Content-Type: text/plain
Content-Disposition: inline
Content-Transfer-Encoding: 7bit
Mime-Version: 1.0
Date: Mon, 3 Feb 2003 10:58:57 -0800 (PST)
From: Jasper Tymesman <jtyme@bboy.com>
To: pgsql-sql@postgresql.org
Subject: Serialized Transaction Locking Issues
Reply-To: jtyme@bboy.com
Message-Id: <20030203185857.7609D3E7C@sitemail.everyone.net>

Hello, 

I'm currently in the midst of working on a serializable transaction which drops indexes on several tables, does a bulk copy, and rebuilds the indexes. Based on what I've read it seemed as though I'd be able to concurrently execute read on queries against these tables, returning results based on the table snapshot from before the serialized transaction began. However, this doesn't work. A simple read-only select query waits until the serialized transaction is finished before returning results. 

Based on the user documentation, specifically (http://www.postgresql.org/docs/view.php?version=7.3&idoc=1&file=explicit-locking.html#LOCKING-TABLES), it would seem that the only issue in PostgreSQL that could block a simple select call would be an ACCESS EXCLUSIVE lock, which is acquired only by DROP TABLE, ALTER TABLE, and VACUUM FULL, none of which I'm using. In fact, I've noticed this exact behavior with DROP INDEX. 

Please excuse my futile attempt to outline two concurrent transactions here: 

testdb=# \d trans_test 
Table "public.trans_test" 
Column | Type | Modifiers 
- -------+--------+----------- 
val integer 
Indexes: idx_trans_test btree(val) 

testdb=# SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE; 
SET 

[TRANSACTION 1] BEGIN; 
BEGIN 
[TRANSACTION 1] SELECT * FROM trans_test; 
val 
- ----- 
1 
2 

[TRANSACTION 2] SELECT * FROM trans_test; 
val 
- ----- 
1 
2 

[TRANSACTION 1] DROP INDEX idx_trans_test; 
DROP INDEX 

[TRANSACTION 2] SELECT * FROM trans_test; 
... (Waiting) 

[TRANSACTION 1] COMMIT; 
COMMIT 

(TRANSACTION 2 returns after COMMIT) 
val 
- ----- 
1 
2 

So is this a glitch or am I missing some nuance of a serializable transaction? In either case I'm eager to figure out whats actually going on. 

Thanks, 

- -justin 


_____________________________________________________________
Sign up for FREE email from bboy.com at http://www.bboy.com

_____________________________________________________________
Select your own custom email address for FREE! Get you@yourchoice.com w/No Ads, 6MB, POP & more! http://www.everyone.net/selectmail?campaign=tag