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  • 在什么情况下使用exist和in

    http://www.itpub.net/thread-406784-4-1.html
    You Asked (Jump to Tom's latest followup)

    Tom:

    can you give me some example at which situation
    IN is better than exist, and vice versa.


    and we said...

    Well, the two are processed very very differently.

    Select * from T1 where x in ( select y from T2 )

    is typically processed as:

    select *
    from t1, ( select distinct y from t2 ) t2
    where t1.x = t2.y;

    The subquery is evaluated, distinct'ed, indexed (or hashed or sorted) and then
    joined to the original table -- typically.


    As opposed to

    select * from t1 where exists ( select null from t2 where y = x )

    That is processed more like:


    for x in ( select * from t1 )
    loop
    if ( exists ( select null from t2 where y = x.x )
    then
    OUTPUT THE RECORD
    end if
    end loop

    It always results in a full scan of T1 whereas the first query can make use of
    an index on T1(x).


    So, when is where exists appropriate and in appropriate?

    Lets say the result of the subquery
    ( select y from T2 )

    is "huge" and takes a long time. But the table T1 is relatively small and
    executing ( select null from t2 where y = x.x ) is very very fast (nice index on
    t2(y)). Then the exists will be faster as the time to full scan T1 and do the
    index probe into T2 could be less then the time to simply full scan T2 to build
    the subquery we need to distinct on.


    Lets say the result of the subquery is small -- then IN is typicaly more
    appropriate.


    If both the subquery and the outer table are huge -- either might work as well
    as the other -- depends on the indexes and other factors.

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  • 原文地址:https://www.cnblogs.com/caroline/p/4093020.html
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