Message-ID: <BAY103-F17FF599D1FD15D1E06780BE76D0@phx.gbl>
From: "T- Bone" <jbowen333@hotmail.com>
To: pgsql-sql@postgresql.org
Subject: Comments on subquery performance
Date: Thu, 17 Feb 2005 10:55:30 -0700
Mime-Version: 1.0
Content-Type: text/plain; format=flowed

(second attempt in two days to post this message...I appologise if for some 
reason a duplicate appears)

Hello all,

I created a query that contains two subqueries and joins and would like some 
feedback on whether:
  1) this approach is logical; and,
  2) if this is an optimal approach (performance wise) to return the records 
I seek.

Essentially, I am attempting to perform a 'lookup' on a value in another 
table 3 times, for three different columns.  I have three columns with 
category codes in tblListings and would like to 'lookup' the actual category 
text in tblCategory.  I have created a functional query that contains two 
subqueries and joins, but am concerned this may not be the fastest (or 
logical?) way to achieve what I seek.

I thought of another approach to create a function to evaluate the records 
on a row-by-row and column-by-column approach, but thought that may prove 
even slower.  I would appreciate any comments on my logic or learning of any 
alternative means that would result in better performance.

I have included the SQL for reference.  Thanks in advance.

Regards,
Jim

----------------------------------------8<-------------------------------------------------

SELECT "CatID1", "CatID2", "CatID3", c1 AS "CatName1", c2 AS "CatName2", 
t6."CatName" AS "CatName3"
	FROM
	(SELECT "CatID1", "CatID2", "CatID3", c1, t4."CatName" AS c2
		FROM
	  (SELECT t1."CatID1", t1."CatID2", t1."CatID3", t2."CatName" AS c1
			FROM "MySchema"."tblListings" t1
			INNER JOIN  "MySchema"."tblCategories" t2
			ON (t1."CatID1" = t2."CatID")) t3
	  	LEFT OUTER JOIN "MySchema"."tblCategories" t4
	  	ON (t3."CatID2" = t4."CatID")) t5
   LEFT OUTER JOIN "MySchema"."tblCategories" t6
   ON (t5."CatID3" = t6."CatID");

----------------------------------------8<-------------------------------------------------

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