Re: ctidscan as an example of custom-scan (Re: [v9.5] Custom Plan API)

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Jul 14, 2015 at 3:07 PM, Alvaro Herrera
> <alvherre(at)2ndquadrant(dot)com> wrote:
>> As a general principle, I think it's a good idea to have a module that's
>> mostly just a skeleton that guides people into writing something real to
>> use whatever API is being tested. It needs to be simple enough that not
>> much need to be deleted when writing the real thing, and complex enough
>> to cover the parts that need covering. If whatever replaces ctidscan is
>> too complex, it will not serve that purpose.
>>
>> My guess is that something whose only purpose is to test the custom scan
>> interface for coverage purposes can be simpler than this module.

> See, I actually think the opposite: I think we've been accumulating a
> reasonable amount of test code that actually serves no really useful
> purpose and is just cruft. Stuff like test_shm_mq and test_decoding
> seem like they actually catches bugs, so I like that, but I think
> stuff like worker_spi is actually TOO simple to be useful in building
> anything real, and it provides no useful test coverage, either. But
> this is all a matter of opinion, of course, and I'll defer to whatever
> the consensus is.

I think this ties into my core unhappiness with the customscan stuff,
which is that I don't believe it's *possible* to do anything of very
great interest with it. I think anything really useful will require
core code modifications and/or hooks that don't exist now. So a finger
exercise like ctidscan, even though it might have some marginal use,
doesn't do much to alleviate that concern. It certainly doesn't seem
like it's a suitable placeholder proving that we aren't breaking any
actual use cases for the feature.

(BTW, if we care about the use cases this has, such as data recovery from
partially-corrupt tables, it would make way more sense and take way less
net new code to just teach TidScan about it.)

regards, tom lane