Re: Forcing the use of particular execution plans

Tim Truman wrote:
> Query:
> SELECT count(*) as count FROM
> (
> SELECT *
> FROM transaction t, merchant m
> WHERE t.merchant_id = m.id
> AND m.id = 198
> AND t.transaction_date >= '20050101'
> AND t.transaction_date <= '20060925'
> AND credit_card_no LIKE '1111%111'
>
> UNION ALL
> SELECT *
> FROM transaction t, merchant m
> WHERE t.merchant_id = m.id
> AND m.parent_merchant_id = 198
> AND t.transaction_date >= '20050101'
> AND t.transaction_date <= '20060925'
> AND credit_card_no LIKE '1111%111'
> ) AS foobar
>

Actually, I think the best course of action is to rewrite the query to a
faster alternative. What you can try is:
SELECT SUM(count) AS count FROM
(
SELECT count(*) AS count
FROM transaction t, merchant m
WHERE t.merchant_id = m.id
AND m.id = 198
AND t.transaction_date >= '20050101'
AND t.transaction_date <= '20060925'
AND credit_card_no LIKE '1111%111'

UNION ALL
SELECT count(*) AS count
FROM transaction t, merchant m
WHERE t.merchant_id = m.id
AND m.parent_merchant_id = 198
AND t.transaction_date >= '20050101'
AND t.transaction_date <= '20060925'
AND credit_card_no LIKE '1111%111'
) AS foobar;

The next optimization is to merge the 2 subqueries into one. If you
schema is such that m.id can not be the same as m.parent_merchant_id I
think your query can be reduced to:
SELECT count(*) AS count
FROM transaction t, merchant m
WHERE t.merchant_id = m.id
AND
(
m.id = 198
OR
m.parent_merchant_id = 198
)
AND t.transaction_date >= '20050101'
AND t.transaction_date <= '20060925'
AND credit_card_no LIKE '1111%111'

If m.id can be the same as m.parent_merchant_id you need something like:
SELECT SUM(
CASE WHEN m.id = m.parent_merchant_id THEN 2 ELSE 1 END
) AS count
FROM transaction t, merchant m
WHERE t.merchant_id = m.id
AND
(
m.id = 198
OR
m.parent_merchant_id = 198
)
AND t.transaction_date >= '20050101'
AND t.transaction_date <= '20060925'
AND credit_card_no LIKE '1111%111'

Jochem