SQL Server Logical/Physical Reads!!!!

  • Dinesh Babu Verma

    Mr or Mrs. 500

    Points: 520

    Summary Info:

    Logical Reads : Reading Data pages from Cache

    Physical Reads : Reading Data pages from Hard Disk

    Buffer Cach Hit Ratio logical reads – physical reads)/logical read * 100%

    Detailed Info:

    Logical Reads:

    Logical read indicates total number of data pages needed to be accessed from data cache to process query. It is very possible that logical read will access same data pages many times, so count of logical read value may be higher than actual number of pages in a table. Usually the best way to reduce logical read is to apply correct index or to rewrite the query.

    Physical Reads

    Physical read indicates total number of data pages that are read from disk. In case no data in data cache, the physical read will be equal to number of logical read. And usually it happens for first query request. And for subsequent same query request the number will be substantially decreased because the data pages have been in data cache.

    Buffer Cash Hit Ratio

    Buffer hit ratio will be calculated based on these two kinds of read as the following formula: (logical reads – physical reads)/logical read * 100%. The high buffer hit ratio (if possible to near 100%) indicates good database performance on SQL Server level. So use information from physical read and buffer hit ratio to measure performance in server level and logical read to measure individual query level

    Execess of the Logical Reads tends high memory Usage, there are some ways by which we can Reduce Logical Reads:

    1. Improper/Useless/Insufficient Indexes: Indexes should be build on the basis of data access or retrieval process if any of the indexes is build on the columns which are not used in a query will leads to High Logical reads and will degrade the performance while reads and writing the data....

    2.Poor Fill Factor/Page Density: Page use should should not be very less. otherwise large number of page will be used for small amount of data which will also leads to High Logical Reads....

    3.Wide Indexes: Indexing on the large number of columns will leads to high logical reads....

    4. Index scanning: if query is leads to index scanning on the table then logical reads will be high...

    Logical Reads count can be get by using follwoing ways

    Below are the ways to check logical Reads:

    1. set statistics io on

    2. sys.dm_exec_query_Stats

    by executing the below statement we can find detailed info about reads/writes

    select * from sys.dm_exec_query_Stats

    3. SQL Profiler: by executing the sql profiler on that database we can find out logical reads..

    there are some other SQL DMV/F which also help us to get logical reads...

    Thanks,

    Dinesh Babu Verma

  • spaghettidba

    SSC Guru

    Points: 105661

    And the question is?

  • Gail Shaw

    SSC Guru

    Points: 1004424

    Is there a question here?

    Gail Shaw
    Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
    SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability

    We walk in the dark places no others will enter
    We stand on the bridge and no one may pass
  • Henrico Bekker

    One Orange Chip

    Points: 27652

    GilaMonster (2/10/2012)


    Is there a question here?

    maybe it was suppose to be an article submission? 😛

    -----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
    This thing is addressing problems that dont exist. Its solution-ism at its worst. We are dumbing down machines that are inherently superior. - Gilfoyle

  • Bipin Singh

    Valued Member

    Points: 72

    Hi,

    I was recently asked in an interview about the topic & had I idea about the same but in absence of proper knowledge I was not able to answer properly. After reading this article I got a very nice idea about the same for thank u very much for posting a very good post.

    Regards

  • mario17

    SSCertifiable

    Points: 6078

    and the question is answered in Perfect manner !!!

    Exactly what I wanted to learn.

    Tx

  • DicksonMulwa

    Old Hand

    Points: 365

    It's still important information. In addition to his summary above, you can easily gather the said stats per query by running the SET STATISTICS IO or SET STATISTICS TIME command before running the query.

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