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Wednesday, June 24, 2009 9:26 AM

Maximum Row Size and Query Hints

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
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Tuesday, April 28, 2009 2:45 PM

Implied Predicates and Query Hints

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:
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Wednesday, March 18, 2009 7:34 AM

OPTIMIZED Nested Loops Joins

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
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Wednesday, March 04, 2009 9:27 AM

Optimizing I/O Performance by Sorting – Part 2

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
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Wednesday, February 25, 2009 9:29 AM

Optimizing I/O Performance by Sorting – Part 1

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
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Tuesday, October 07, 2008 2:44 PM

Random Prefetching

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
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Thursday, May 15, 2008 9:39 AM

Query Processing Presentation

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,
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Attachment(s): QPTalk.pdf
Tuesday, May 06, 2008 11:54 AM

Conversion and Arithmetic Errors: Change between SQL Server 2000 and 2005

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
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Monday, April 28, 2008 4:23 PM

Conversion and Arithmetic Errors

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
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Wednesday, May 02, 2007 11:48 AM

Query Plans and Read Committed Isolation Level

Last week I looked at how concurrent updates may cause a scan running at read committed isolation level to return the same row multiple times or to miss a row entirely. This week I'm going to take a look at how concurrent updates may affect slightly more
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Monday, December 04, 2006 6:14 PM

Semi-join Transformation

In several of my prior posts, I’ve given examples of semi-joins. Recall that semi-joins essentially return a row from one input if we can find at least one matching row from the other input. Here is a simple example: create table T1 ( a int , b int )
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Thursday, November 16, 2006 8:08 PM

Parallel Hash Join

SQL Server uses one of two different strategies to parallelize a hash join . The more common strategy uses hash partitioning. In some cases, we use broadcast partitioning; this strategy is often called a “broadcast hash join.” Hash Partitioning The more
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Wednesday, November 08, 2006 7:04 PM

Parallel Nested Loops Join

SQL Server parallelizes a nested loops join by distributing the outer rows (i.e., the rows from the first input) randomly among the nested loops threads. For example, if we have two threads running a nested loops join, we send about half of the rows to
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Wednesday, August 30, 2006 2:02 PM

Subqueries: ANDs and ORs

In my “Introduction to Joins” post , I gave an example of how we can use a semi-join to evaluate an EXISTS subquery. Just to recap, here is another example: create table T1 ( a int , b int ) create table T2 ( a int ) create table T3 ( a int ) select *
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Wednesday, August 23, 2006 1:23 PM

Subqueries in CASE Expressions

In this post, I’m going to take a look at how SQL Server handles subqueries in CASE expressions. I’ll also introduce some more exotic join functionality in the process. Scalar expressions For simple CASE expressions with no subqueries, we can just evaluate
Posted by craigfr | 5 Comments
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