Re: [HACKERS] Interval for launching the table sync worker

On 19/04/17 15:57, Masahiko Sawada wrote:
> On Wed, Apr 19, 2017 at 10:07 PM, Petr Jelinek
> <petr.jelinek@2ndquadrant.com> wrote:
>> On 19/04/17 14:42, Masahiko Sawada wrote:
>>> On Wed, Apr 19, 2017 at 5:12 PM, Kyotaro HORIGUCHI
>>> <horiguchi.kyotaro@lab.ntt.co.jp> wrote:
>>>> At Tue, 18 Apr 2017 18:40:56 +0200, Petr Jelinek <petr.jelinek@2ndquadrant.com> wrote in
<f64d87d1-bef3-5e3e-a999-ba302816a0ee@2ndquadrant.com>
>>>>> On 18/04/17 18:14, Peter Eisentraut wrote:
>>>>>> On 4/18/17 11:59, Petr Jelinek wrote:
>>>>>>> Hmm if we create hashtable for this, I'd say create hashtable for the
>>>>>>> whole table_states then. The reason why it's list now was that it seemed
>>>>>>> unnecessary to have hashtable when it will be empty almost always but
>>>>>>> there is no need to have both hashtable + list IMHO.
>>>>
>>>> I understant that but I also don't like the frequent palloc/pfree
>>>> in long-lasting context and double loop like Peter.
>>>>
>>>>>> The difference is that we blow away the list of states when the catalog
>>>>>> changes, but we keep the hash table with the start times around.  We
>>>>>> need two things with different life times.
>>>>
>>>> On the other hand, hash seems overdone. Addition to that, the
>>>> hash-version leaks stale entries while subscriptions are
>>>> modified. But vacuuming them costs high.
>>>>
>>>>> Why can't we just update the hashtable based on the catalog? I mean once
>>>>> the record is not needed in the list, the table has been synced so there
>>>>> is no need for the timestamp either since we'll not try to start the
>>>>> worker again.
>>>
>>> I guess the table sync worker for the same table could need to be
>>> started again. For example, please image a case where the table
>>> belonging to the publication is removed from it and the corresponding
>>> subscription is refreshed, and then the table is added to it again. We
>>> have the record of the table with timestamp in the hash table when the
>>> table sync in the first time, but the table sync after refreshed could
>>> have to wait for the interval.
>>>
>>
>> But why do we want to wait in such case where user has explicitly
>> requested refresh?
>>
> 
> Yeah, sorry, I meant that we don't want to wait but cannot launch the
> tablesync worker in such case.
> 
> But after more thought, the latest patch Peter proposed has the same
> problem. Perhaps we need to scan always whole pg_subscription_rel and
> remove the entry if the corresponding table get synced.
> 

Yes that's what I mean by "Why can't we just update the hashtable based
on the catalog". And if we do that then I don't understand why do we
need both hastable and linked list if we need to update both based on
catalog reads anyway.

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