RE: Performance in subquery

Two things come to mind.

1) Make sure you have run VACUUM ANALYZE on your database.

2) Use a join rather than a sub-select, e.g.

SELECT
p.prod_name
FROM
products p, prod_attr pa
WHERE
p.prod_id = pa.prod_id AND
pa.prod_attr_text LIKE '%Linux%';

A word of caution though... Because you are using a wildcard in the first
position of your search string ('%Linux%'), you preclude the use of any
index and force a sequential scan!

Hope this helps,

Phil Culberson
DAT Services

-----Original Message-----
From: Brian Haney [mailto:brian(at)ibsystems(dot)com]
Sent: Monday, April 17, 2000 12:52 PM
To: pgsql-general(at)postgresql(dot)org
Subject: [GENERAL] Performance in subquery

I have multivendor product catalog and want to be able to search for
arbitrary text in a table of product attributes. Two of the several tables
are:

Table = products
+----------------------------------+----------------------------------+-----
--+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-----
--+
| prod_id | int4 not null default nextval |
4 |
| vendor_id | int4 |
4 |
| category_id | int4 |
4 |
| prod_name | text not null |
var |
| prod_description | text not null |
var |
| prod_price_low | numeric | 30.6
|
| prod_price_high | numeric | 30.6
|
| prod_promo | text |
var |
| prod_promo_url | text |
var |
| prod_datasht_url | text |
var |
| prod_buynow_url | text |
var |
| prod_asp_url | text |
var |
| prod_active | bool default 'false' |
1 |
| last_modified | timestamp |
4 |
| prod_demo_url | text |
var |
| prod_ephone | text |
var |
+----------------------------------+----------------------------------+-----
--+
Index: products_pkey

Table = prod_attr
+----------------------------------+----------------------------------+-----
--+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-----
--+
| attr_id | int4 not null |
4 |
| prod_id | int4 not null |
4 |
| prod_attr_text | text not null |
var |
+----------------------------------+----------------------------------+-----
--+
Indices: prod_attr_pkey
prod_attr_text_idx

The products table has 538 records and the prod_attr table has 7870.

When I enter the query:

SELECT prod_id FROM prod_attr WHERE prod_attr_text LIKE '%Linux%';

it performs quite well to give me a list of the product IDs almost
instantaneously.

But when I query:

select prod_name from products where prod_id in ( SELECT prod_id FROM
prod_attr
WHERE prod_attr_text LIKE '%Linux%');

It takes over 30 seconds to get the results. Here are the EXPLAINs:

First query:

Seq Scan on prod_attr (cost=317.84 rows=1 width=4)

Second query:

Seq Scan on products (cost=31.75 rows=538 width=12)
SubPlan
-> Seq Scan on prod_attr (cost=317.84 rows=1 width=4)

As you can see, I have created and index for prod_attr
(prod_attr_text), but it has had no discernable effect.

The explains imply to me that the real cost of the second query is
scanning the 7870 records, but then I would expect the second query to
take only slightly longer than the first. I suspect something else is
causing the second query to be so doggone slow.

Any help would be greatly appreciated.

--
Brian Haney VP Engineering/CTO
brian(at)ibsystems(dot)com Internet Business Systems, Inc.