> 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.
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