From: Tom Lane <tgl@sss.pgh.pa.us>
To: Robert Haas <robertmhaas@gmail.com>
cc: Amit Langote <amitlangote09@gmail.com>,
        Justin Pryzby <pryzby@telsasoft.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: FailedAssertion("pd_idx == pinfo->nparts",
 File: "execPartition.c", Line: 1689)
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Comments: In-reply-to Robert Haas <robertmhaas@gmail.com>
	message dated "Wed, 05 Aug 2020 15:59:54 -0400"
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Date: Wed, 05 Aug 2020 16:19:08 -0400
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Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Aug 5, 2020 at 2:22 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> In the longer term, it's annoying that we have no test methodology
>> for this other than "manually set a breakpoint here".  If we're going
>> to allow plan-relevant DDL changes to happen with less than full table
>> lock, I think we need to improve that.  I spent a little bit of time
>> just now trying to build an isolationtester case for this, and failed
>> completely.  So I wonder if we can create some sort of test module that
>> allows capture of a plan tree and then execution of that plan tree later
>> (even after relcache inval would normally have forced replanning).
>> Obviously that could not be a normal SQL-accessible feature, because
>> some types of invals would make the plan completely wrong, but for
>> testing purposes it'd be mighty helpful to check that a stale plan
>> still works.

> That's an interesting idea. I don't know exactly how it would work,
> but I agree that it would allow useful testing that we can't do today.

After thinking about it for a little bit, I'm envisioning a test module
that can be loaded into a session, and then it gets into the planner_hook,
and what it does is after each planner execution, take and release an
advisory lock with some selectable ID.  Then we can construct
isolationtester specs that do something like

	session 1				session 2

	LOAD test-module;
	SET custom_guc_for_lock_id = n;
	prepare test tables;

						SELECT pg_advisory_lock(n);
		
	SELECT victim-query-here;
	... after planning, query blocks on lock

						perform DDL changes;
						SELECT pg_advisory_unlock(n);

	... query executes with now-stale plan

			regards, tom lane