether they’re PNGs, JPEGs, or any other type of picture file? Search for “penguin kind:picture”. We’ve looked at Windows’ advanced search operators before, so check out our in-depth guide for more information. The Advanced Query Syntax gives you access to options that aren’t available in the graphical interface. Creating Saved Searches Windows allows you to take searches you’ve made and save them as a file. You can then quickly perform the search later by double-clicking the file. The file functions almost like a virtual folder that contains the files you specify. For example, let’s say you wanted to create a saved search that shows you all the new files created in your indexed folders within the last week. You could perform a search for “datecreated:this week”, then click the Save search button on the toolbar or ribbon. You’d have a new virtual folder you could quickly check to see your recent files. One of the best things about Windows search is that it’s available entirely from the keyboard. Just press the Windows key, start typing the name of the file or program you want to open, and press Enter to quickly open it. Windows 8 made this much more obnoxious with its non-unified search, but unified search is finally returning with Windows 8.1.     

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  • Can't connect to wifi after 13.10 update - Qualcomm Atheros AR93xx Wireless Network Adapter [168c:0030] (rev 01)

    - by user1952119
    I updated Ubuntu to 13.10 last night and wifi was working fine until I went to wake up my computer this morning and no WiFi connection. I am on a iMac with Qualcomm Atheros AR93xx Wireless Network Adapter [168c:0030] (rev 01). The only solutions I have seen so far are for broadcomm WiFi drivers. There is no wifi driver in addition drivers only graphics drivers. Not sure how to fix this, Any help would be greatly appreciated. UPDATE: WiFi works after reboot but doesn't after being put to sleep nor does Ethernet connect after sleep.

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  • Is there a significant hit to a non .com TLDs exact match domain (EMD) names after Google's Panda update?

    - by ElHaix
    In this article, there is a good overview of exact match domain names and how they affect SEO after Google's Panda update. The last graph shows the Non-com EMD Influence, where it is suggested that a .com tld will perform better than a non-.com one. However, let's consider local search. In the US, .com's work great. However, let's say you're in Canada, and you have a .ca EMD, all with local, Canadian results. Would the expectation be that the .com equivalent still perform better? As a user I would expect the .ca results to be more relevant, and I'm wondering if anyone else has experience with this?

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  • SQL Server 2012 RTM Cumulative Update #10 is available!

    - by AaronBertrand
    The SQL Server team has released CU #10 for SQL Server 2012 RTM. KB article: http://support.microsoft.com/kb/2891666 Build # is 11.0.2420 This build has 4 fixes For most customers, this cumulative update hardly justifies the download, never mind patching and regression testing, at least IMHO. Of the four fixes, two involve SSAS, one involves SSRS, and one involves the database engine tuning advisor. Relevant for builds 11.0.2100 -> 11.0.2419. Do not attempt to install on SQL Server 2012 SP1 (any...(read more)

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  • Why does Adobe Acrobat Reader require a restart on update?

    - by White Phoenix
    Why does Adobe Acrobat Reader require a restart when you update it? It's a piece of software that reads PDFs. Just what else is it installing/updating that requires a total OS reboot to "complete" the installation? A bit of Googling indicates that it's to allow the OS to install the Adobe "Speed Launcher" - is this it or are there other components that are more tightly into the OS that it must absolutely have to have closed so that it can update them?

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  • How to get Magento to update order status when PayPal returns IPN message?

    - by Nick
    When someone checks out in Magento with PayPal, and PayPal flags their payment for review, Magento correctly sets the order status to "Payment Review". However, if after a day or two PayPal decides the order is OK, it sends an IPN message to Magento with the proper payment status of "Pending" and pending reason of "authorization". I can see this IPN message in Magento's paypal logs (and can simulate it with the sandbox), however, when Magento receives this message it does not update its order status. Why not and how can this be fixed? I am using Magento 1.5.1.0.

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  • Should I update blog posts or rewrite them as technology (and me) changes?

