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: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Amit Langote <amitlangote09(at)gmail(dot)com>, Rajkumar Raghuwanshi <rajkumar(dot)raghuwanshi(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: postgres_fdw : altering foreign table not invalidating prepare statement execution plan.
Date: 2016-10-07 01:26:04
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On 2016/10/06 21:55, Etsuro Fujita wrote:
> On 2016/10/06 20:17, Amit Langote wrote:
>> On 2016/10/05 20:45, Etsuro Fujita wrote:
>>> On 2016/10/05 14:09, Ashutosh Bapat wrote:
>>>> IMO, maintaining that extra function and
>>>> the risk of bugs because of not keeping those two functions in sync
>>>> outweigh the small not-so-frequent gain.
>>> The inefficiency wouldn't be negligible in the case where there are large
>>> numbers of cached plans. So, I'd like to introduce a helper function that
>>> checks the dependency list for the generic plan, to eliminate most of the
>>> duplication.
>> I too am inclined to have a PlanCacheForeignCallback().
>> Just one minor comment:
> Thanks for the comments!
> I noticed that we were wrong. Your patch was modified so that
> dependencies on FDW-related objects would be extracted from a given plan
> in create_foreignscan_plan (by Ashutosh) and then in
> set_foreignscan_references by me, but that wouldn't work well for INSERT
> cases. To fix that, I'd like to propose that we collect the dependencies
> from the given rte in add_rte_to_flat_rtable, instead.

I see. So, doing it from set_foreignscan_references() fails to capture
plan dependencies in case of INSERT because it won't be invoked at all
unlike the UPDATE/DELETE case.

> Attached is an updated version, in which I removed the
> PlanCacheForeignCallback and adjusted regression tests a bit.
>>> If some writable FDW consulted foreign table/server/FDW options in
>>> AddForeignUpdateTarget, which adds the extra junk columns for
>>> UPDATE/DELETE to the targetList of the given query tree, the rewritten
>>> query tree would also need to be invalidated. But I don't think such an
>>> FDW really exists because that routine in a practical FDW wouldn't change
>>> such columns depending on those options.
> I had second thoughts about that; since the possibility wouldn't be zero,
> I added to extract_query_dependencies_walker the same code I added to
> add_rte_to_flat_rtable.

And here, since AddForeignUpdateTargets() could possibly utilize foreign
options which would cause *query tree* dependencies. It's possible that
add_rte_to_flat_rtable may not be called before an option change, causing
invalidation of any cached objects created based on the changed options.
So, must record dependencies from extract_query_dependencies as well.

> After all, the updated patch is much like your version, but I think your
> patch, which modified extract_query_dependencies_walker only, is not
> enough because a generic plan can have more dependencies than its query
> tree (eg, consider inheritance).

Agreed. I didn't know at the time that extract_query_dependencies is only
able to capture dependencies using the RTEs in the *rewritten* query tree;
it wouldn't have gone through the planner at that point.

I think this (v4) patch is in the best shape so far.


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