logical read vs physical read

 

Introduction

• Normally, When we request a data for a query(i.e: SELECT), The Database engine fetches the data page(s) from the physical disk and loads into the Buffer Cache called "Physical Read".
• "Physical Reads" occurrs only when data pages are not there in "Buffer cache".
• Then , The data page(s) are retrived from the Buffer cache called "Logical Read"
• If the data pages already there in Buffer Cache then, "Logical Reads" will be performed directly, no Physical read will be there. 
• So, Disk IO reduced by using the "Logical Read", Because, The data pages retrived from Buffer Cache, not from Physical Disk .

 

http://www.dotnetfunda.com/articles/article1343-logical-and-physical-reads-how-it39s-working-and-how-to-manage.aspx