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A copy of my content from SQLBlog.com plus occasional new content.
Showing posts with label Undocumented. Show all posts
Showing posts with label Undocumented. Show all posts

Friday, 27 August 2010

Sorting, Row Goals, and the TOP 100 Problem

Sorting, Row Goals, and the TOP 100 Problem

When you write a query to return the first few rows from a potential result set, you’ll often use the TOP clause.

To give a precise meaning to the TOP operation, it will normally be accompanied by an ORDER BY clause. Together, the TOP…ORDER BY construction can be used to precisely identify which top ‘n’ rows should be returned.

Wednesday, 18 August 2010

Inside the Optimizer: Row Goals In Depth

Inside the Optimizer: Row Goals In Depth

Background

One of the core assumptions made by the SQL Server query optimizer cost model is that clients will eventually consume all the rows produced by a query.

This results in plans that favour the overall execution cost, though it may take longer to begin producing rows.

Wednesday, 11 August 2010

The Impact of Non-Updating Updates

The Impact of Non-Updating Updates

From time to time, I encounter a system design that always issues an UPDATE against the database after a user has finished working with a record — without checking to see if any of the data was in fact altered.

The prevailing wisdom seems to be “the database will sort it out”. This raises an interesting question: How smart is SQL Server in these circumstances?

In this post, I’ll look at a generalisation of this problem: What is the impact of updating a column to the value it already contains?

The specific questions I want to answer are:

  • Does this kind of UPDATE generate any log activity?
  • Do data pages get marked as dirty (and so eventually get written out to disk)?
  • Does SQL Server bother doing the update at all?

Thursday, 5 August 2010

Iterators, Query Plans, and Why They Run Backwards

Iterators, Query Plans, and Why They Run Backwards

Iterators

SQL Server uses an extensible architecture for query optimization and execution, using iterators as the basic building blocks.

Iterators are probably most familiar in their graphical showplan representation, where each icon represents a single iterator. They also show up in XML query plan output as RelOp nodes:

Each iterator performs a single simple function, such as applying a filtering condition, or performing an aggregation. It can represent a logical operation, a physical operation, or (most often) both.