Message-ID: <451A6AA2.9060003@oli.tudelft.nl>
Date: Wed, 27 Sep 2006 14:12:18 +0200
From: Jochem van Dieten <jochemd@oli.tudelft.nl>
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To: pgsql-performance@postgresql.org
Subject: Re: Forcing the use of particular execution plans
References: <00501c6e1ff$c80022c0$0340050a@sdg>
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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