Re: postgres_fdw : altering foreign table not invalidating prepare statement execution plan.

From: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: tgl(at)sss(dot)pgh(dot)pa(dot)us, amitlangote09(at)gmail(dot)com, rajkumar(dot)raghuwanshi(at)enterprisedb(dot)com, pgsql-hackers(at)postgresql(dot)org
Subject: Re: postgres_fdw : altering foreign table not invalidating prepare statement execution plan.
Date: 2016-04-05 10:46:04
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On 2016/04/05 18:44, Kyotaro HORIGUCHI wrote:
> At Tue, 5 Apr 2016 14:24:52 +0900, Amit Langote wrote:
>> On 2016/04/05 0:23, Tom Lane wrote:
>>> Amit Langote <amitlangote09(at)gmail(dot)com> writes:
>>>> Hm, some kind of PlanInvalItem-based solution could work maybe?
>>> Hm, so we'd expect that whenever an FDW consulted the options while
>>> making a plan, it'd have to record a plan dependency on them? That
>>> would be a clean fix maybe, but I'm worried that third-party FDWs
>>> would fail to get the word about needing to do this.
>> I would imagine that that level of granularity may be a little too much; I
>> mean tracking dependencies at the level of individual FDW/foreign
>> table/foreign server options. I think it should really be possible to do
>> the entire thing in core instead of requiring this to be made a concern of
>> FDW authors. How about the attached that teaches
>> extract_query_dependencies() to add a foreign table and associated foreign
>> data wrapper and foreign server to invalItems. Also, it adds plan cache
>> callbacks for respective caches.
> It seems to work but some renaming of functions would needed.

Yeah, I felt the names were getting quite long, too :)

>> One thing that I observed that may not be all that surprising is that we
>> may need a similar mechanism for postgres_fdw's connection cache, which
>> doesn't drop connections using older server connection info after I alter
>> them. I was trying to test my patch by altering dbaname option of a
>> foreign server but that was silly, ;). Although, I did confirm that the
>> patch works by altering use_remote_estimates server option. I could not
>> really test for FDW options though.
>> Thoughts?
> It seems a issue of FDW drivers but notification can be issued by
> the core. The attached ultra-POC patch does it.
> Disconnecting of a fdw connection with active transaction causes
> an unexpected rollback on the remote side. So disconnection
> should be allowed only when xact_depth == 0 in
> pgfdw_subxact_callback().
> There are so many parameters that affect connections, which are
> listed as PQconninfoOptions. It is a bit too complex to detect
> changes of one or some of them. So, I try to use object access
> hook even though using hook is not proper as fdw interface. An
> additional interface would be needed to do this anyway.
> With this patch, making any change on foreign servers or user
> mappings corresponding to exiting connection causes
> disconnection. This could be moved in to core, with the following
> API like this.
> reoutine->NotifyObjectModification(ObjectAccessType access,
> Oid classId, Oid objId);
> - using object access hook (which is put by the core itself) is not bad?
> - If ok, the API above looks bad. Any better API?

Although helps achieve our goal here, object access hook stuff seems to be
targeted at different users:

* Hook on object accesses. This is intended as infrastructure for security
* and logging plugins.
object_access_hook_type object_access_hook = NULL;

I just noticed the following comment above GetConnection():

* XXX Note that caching connections theoretically requires a mechanism to
* detect change of FDW objects to invalidate already established connections.
* We could manage that by watching for invalidation events on the relevant
* syscaches. For the moment, though, it's not clear that this would really
* be useful and not mere pedantry. We could not flush any active connections
* mid-transaction anyway.

So, the hazard of flushing the connection mid-transaction sounds like it
may be a show-stopper here, even if we research some approach based on
syscache invalidation. Although I see that your patch takes care of it.

> By the way, AlterUserMapping seems missing calling
> InvokeObjectPostAlterHook. Is this a bug?

Probably, yes.


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