1) "SELECT MIN(Id) FROM Customer" and "SELECT MAX(Id) FROM Customer"
are both fast, but "SELECT MIN(Id), MAX(Id) FROM Customer" is slow,
taking 5 seconds. Why?
Derby knows how to use an index to quickly find a minimum or a
maximum (by traversing down one side of the B-tree or the other). It
doesn't know how to do both in the same query, which would take two traversals.
2) "SELECT * FROM Customer ORDER BY Id" is fast but "SELECT * FROM
Customer ORDER BY Id DESC" is slow. Why? Can't it scroll the index backwards?
Nope - Derby doesn't do backwards scrolling. I think the physical
data layout could support it (i.e. there are backwards pointers), but
backwards scrolling has never been implemented. Could someone more
familiar with the store confirm this (Mike?).
- Jeff Lichtman
[EMAIL PROTECTED]
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