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

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.

Considering the anticipated complexity when many subscriptions
exist in the list, and complexity to remove stale entries in the
hash, I'm inclining toward poroposing just to add 'worker_start'
in pg_subscription_rel. We already have the similars in
pg_stat_activity and pg_stat_replication.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center