Re: LOCK TABLE oddness in PLpgSQL function called via JDBC

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
>