Re: Query using SeqScan instead of IndexScan - Mailing list 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@clickspace.com

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

http://www.clickspace.com