The Row Count Spool operator is a simple component in an execution plan that can make an enormous difference to SQL query plans that would otherwise read a table on every row of the inner input to a nested loop. As this is a common type of query, it illustrates the power that a knowledge of your showplan operators can give you.… Read more
If you've ever wondered what happens to your queries if you're not conscientious about creating indexes, then Fabiano Amorim has yet another nugget of Execution Plan wisdom for you. The Query Optimizer's solution is to use the Nonclustered Index Spool, and we're about to learn why.… Read more
We soon learn, in SQL Server, that heaps are a bad thing, without necessarily understanding how or why. Jonathan Lewis is an Oracle expert who doesn't like to take such strictures for granted, especially when they don't apply to Oracle. Jonathan discovers much about how SQL Server places data, and concludes from his experiments that heaps perform badly in SQL Server because you cannot specify a fill factor for them.… Read more
SQL Server's UPDATE statement is apparently simple, but complications such as the FROM clause can cause puzzlement. Bob Sheldon starts simply, and introduces the more complex forms painlessly. … Read more
The 14 chapters of "The Art of XSD", written by MVP Jacob Sebastian, will take the reader step-by-step all the way from the basics of XML Schema design all the way to advanced topics on SQL Server XML Schema Collections. Reviewer Hima Bindu Vejella gives it an 8/10 rating, and gives us an excellent distilled description of what the book has to offer.… Read more
Continuing to illuminate the depths of SQL Server's Query Optimizer, Fabiano shines a light on the sixth major Showplan Operator on his list: the Lazy Spool. What does the Lazy Spool do that's so special, how does the Query Optimizer use it, and why is it so Lazy? Fabiano explains all...… Read more
A well-known Oracle expert records faithfully his struggles with the unfamiliar : SQL Server. He now sets himself the task of creating a table with a million rows of random data. As one might expect, it is the lack of familiarity with the workarounds and tricks of SQL Server that trips him up. His journey brings us fresh insights, and a glimpse at the alternative-universe of Oracle.… Read more
For the fifth part of Fabiano's mission to describe the major Showplan Operators used by SQL Server's Query Optimiser, he introduces the spool operators and particularly the Eager Spool, explains blocking and non-blocking and then describes how the Halloween Problem is avoided.… Read more
Fabiano continues in his mission to describe the major Showplan Operators used by SQL Server's Query Optimiser. This week he meets a star, the Key Lookup, a stalwart performer, but most famous for its role in ill-performing queries where an index does not 'cover' the data required to execute the query. If you understand why, and in what circumstances, key lookups are slow, it helps greatly with optimising query performance. … Read more
The third part of Fabiano's mission to describe the major Showplan Operators used by SQL Server's Query Optimiser continues with the 'Compute Scalar' operator. Fabiano shows how a tweak to SQL to avoid a 'Compute Scalar' step can improve its performance.… Read more
When a SQL expert moves from Oracle to SQL Server, he can spot obvious strengths and weaknesses in the product that are too familiar to be apparent to the SQL Server DBA. Jonathan Lewis is one such expert: In this article he records his train of thought whilst investigating the mechanics of the SQL Server database engine. The result makes interesting reading.… Read more
Fabiano continues in his mission to describe, one week at a time, all the major Showplan Operators used by SQL Server's Query Optimiser to build the Query Plan. This week he gets the Concatenation operator .… Read more
As part of his mission to explain the Query Optimiser in practical terms, Fabiano attempts the feat of describing, one week at a time, all the major Showplan Operators used by SQL Server's Query Optimiser to build the Query Plan. He starts with Assert… Read more
The Transaction Log provides the means by which either the complete set of tasks of a database transaction are performed or none of them are. It ensures that , via rollback, only valid data is written out to the database, and it allows transactions to be played back to recreate the system state right before a failure. Robert Sheldon explains the various basic tasks involved in managing the transaction log.… Read more
Change Data Capture records INSERTs, UPDATEs, and DELETEs applied to SQL Server tables, and makes a record available of what changed, where, and when, in simple relational 'change tables' rather than in an esoteric chopped salad of XML. These change tables contain columns that reflect the column structure of the source table you have chosen to track, along with the metadata needed to understand the changes that have been made. Pinal Dave explains all, with plenty of examples in a simple introduction.… Read more
Filestream allows us to store and manage unstructured datain SQL Server more easily. Initially, the accounts of FILESTREAM assumed prodigious powers of concentration and cognition, and we mortals all recoiled numbly. However, it became clear that we were missing out on some extraordinarily useful functionality, so we asked Jacob Sebastian to come up with a simple and clear-cut account of the FILESTREAM feature in SQL Server 2008. You'll agree he has managed the feat superbly.… Read more
The FOR XML clause in SQL Server causes a lot of difficulty, mainly because it is rather poorly explained in Books-on-Line. We challenged Bob Sheldon to make it seem simple. Here is his sublime response. … Read more
What is an XML Schema, and why is it important? How did XSD develop from its precursors, DTD and XDR? What is XML capable of in the recent versions of SQL Server? Jacob Sebastian gives a succinct answer in this extract from his new book.… Read more
The whole subject of the Three-Valued (also known as ternary, trivalent or 3VL) Logic of SQL tends to trip people up. This is hardly surprising in view of the fact that it involves an esoteric Polish mathematician and because it behaves differently in the DDL (Data Declaration Language) and the DML (Data Manipulation Language). In response to requests, Joe Celko comes to the rescue and makes it all seem simple.… Read more
When discussing the normalisation process, it is always the First Normal Form that causes the most grief and confusion. Anith Sen takes up the challenge to explain, in simple terms, exactly what the First Normal Form really is, and why it is so important for Database Design. Along the way, he dispels some of the myths that have grown up around 1NF.… Read more
