Re: [HACKERS] Vacuum: allow usage of more than 1GB of work mem

On Thu, Feb 8, 2018 at 12:13 AM, Claudio Freire <klaussfreire@gmail.com> wrote:
> On Wed, Feb 7, 2018 at 11:29 PM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>> Claudio Freire wrote:
>>> On Wed, Feb 7, 2018 at 8:52 PM, Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>>> >> Waiting as you say would be akin to what the patch does by putting
>>> >> vacuum on its own parallel group.
>>> >
>>> > I don't think it's the same.  We don't need to wait until all the
>>> > concurrent tests are done -- we only need to wait until the transactions
>>> > that were current when the delete finished are done, which is very
>>> > different since each test runs tons of small transactions rather than
>>> > one single big transaction.
>>>
>>> Um... maybe "lock pg_class" ?
>>
>> I was thinking in first doing
>>   SELECT array_agg(DISTINCT virtualtransaction) vxids
>>     FROM pg_locks \gset
>>
>> and then in a DO block loop until
>>
>>    SELECT DISTINCT virtualtransaction
>>      FROM pg_locks
>> INTERSECT
>>    SELECT (unnest(:'vxids'::text[]));
>>
>> returns empty; something along those lines.
>
> Isn't it the same though?
>
> I can't think how a transaction wouldn't be holding at least an access
> share on pg_class.

Never mind, I just saw the error of my ways.

I don't like looping, though, seems overly cumbersome. What's worse?
maintaining that fragile weird loop that might break (by causing
unexpected output), or a slight slowdown of the test suite? I don't
know how long it might take on slow machines, but in my machine, which
isn't a great machine, while the vacuum test isn't fast indeed, it's
just a tiny fraction of what a simple "make check" takes. So it's not
a huge slowdown in any case.

I'll give it some thought, maybe there's a simpler way.