Skip site navigation (1) Skip section navigation (2)

Query using SeqScan instead of IndexScan

From: Brendan Duddridge <brendan(at)clickspace(dot)com>
To: PostgreSQL Performance <pgsql-performance(at)postgresql(dot)org>
Subject: Query using SeqScan instead of IndexScan
Date: 2006-03-30 03:12:28
Message-ID: 2F1712F1-E5E2-455E-A291-06F300A66549@clickspace.com (view raw or flat)
Thread:
Lists: pgsql-performance
Hi,

I have a query that is using a sequential scan instead of an index  
scan. I've turned off sequential scans and it is in fact faster with  
the index scan.

Here's my before and after.

Before:

ssdev=# SET enable_seqscan TO DEFAULT;
ssdev=# explain analyze select cp.product_id
		from category_product cp, product_attribute_value pav
		where cp.category_id = 1001082 and cp.product_id = pav.product_id;

                                                                         
            QUERY PLAN
------------------------------------------------------------------------ 
------------------------------------------------------------------------ 
------------------------------
Hash Join  (cost=25.52..52140.59 rows=5139 width=4) (actual  
time=4.521..2580.520 rows=19695 loops=1)
    Hash Cond: ("outer".product_id = "inner".product_id)
    ->  Seq Scan on product_attribute_value pav  (cost=0.00..40127.12  
rows=2387312 width=4) (actual time=0.039..1469.295 rows=2385846 loops=1)
    ->  Hash  (cost=23.10..23.10 rows=970 width=4) (actual  
time=2.267..2.267 rows=1140 loops=1)
          ->  Index Scan using x_category_product__category_id_fk_idx  
on category_product cp  (cost=0.00..23.10 rows=970 width=4) (actual  
time=0.122..1.395 rows=1140 loops=1)
                Index Cond: (category_id = 1001082)
Total runtime: 2584.221 ms
(7 rows)


After:

ssdev=# SET enable_seqscan TO false;
ssdev=# explain analyze select cp.product_id
		from category_product cp, product_attribute_value pav
		where cp.category_id = 1001082 and cp.product_id = pav.product_id;

                                                                         
               QUERY PLAN
------------------------------------------------------------------------ 
------------------------------------------------------------------------ 
-------------------------------------
Nested Loop  (cost=0.00..157425.22 rows=5139 width=4) (actual  
time=0.373..71.177 rows=19695 loops=1)
    ->  Index Scan using x_category_product__category_id_fk_idx on  
category_product cp  (cost=0.00..23.10 rows=970 width=4) (actual  
time=0.129..1.438 rows=1140 loops=1)
          Index Cond: (category_id = 1001082)
    ->  Index Scan using product_attribute_value__product_id_fk_idx  
on product_attribute_value pav  (cost=0.00..161.51 rows=61 width=4)  
(actual time=0.016..0.053 rows=17 loops=1140)
          Index Cond: ("outer".product_id = pav.product_id)
Total runtime: 74.747 ms
(6 rows)

There's quite a big difference in speed there. 2584.221 ms vs. 74.747  
ms.

Any ideas what I can do to improve this without turning sequential  
scanning off?

Thanks,

____________________________________________________________________
Brendan Duddridge | CTO | 403-277-5591 x24 |  brendan(at)clickspace(dot)com

ClickSpace Interactive Inc.
Suite L100, 239 - 10th Ave. SE
Calgary, AB  T2G 0V9

http://www.clickspace.com

Responses

pgsql-performance by date

Next:From: Brendan DuddridgeDate: 2006-03-30 03:20:17
Subject: Re: Query using SeqScan instead of IndexScan
Previous:From: Craig A. JamesDate: 2006-03-30 03:01:26
Subject: Re: Decide between Postgresql and Mysql (help of

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group