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

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.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center