logical vs physical reads

what is the difference between logical reads and physical reads?
yanci1179Asked:
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Guy Hengel [angelIII / a3]Billing EngineerCommented:
physical reads means the data has been read from disk into memory
logical reads means the the data was already in memory

1 logical reads means 1 page of data, ie 8KB
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yanci1179Author Commented:
i read that in books online....but it didn't really mean much.  Ideally which one should be lower?
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Brendt HessSenior DBACommented:
In a perfect world, physical reads would be equal to the number of data blocks in your database, and all other reads would be logical.  So, by preference logical reads should be the higher of the two.
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Guy Hengel [angelIII / a3]Billing EngineerCommented:
perfectly, the physical reads should be 0. also, logical reads as low as possible.
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yanci1179Author Commented:
let me see if I understand this logical data read from what is already in cache.  if it's not there then it has to read the physical tables and retrieve the data?
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Brendt HessSenior DBACommented:
Absolutely correct.
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