    - by Rachel
    I started a programming blog earlier this year, and since I started it some things have changed. Some changes are due to technology changing, some changes are due to my code libraries improving, and some (ok, probably most) are due to me changing as I learn more. I want to go back and completely re-write certain blog posts. Is it better to rewrite posts to remove old information and update them with new stuff, or to create entirely new posts and possibly take down old ones? I'm not talking about small changes to the code, or an extra few sentences, but complete rewrites with new code, new information, etc. Some things to consider are comments on the post, subscribers who receive updates when new posts are created, and user bookmarks.

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  • Cardinality Estimation Bug with Lookups in SQL Server 2008 onward

    - by Paul White
    Cost-based optimization stands or falls on the quality of cardinality estimates (expected row counts).  If the optimizer has incorrect information to start with, it is quite unlikely to produce good quality execution plans except by chance.  There are many ways we can provide good starting information to the optimizer, and even more ways for cardinality estimation to go wrong.  Good database people know this, and work hard to write optimizer-friendly queries with a schema and metadata (e.g. statistics) that reduce the chances of poor cardinality estimation producing a sub-optimal plan.  Today, I am going to look at a case where poor cardinality estimation is Microsoft’s fault, and not yours. SQL Server 2005 SELECT th.ProductID, th.TransactionID, th.TransactionDate FROM Production.TransactionHistory AS th WHERE th.ProductID = 1 AND th.TransactionDate BETWEEN '20030901' AND '20031231'; The query plan on SQL Server 2005 is as follows (if you are using a more recent version of AdventureWorks, you will need to change the year on the date range from 2003 to 2007): There is an Index Seek on ProductID = 1, followed by a Key Lookup to find the Transaction Date for each row, and finally a Filter to restrict the results to only those rows where Transaction Date falls in the range specified.  The cardinality estimate of 45 rows at the Index Seek is exactly correct.  The table is not very large, there are up-to-date statistics associated with the index, so this is as expected. The estimate for the Key Lookup is also exactly right.  Each lookup into the Clustered Index to find the Transaction Date is guaranteed to return exactly one row.  The plan shows that the Key Lookup is expected to be executed 45 times.  The estimate for the Inner Join output is also correct – 45 rows from the seek joining to one row each time, gives 45 rows as output. The Filter estimate is also very good: the optimizer estimates 16.9951 rows will match the specified range of transaction dates.  Eleven rows are produced by this query, but that small difference is quite normal and certainly nothing to worry about here.  All good so far. SQL Server 2008 onward The same query executed against an identical copy of AdventureWorks on SQL Server 2008 produces a different execution plan: The optimizer has pushed the Filter conditions seen in the 2005 plan down to the Key Lookup.  This is a good optimization – it makes sense to filter rows out as early as possible.  Unfortunately, it has made a bit of a mess of the cardinality estimates. The post-Filter estimate of 16.9951 rows seen in the 2005 plan has moved with the predicate on Transaction Date.  Instead of estimating one row, the plan now suggests that 16.9951 rows will be produced by each clustered index lookup – clearly not right!  This misinformation also confuses SQL Sentry Plan Explorer: Plan Explorer shows 765 rows expected from the Key Lookup (it multiplies a rounded estimate of 17 rows by 45 expected executions to give 765 rows total). Workarounds One workaround is to provide a covering non-clustered index (avoiding the lookup avoids the problem of course): CREATE INDEX nc1 ON Production.TransactionHistory (ProductID) INCLUDE (TransactionDate); With the Transaction Date filter applied as a residual predicate in the same operator as the seek, the estimate is again as expected: We could also force the use of the ultimate covering index (the clustered one): SELECT th.ProductID, th.TransactionID, th.TransactionDate FROM Production.TransactionHistory AS th WITH (INDEX(1)) WHERE th.ProductID = 1 AND th.TransactionDate BETWEEN '20030901' AND '20031231'; Summary Providing a covering non-clustered index for all possible queries is not always practical, and scanning the clustered index will rarely be optimal.  Nevertheless, these are the best workarounds we have today. In the meantime, watch out for poor cardinality estimates when a predicate is applied as part of a lookup. The worst thing is that the estimate after the lookup join in the 2008+ plans is wrong.  It’s not hopelessly wrong in this particular case (45 versus 16.9951 is not the end of the world) but it easily can be much worse, and there’s not much you can do about it.  Any decisions made by the optimizer after such a lookup could be based on very wrong information – which can only be bad news. If you think this situation should be improved, please vote for this Connect item. © 2012 Paul White – All Rights Reserved twitter: @SQL_Kiwi email: [email protected]

