should i prefer join or left join over exists or in

  • hi,

    I have seen people use exists when exists is required and no cols are required in select.

    But I notice sometimes exists slows down and join performs better

    q1) is there any news i should know ,  that one should not use exists or in , and should go for  join etc etc

     

    yours sincerely

     

     

  • If relationship is 1 - 1 -> inner join

    If relationship is 1 - n -> exists

    Johan

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  • I've always thought you should only use inner join if you want to return columns from the other table, but this link suggests join is preferable if there is an index on the join column(s). https://www.mssqltips.com/sqlservertip/6659/sql-exists-vs-in-vs-join-performance-comparison/

    I used to refer to articles by Gail Shaw, but they don't seem to be working and could be out of date now. When it comes to left join vs not exists, I usually use not exists. https://erikdarling.com/why-not-exists-makes-more-sense-than-left-joins/

  • Ed B wrote:

    I've always thought you should only use inner join if you want to return columns from the other table, but this link suggests join is preferable if there is an index on the join column(s). https://www.mssqltips.com/sqlservertip/6659/sql-exists-vs-in-vs-join-performance-comparison

    That is NOT what that article suggests.  The article discusses a methodology for comparing various alternatives and determining which is more efficient.  While it did find that a join performed better in this VERY SPECIFIC CASE, it did not draw any conclusions about how that might generalize.

    Drew

    • This reply was modified 1 month ago by  drew.allen.
    • This reply was modified 1 month ago by  drew.allen.

    J. Drew Allen
    Business Intelligence Analyst
    Philadelphia, PA

  • 100% agree with Drew here and in your specific scenario, JOIN may perform better or worse. My advice is to test on your system as is (ie don't add extra indexes, don't partition the table, don't change anything at the DDL level) but do a test on a test system and verify the performance differences. Also, do more than 1 test of each type so you can get an average run time, not just a single run. First run will create a new plan, running it a second time will reuse the plan, so second run will be a little bit faster from start to finish (sometimes the difference though is so close to 0.000 seconds you can't tell), so I usually do 3 to 4 runs of each query so I can get a good average run time and can say that method A is faster for this specific query.

    I did a live demo one time trying to show how indexes helped performance of SELECTs but how HEAPs were faster for inserts and I ran it on my personal laptop during a presentation. ALL the times I ran it previously, it worked how I described. In the actual live presentation, Windows updates kicked in during the HEAP insert and HEAP was about 5 times slower than inserting into a table with 2 indexes (1 clustered, 1 non-clustered). Made me look like a liar, but I found the issue and corrected it for future demos (update prior to the demo and use a VM that is internet-disabled or use screenshots to show things).

    The above is all just my opinion on what you should do. 
    As with all advice you find on a random internet forum - you shouldn't blindly follow it.  Always test on a test server to see if there is negative side effects before making changes to live!
    I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.

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