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very slow simple query - outer join makes it quicker

From: "Gerard Isdell" <gerard(dot)isdell(at)kinetic(dot)co(dot)uk>
To: pgsql-performance(at)postgresql(dot)org
Subject: very slow simple query - outer join makes it quicker
Date: 2004-04-19 10:59:48
Message-ID: WorldClient-F200404191159.AA59480054@kinetic.co.uk (view raw or flat)
Thread:
Lists: pgsql-performance
Hi Everyone 

I am new to this group and relatively new to Postgres, having used MSSQL 7 
up until now.

One table in my database in returning even the simplest of queries extremely 
slowly. The table is user table, and even the select userid from users takes 
over 20 seconds to run. There are about 2000 records in the table. 


The EXPLAIN ANALYZE on this table produces this output:
Seq Scan on users  (cost=0.00..89482.63 rows=1463 width=4) (actual 
time=68.836..40233.463 rows=1465 loops=1)
Total runtime: 40234.965 ms


SELECT USERID FROM USERS  produces this:
1465 rows fetched (25.28 sec)

The userid field is the primary key and has an index on it with this ddl: 
ALTER TABLE "public"."users" ADD CONSTRAINT "users_pkey" PRIMARY KEY 
("userid");

There are other tables, such as the messages table, that have 10s of 
thousands of rows and they return records much more quickly.


There must be something seriously wrong for simple queries like this to take 
so long.   

I should say that we are using the OpenFTS text search on the users  table.

In many cases to make the queries run at reasonable speeds I do an outer 
join on another table, and surprisingly these results come back very quickly

Can anybody help me in diagnosing this problem.


Gerard Isdell


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