Re: Locking in PostgreSQL?

On 6-Dec-06, at 8:20 AM, Jens Schipkowski wrote:

> On Wed, 06 Dec 2006 13:29:37 +0100, Dave Cramer <pg@fastcrypt.com>
> wrote:
>
>> Unless you specifically ask for it postgresql doesn't lock any
>> rows when you update data.
>>
> Thats not right. UPDATE will force a RowExclusiveLock to rows
> matching the WHERE clause, or all if no one is specified.
> @Joost Kraaijeveld: Yes, because there is no EntryExclusiveLock or
> something like that. Roughly you can say, each UPDATE statement
> iterates through the affected table and locks the WHERE clause
> matching records (rows) exclusivly to prevent data inconsistancy
> during the UPDATE. After that your rows will be updated and the
> lock will be repealed.
> You can see this during an long lasting UPDATE by querying the
> pg_locks with joined pg_stats_activity (statement must be enabled).

Apparently I've completely misunderstood MVCC then.... My
understanding is that unless you do a select ... for update then
update the rows will not be locked .

Dave
>
>> Dave
>> On 6-Dec-06, at 2:04 AM, Joost Kraaijeveld wrote:
>>
>>> Does PostgreSQL lock the entire row in a table if I update only 1
>>> column?
>>>
>>>
>>> --Groeten,
>>>
>>> Joost Kraaijeveld
>>> Askesis B.V.
>>> Molukkenstraat 14
>>> 6524NB Nijmegen
>>> tel: 024-3888063 / 06-51855277
>>> fax: 024-3608416
>>> web: www.askesis.nl
>>>
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>>
>>
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>> TIP 9: In versions below 8.0, the planner will ignore your desire to
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>>        match
>
>
>
> CU,
> Jens
>
> --
> **
> Jens Schipkowski
>
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