Let me describe my problem with my Firebird database:

Two tables:
Book (33043 records)
Borrower (1607 records)

The first query took 21 seconds last time I ran it. The second query took
less than 1 second. For the second query I removed one line that accessed a
location field. This field is a location id field that can have 6 different
values (1 to 6). In the book table, 25014 records have a value of 1 for this
field.

Why does this line increase the speed of the query so much? Is there a way
to work around this so that I can still restrict the query to take records
with a value of 1 but without the query taking so long?

QUERY 1 (slow!)
------------------
SELECT
B.BORROWER_ID,
BR.NAME
FROM
BOOK B,
BORROWER BR
WHERE
B.BORROWER_ID = BR.BORROWER_ID AND
B.DATE_DUE_BACK IS NOT NULL AND
B.DATE_DUE_BACK >= '05/10/2002' AND
B.DATE_DUE_BACK <= '06/07/2002' AND
B.LOCATION_ID = 1
GROUP BY
B.BORROWER_ID,
BR.NAME
ORDER BY
BR.NAME

QUERY 2 (fast! - the blank line is the difference between the two queries)
------------------
SELECT
B.BORROWER_ID,
BR.NAME
FROM
BOOK B,
BORROWER BR
WHERE
B.BORROWER_ID = BR.BORROWER_ID AND
B.DATE_DUE_BACK IS NOT NULL AND
B.DATE_DUE_BACK >= '05/10/2002' AND
B.DATE_DUE_BACK <= '06/07/2002'

GROUP BY
B.BORROWER_ID,
BR.NAME
ORDER BY
BR.NAME
----------------------------

Thanks!
Stephen