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Including my content from SQLBlog.com and some from SQLPerformance.com
Showing posts with label Execution Plan. Show all posts
Showing posts with label Execution Plan. Show all posts

Thursday 4 August 2011

Avoiding Uniqueness for Performance

Avoiding Uniqueness for Performance

In my last post, Enforcing Uniqueness for Performance, I showed how using a unique index could speed up equality seeks by around 40%.

Friday 29 July 2011

Enforcing Uniqueness for Performance

Enforcing Uniqueness for Performance

A little while back, I posted a short series on seeks and scans:

One of the things I highlighted in the middle post was the difference between a singleton seek and a range scan:

  • A singleton equality seek always retrieves exactly one row, and is guaranteed to do so because a unique index exists to enforce it.

  • A range scan seeks down the B-tree to a starting (or ending) point, and scans forward (or backward) from that point using the next or previous page pointers.

Today’s short post shows how much faster a singleton seek is, compared with a range scan, even when both return exactly the same number of records.

Thursday 7 July 2011

Bitmap Magic (or… how SQL Server uses bitmap filters)

Bitmap Magic (or… how SQL Server uses bitmap filters)


Can a parallel query use less CPU than the same serial query, while executing faster?

The answer is yes! To demonstrate, I’ll use the following two (heap) tables, each containing a single column typed as integer:

Tables #BuiltInt and #Probe

Wednesday 28 July 2010

The “Segment Top” Query Optimization

The Segment Top Query Optimization

A question that often comes up on the forums is how to get the first or last row from each group of records in a table. This post describes a clever query plan optimisation that SQL Server can use for these types of query.