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.
-- 
Mike Holden


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