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  • How to make a Table of Content auto-update?

    - by Dan
    I am using Word 2007, but saving my documents in .doc (as opposed to .docx) formats because that's company policy. I have the ToC set up fine, but is there a way to have it update automatically (at document open, save or otherwise)? Word help suggests that it should update upon opening the document, but that doesn't seem to happen. Any ideas?

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  • How should I update Ajax Control Toolkit in VS 2010?

    - by Soham
    Suppose if there is a new version available of Ajax Control Toolkit then how should I install/update it in my visual studio 2010 which has already install older version of same toolkit? I would like to install new one while older toolkit would totally uninstalled, because mostly new toolkit always has all controls that were in the old toolkit & also some new controls. Then 1) Should I've to remove .dll file in my toolkit installation folder and place new .dll file there? If I do so then will VS 2010 automatically delete older entries of toolkit controls in .NET Components list and place new control there? Or 2) Should I've to uninstall old toolkit manually i.e delete .dll file and uncheck all entries in .NET Components list & after that install new one from scratch?

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  • Why ubuntu does not use the kernels installed by automatic update?

    - by Guillaume Coté
    I used the script describe in this question to list the kernel installed on the computer : How do I to remove or hide old kernel versions, to clean up the boot menu? In the 3.2.0, I have 33, 34, 35, 36, 37, 38, 39, 41, 43, 44, 45 and 48. I would expect to be running 3.2.0-48 after a reboot, but I am still running 3.2.0-32. Why the kernels installed by auto update are not used? [I am running 12.04 LTS] /boot/grub/menu.lst was modified for the last time June 16 2013, it contains 3.2.0-32-generic 2.6.32-45-generic 2.6.32-44-generic 2.6.32-43-generic A recovery for each of those and a memtest. I would have expected the kernel between 3.2.0-33 to 3.2.0-48 to be in this file before 3.2.0-32.

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  • How do I allow a standard user to update the kernel modules for VMware?

    - by GUI Junkie
    I've set up VMWare Player for my wife. Every once in a while (notably after a kernel update), the VMWare Player needs to be compiled into the kernel. My spouse does not have su- activated (if anybody screws up the OS, it's going to be me). I'd like to give her permission to do this, but only for that program. Is this possible? Can it be done safely (for the OS)? How can it be done? Edit: I tried to add the following to visudo guijunkette ALL=(ALL) NOPASSWD: /usr/bin/vmware-modconfig When running with her user, after typing her password, the following error occurred: So, in the end, no cigar. Edit: I've given up on VMWare Player and moved to VirtualBox. Made a clean install.

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  • Le SP 1 de Windows 7 et Windows Server 2008 R2 disponible, Microsoft recommande l'installation via Windows Update

    Le SP 1 de Windows 7 et Windows Server 2008 R2 disponible Microsoft recommande l'installation via Windows Update Mise à jour du 23/02/11 Une semaine après sa disponibilité pour les abonnés MSDN/TechNet, le Service Pack 1 de Windows 7 et de Windows Server 2008 R2 est disponible pour tous. Ce SP1 est avant tout un pack cumulatif des mises à jours publiées jusqu'en décembre 2010. Pour Windows Server 2008 R2, il ajoute cependant « deux nouvelles fonctionnalités majeures », souligne justement Jean-Michel Ormes sur son blog Développez. « RemoteFX est une plateforme d'accélération graphique qui permet aux utilisateurs distan...

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