Re: ALTER TABLE lock strength reduction patch is unsafe Reply-To:

On 8 March 2014 11:14, Simon Riggs <simon@2ndquadrant.com> wrote:
> On 7 March 2014 09:04, Simon Riggs <simon@2ndquadrant.com> wrote:
>
>> The right thing to do here is to not push to the extremes. If we mess
>> too much with the ruleutil stuff it will just be buggy. A more
>> considered analysis in a later release is required for a full and
>> complete approach. As I indicated earlier, an 80/20 solution is better
>> for this release.
>>
>> Slimming down the patch, I've removed changes to lock levels for
>> almost all variants. The only lock levels now reduced are those for
>> VALIDATE, plus setting of relation and attribute level options.
>>
>> VALIDATE is implemented by calling pg_get_constraintdef_mvcc(), a
>> slightly modified variant of pg_get_constraintdef that uses the
>> transaction snapshot. I propose this rather than Noah's solution
>> solely because this will allow any user to request the MVCC data,
>> rather than implement a hack that only works for pg_dump. I will post
>> the patch later today.
>
> Implemented in attached patch, v22
>
> The following commands (only) are allowed with
> ShareUpdateExclusiveLock, patch includes doc changes.
>
> ALTER TABLE ... VALIDATE CONSTRAINT constraint_name
> covered by isolation test, plus verified manually with pg_dump
>
> ALTER TABLE ... ALTER COLUMN ... SET STATISTICS
> ALTER TABLE ... ALTER COLUMN ... SET (...)
> ALTER TABLE ... ALTER COLUMN ... RESET (...)
>
> ALTER TABLE ... CLUSTER ON ...
> ALTER TABLE ... SET WITHOUT CLUSTER
> ALTER TABLE ... SET (...)
> covered by isolation test
>
> ALTER TABLE ... RESET (...)
>
> ALTER INDEX ... SET (...)
> ALTER INDEX ... RESET (...)
>
> All other ALTER commands take AccessExclusiveLock
>
> I commend this patch to you for final review; I would like to commit
> this in a few days.

I'm planning to commit this today at 1500UTC barring objections or
negative reviews.

-- Simon Riggs                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services