SET STATISTICS IO v7.0
You can use SET STATISTICS IO ON|OFF to help gauge the I/O performance 
of any query you are writing. When you issue this command prior to 
execution of a query, you receive information about the logical reads, 
physical reads, read ahead reads, and scan count. These four values can 
show you what is happening behind the scenes of your query as it 
pertains to I/O.

* Logical reads--indicates how many pages were accessed in order to 
complete the query; these reads are from the data cache

* Physical reads--indicates how many pages were read from disk; this 
value is always equal to or less than logical reads

* Read ahead reads--indicates how many pages were preloaded into the 
data cache using the read ahead mechanisms

* Scan count--indicates the number of times that a corresponding table 
was accessed

A short example will show you some of the data.

SET STATISTICS IO ON 
GO 
SELECT * FROM pubs.dbo.titleauthors

You would see output similar to the following:

au_id       title_id au_ord royaltyper  
----------- -------- ------ -----------  
172-32-1176 PS3333   1      100 
. 
. 
 
Table 'titleauthor'. Scan count 1, logical reads 2,  
physical reads 2, read-ahead reads 0.

Execute the query again and you will see:

Table 'titleauthor'. Scan count 1, logical reads 2,  
physical reads 0, read-ahead reads 0.

Notice that the physical reads are now zero (0). This is because the 
first query had to physically read the pages from disk and load them 
into data cache. The second time the query was executed, the pages were 
already residing within the data cache. Hence, no physical reads were 
necessary.

NOTE: The concept of SET STATISTICS IO is abbreviated here, as there is 
quite a bit of information attached to the meaning of each returned 
value. For more information, consult MSDN SQL Server Books Online.
http://click.online.com/Click?q=eb-HlLoQkDSIXos2eRBoa0IYZIn9dRR 

CORRECTION
In a previous SQL Server mail "Hash join strategies" we gave a value 
of 2 mod 2 = 1. It should have read 2 mod 2 = 0. We apologize for any 
inconvenience this may have caused.

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