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

Wednesday, 18 January 2012

Dynamic Seeks and Hidden Implicit Conversions

Dynamic Seeks and Hidden Implicit Conversions

A LIKE predicate with only a trailing wildcard can usually use an index seek, as the following AdventureWorks sample database query shows:

SELECT 
    P.[Name]
FROM Production.Product AS P
WHERE 
    P.[Name] LIKE N'D%';

Tuesday, 9 August 2011

SQL Server, Seeks, and Binary Search

SQL Server, Seeks, and Binary Search

The following table summarizes the results from my last two articles, Enforcing Uniqueness for Performance and Avoiding Uniqueness for Performance. It shows the CPU time used when performing 5 million clustered index seeks into a unique or non-unique index:

Test summary

In test 1, making the clustered index unique improved performance by around 40%.

In test 2, making the same change reduced performance by around 70% (on 64-bit systems – more on that later).

Thursday, 17 February 2011

Seeking Without Indexes

Seeking Without Indexes

A seek can contain one or more seek predicates, each of which can either identify (at most) one row in a unique index (a singleton lookup) or a range of values (a range scan).

When looking at an execution plan, we often need to look at the details of the seek operator in the Properties window to see how many operations it is performing, and what type of operation each one is.

As seen in the first post of this mini-series, When is a Seek not a Seek? the number of hidden seeking operations can have an appreciable impact on performance.

So…is it a Seek or a Scan?

So…is it a Seek or a Scan?

You might be most familiar with the terms ‘Seek’ and ‘Scan’ from the graphical plans produced by SQL Server Management Studio (SSMS). You might look to the SSMS tool-tip descriptions to explain the differences between them:

Scan and Seek tooltips

Both mention scans and ranges (nothing about seeks) and the Index Seek description maybe implies that it will not scan the index entirely (which isn’t necessarily true). Not massively helpful.

Wednesday, 16 February 2011

When is a Seek not a Seek?

When is a Seek not a Seek?

The following script creates a single-column clustered table containing the integers from 1 to 1,000 inclusive.

IF OBJECT_ID(N'tempdb..#Test', N'U') IS NOT NULL
BEGIN
    DROP TABLE #Test
END;
GO
CREATE TABLE #Test
(
    id integer PRIMARY KEY CLUSTERED
);
INSERT #Test
    (id)
SELECT
    V.number
FROM master.dbo.spt_values AS V
WHERE
    V.[type] = N'P'
    AND V.number BETWEEN 1 AND 1000;

Let’s say we are given the following task:

Find the rows with values from 100 to 170, excluding any values that divide exactly by 10.

Thursday, 23 September 2010

A Tale of Two Index Hints

A Tale of Two Index Hints

If you look up Table Hints in the official documentation, you’ll find the following statements:

If a clustered index exists, INDEX(0) forces a clustered index scan and INDEX(1) forces a clustered index scan or seek.

If no clustered index exists, INDEX(0) forces a table scan and INDEX(1) is interpreted as an error.

The interesting thing there is that both hints can result in a scan. If that is the case, you might wonder if there is any effective difference between the two.

This blog entry explores that question, and highlights an optimizer quirk that can result in a much less efficient query plan when using INDEX(0). I’ll also cover some stuff about ordering guarantees.