...or as a cross product:
SELECT MIN(A.Id), MAX(B.Id) FROM Customer A, Customer B
This might be fairly efficient, given that each branch of the cross
product only returns a single row. I guess this could be a possible
internal translation as well.
Roy
In this case Derby would do the cross product before calculating the
MIN and MAX values, so the performance would be worse than the original query.
To do this as two separate scans, Derby would have to be changed to
know that MIN and MAX results are independent of the number of values
fed to them. For example, the optimization you're proposing would get
the wrong answer for a query like this:
SELECT SUM(A.balance), SUM(B.balance) FROM Customers A, Suppliers B
Although it could get the right answer for this:
SELECT SUM(DISTINCT A.balance), SUM(DISTINCT B.balance) FROM
Customers A, Suppliers B
Figuring out all the cases where a Cartesian product could be split
into two separate scans could be tricky.
- Jeff Lichtman
[EMAIL PROTECTED]
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