Re: print_path is missing GatherMerge and CustomScan support

(2018/07/19 14:11), Ashutosh Bapat wrote:
> On Thu, Jul 19, 2018 at 5:37 AM, Michael Paquier<michael@paquier.xyz>  wrote:
>> On Wed, Jul 18, 2018 at 12:15:25PM +0530, Ashutosh Bapat wrote:
>>> Yes that's right. Thanks for taking care of it.
>>
>> Okay, I have pushed a fix for this one as that's wrong and
>> back-patched to v11.  The coverage of reparameterize_path_by_child is
>> actually quite poor if you look at the reports:
>> https://coverage.postgresql.org/src/backend/optimizer/util/pathnode.c.gcov.html
>>
>> Could it be possible to stress that more?  This way mistakes like this
>> one could have been avoided from the start.
>
> I had extensive testcases in my original patch-set to exercise that
> code but 1. that testset was too extensive; even today
> partition_join.sql is a separate testcase and it's quite large. 2.
> that function returns NULL rather than throwing an error, if it can
> not produce a parameterized path. So, unless we check whether each of
> those paths get created no test is useful and that can only be done
> through an EXPLAIN OUTPUT which means that the testcase becomes
> fragile. I fine if we want to add more tests just to cover the code if
> those are not as fragile and do not blow up partition_join too much.

It would not be possible to cover these cases:

         case T_GatherPath:
             {
                 GatherPath *gpath;

                 FLAT_COPY_PATH(gpath, path, GatherPath);
                 REPARAMETERIZE_CHILD_PATH(gpath->subpath);
                 new_path = (Path *) gpath;
             }
             break;

         case T_GatherMergePath:
             {
                 GatherMergePath *gmpath;

                 FLAT_COPY_PATH(gmpath, path, GatherMergePath);
                 REPARAMETERIZE_CHILD_PATH(gmpath->subpath);
                 new_path = (Path *) gmpath;
             }
             break;

because we currently don't consider gathering partial child-scan or 
child-join paths.  I think we might consider that in future, though.

Best regards,
Etsuro Fujita