Jari Fredriksson wrote:
>> James Harper wrote:
>>> The subquery returns a very small result set (0 or 1,
>>> assuming you use DISTINCT) and so isn't too inefficient.
>>> It's when you say 'WHERE NOT EXISTS (SOME QUERY WITH
>>> LOTS OF RESULTS)' that you start to really bog down
>> True, but if the outer query contains a very large number
>> of records, the inner query is executed over and over and
>> over and ...
>>
>> This multiplies the small execution time by a large
>> number.
>>
>> Think of it like a WHILE loop that executes a block of
>> code each time round. The inner block of code might be
>> quick, but if the loop goes round several million times,
>> the cumulative time becomes significant.
>>
>> One general rule of thumb of SQL execution is that bulk
>> processing is almost always faster than processing
>> records individually in a loop.
>
> All right then. Then there is this version:
>
> INSERT INTO Filename(Name)
> SELECT DISTINCT Name
> FROM batch AS
> WHERE Name NOT IN
> (
> SELECT DISTINCT Name
> FROM Filename
> );
>
> I guess it depends on RDMS, but I wonder it it executes the subquery more
> than once.
>
>
>
The keyword EXPLAIN before the query should show how mysql would react.
There's some tools also mysql-administrator which can be use to detect / search
point of slow
--
Bruno Friedmann
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