Yesterday, a reader posted a question asking me to comment on SQL Server's algorithm for choosing implicit conversions. When SQL Server encounters an expression with mismatched types, it has two options. It can execute the query with an implicit conversion...

In this post about the sys.dm_db_index_usage_stats and sys.dm_db_index_operational_stats DMVs, I wrote: Another less important difference between these DMVs is that sys.dm_db_index_usage_stats only reports on indexes that have been used at least once...

In my last post (yes, that was two months ago), I gave an example of how a query hint could cause a query to fail. In this post, I'll give another example of how query hints can cause problems. As with my last post, this post was inspired by a question...

In this post, I want to take a look at how two seemingly unrelated features of SQL Server can interact to cause a problem. The idea for this post came from a question submitted by a reader. Let's begin. Consider the following trivial schema and query...

In my past two posts, I explained how SQL Server may add a sort to the outer side of a nested loops join and showed how this sort can significantly improve performance . In an earlier post , I discussed how SQL Server can use random prefetching to improve...

In my last post, I discussed how SQL Server can use sorts to transform random I/Os into sequential I/Os. In this post, I'll demonstrate directly how such a sort can impact performance. For the following experiments, I'll use the same 3 GByte database...

In this post from last year, I discussed how random I/Os are slower than sequential I/Os (particularly for conventional rotating hard drives). For this reason, SQL Server often favors query plans that perform sequential scans of an entire table over plans...

SQL Server includes two DMVs - sys.dm_db_index_usage_stats and sys.dm_db_index_operational_stats - that are extremely useful for monitoring which indexes are used as well as how and when they are used. Both DMVs report similar statistics on information...

In my last post , I explained the importance of asynchronous I/O and described how SQL Server uses sequential read ahead to boost the performance of scans. In this post, I'll discuss how SQL Server uses random prefetching. Let's begin with a simple example...

Balancing CPU and I/O throughput is essential to achieve good overall performance and to maximize hardware utilization. SQL Server includes two asynchronous I/O mechanisms - sequential read ahead and random prefetching - that are designed to address this...

In this post on partitioned tables, I mentioned that SQL Server 2008 has a much more efficient implementation of dynamic partition elimination as compared to SQL Server 2005. In response, a reader posted this comment asking how much dynamic partition...

In my last post , I looked at how SQL Server 2008 handles scans on partitioned tables. I explained that SQL Server 2008 treats partitioned tables as tables with a logical index on the partition id column and that SQL Server 2008 implements partition elimination...

In this post , I introduced how SQL Server 2005 implements query plans on partitioned tables. If you've read that post or used partitioned tables, you may recall that SQL Server 2005 uses a constant scan operator to enumerate the list of partition ids...

Consider the following query: CREATE TABLE T1 (A INT, B1 INT, B2 INT) CREATE TABLE T2 (A INT, B INT) SELECT * FROM T1 WHERE (SELECT SUM(T2.B) FROM T2 WHERE T2.A = T1.A) BETWEEN T1.B1 AND T1.B2 Observe that the subquery in this query only needs to be evaluated...

In my last couple of posts, I wrote about how explicit conversions can lead to errors. In this post, I'm going to take a look at some issues involving implicit conversions. SQL Server adds implicit conversions whenever you mix columns, variables, and...

Last week, I had the opportunity to talk to the New England SQL Server Users Group . I would like to thank the group for inviting me, Adam Machanic for organizing the event, and Red Gate for sponsoring it. My talk was an introduction to query processing...

In this post from last week, I gave an example of a query with a conversion where the optimizer pushes the conversion below a join. The result is that the conversion may be run on rows that do not join which could lead to avoidable failures. I ran this...

Let's take a look at a simple query: CREATE TABLE T1 (A INT, B CHAR(8)) INSERT T1 VALUES (0, '0') INSERT T1 VALUES (1, '1') INSERT T1 VALUES (99, 'Error') SELECT T1.A, CONVERT(INT, T1.B) AS B_INT FROM T1 There is no way to convert the string "Error" into...

In my previous post , I discussed the ROW_NUMBER ranking function which was introduced in SQL Server 2005. In this post, I'll take a look at the other ranking functions - RANK, DENSE_RANK, and NTILE. Let's begin with RANK and DENSE_RANK. These functions...

SQL Server 2005 introduced four new functions, ROW_NUMBER, RANK, DENSE_RANK, and NTILE that are collectively referred to as ranking functions. These functions differ from ordinary scalar functions in that the result that they produce for a given row depends...

In a prior post , I introduced the notion that update plans consist of two parts: a read cursor that identifies the rows to be updated and a write cursor that actually performs the updates. Logically speaking, SQL Server must execute the read cursor and...

A few months ago, I wrote a post describing how SQL Server maintains unique indexes while avoiding false uniqueness violations. In this post, I'm going to look at how SQL Server maintains unique indexes that were created with the WITH IGNORE_DUP_KEY clause...

In some of my past posts, I've discussed how SQL Server implements aggregation including the stream aggregate and hash aggregate operators. I also used hash aggregation as an initial example in my introductory post on parallel query execution . In this...

In this post, I will finish the discussion of recursive CTEs that I began in my last post. I will continue to use the CTE examples from Books Online . To run these examples, you'll need to install the Adventure Works Cycles OLTP sample database . In my...

One of the most important uses of CTEs is to write recursive queries. In fact, CTEs provide the only means to write recursive queries. As I noted last week , there are several excellent CTE examples, including recursive CTE examples, in Books Online ...