Message-ID: <3BBA63DE.3030004@xythos.com>
Date: Tue, 02 Oct 2001 18:03:26 -0700
From: Barry Lind <barry@xythos.com>
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To: Dave Harkness <daveh@MEconomy.com>
CC: pgsql-jdbc@postgresql.org
Subject: Re: LOCK TABLE oddness in PLpgSQL function called via JDBC
References: <5.1.0.14.2.20011002123507.00ac2ff8@mail.meconomy.com>
	<5.1.0.14.2.20011002135313.00ac2ff8@mail.meconomy.com>
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Dave,

I don't know why you are seeing these problems with the lock table.  But 
the select for update should work for you.  (In my product I have done 
exactly the same thing you are trying to do using select for update with 
success).

I would add one minor comment on your description of the behavior of 
using select for update:

The select for update will block other 'select for updates' or 
'updates'.  It does not block other simple selects.  But that is fine 
for the purposes here.

thanks,
--Barry


Dave Harkness wrote:

> At 01:45 PM 10/2/2001, Barry Lind wrote:
> 
>> Dave,
>>
>> First off, are you running with autocommit turned off in JDBC?  By 
>> default autocommit is on, and thus your lock is removed as soon as it 
>> is aquired.
> 
> 
> I've tried it with auto-commit ON and OFF. With it off, I've tried it 
> with READ_COMMITTED and SERIALIZABLE. All produce the same result.
> 
> However, my understanding is that each JDBC statement is executed within 
> a single transaction when auto-commit is ON. I'm executing only one 
> statement:
> 
>     select next_id_block(?, ?)
> 
> While the function does indeed execute multiple statements itself, 
> aren't they all done inside a single transaction? If not, I must rethink 
> our strategy as I had assumed that the PLpgSQL functions I wrote would 
> be transactional.
> 
>> Secondly, you don't need a table lock, you just need to lock the row 
>> between the select and the update.  You should use 'select for update' 
>> to do this.  That way when you issue the select to get the current 
>> value, it will lock the row, preventing other select for update 
>> requests from completing until the lock is released.  That way the 
>> select and the update can be assured that no one else is changing the 
>> data.
> 
> 
> THANK YOU! That's what I thought, but the documentation was a bit light 
> on the subject of SELECT ... FOR UPDATE. So to mirror it back to you, if 
> I do
> 
>     next_id_block ( count )
>     (1)   read idfactory row FOR UPDATE
> 
>     (2)   update idfactory row
>               increment next_id by count
>               increment change_num by 1
>           where change_num is equal to that read in (1)
> 
>     (3)   return next_id read in (1)
> 
> is it safe to assume that the update in (2) will ALWAYS succeed since it 
> would be impossible for any other transaction to read or update the row 
> once it was selected for update?
> 
> Thanks for your help.
> 
> Peace,
> Dave
>