Re: Fixing findDependentObjects()'s dependency on scan order (regressions in DROP diagnostic messages)

On 06.12.2018 11:52, Peter Geoghegan wrote:
> On Wed, Dec 5, 2018 at 10:35 PM Andrey Lepikhov
> <a.lepikhov@postgrespro.ru> wrote:
>> This solution changes pg_depend relation for solve a problem, which
>> exists only in regression tests.  Very rarely it can be in the
>> partitioning cases. Or is it not?
> 
> I don't think it's a matter of how rarely this will happen. We're
> trying to avoid these diagnostic message changes because they are
> wrong. I don't think that there is much ambiguity about that.  Still,
> it will happen however often the user drops objects belonging to
> partition children, which could be quite often.

I want to say that we need to localize the rules for the order of the 
diagnostic messages as much as possible in dependency.c.
> 
>> I think this decision is some excessive.
>> May be you consider another approach:
>> 1. Order of dependencies in 'DROP ... CASCADE' case is a problem of test
>> tools, not DBMS. And here we can use 'verbose terse'.
>> 2. Print all dependencies in findDependentObjects() on a drop error (see
>> attachment as a prototype).
> 
> You didn't include changes to the regression test output, which seems
> like a big oversight...

It was done knowingly to show the differences in messages that 
introduces this approach.

> messages that are represented in the regression tests. Anyway, I don't
> think it's acceptable to change the messages like this. It makes them
> much less useful.

May you clarify this? If I understand correctly, your solution is that 
user receives a report about the last inserted internal dependency on 
the object. Why the full info about internal dependencies of the object 
much less useful?
> 
> These stability issue keeps coming up, which makes a comprehensive
> approach seem attractive to me. At least 95% of the test instability
> comes from pg_depend.
> 

During the retail index tuple deletion project (heap cleaner subsystem) 
we have non-fixed order of tuples at database relations. This caused to 
unsteady order of text strings in some check-world test results.
Thus, I realized that the order of messages in the test results is 
mostly a game of chance. For this reason I think it is necessary to find 
more general solution of the messages ordering problem.

-- 
Andrey Lepikhov
Postgres Professional
https://postgrespro.com
The Russian Postgres Company