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sqlt-sqlsql-server-2000performancequery-analyzer

How are logical reads calculated?

I've read the definition of logical reads from:

http://msdn.microsoft.com/en-us/library/ms184361.aspx

it says:

"Number of pages read from the data cache."

I have two tables (Row count of table_1 is 141, and table_2 is 16.811), when I run those two queries, it gives the following result. 

SELECT * FROM Table_1

results
Scan count 1, logical reads 6, physical reads 0, read-ahead reads 0.

SELECT * FROM Table_2

results
scan count 1, logical reads 701, physical reads 0, read-ahead reads 0

if logic reads is "Number of pages read from the data cache." then what is a page ? How it is calculated ?

Solution

  • A page is a minimal physical data unit SQL Server works with.

    A page is 8K long and may contain several table records, index records and other information.

    Even if a row is 10 bytes long, the whole page needs to be read.

    In your case, a page contains about 20 rows in average.