út 14. 1. 2020 v 22:09 odesílatel Tom Lane <tgl@sss.pgh.pa.us> napsal:
Pavel Stehule <pavel.stehule@gmail.com> writes: > [ parser-support-function-with-demo-20191128.patch ]
TBH, I'm still not convinced that this is a good idea. Restricting the support function to only change the function's return type is safer than the original proposal, but it's still not terribly safe. If you change the support function's algorithm in any way, how do you know whether you've broken existing stored queries? If the support function consults external resources to make its choice (perhaps checking the existence of a cast), where could we record that the query depends on the existence of that cast? There'd be no visible trace of that in the query parsetree.
This risk is real and cannot be simply solved without more complications.
Can be solution to limit and enforce this functionality only for extensions that be initialized from shared_preload_libraries or local_preload_libraries?
I'm also still not convinced that this idea allows doing anything that can't be done just as well with polymorphism. It would be a really bad idea for the support function to be examining the values of the arguments (else what happens when they're not constants?). So all you can do is look at their types, and then it seems like the things you can usefully do are pretty much like polymorphism, i.e. select some one of the input types, or a related type such as an array type or element type. If there are gaps in what you can express with polymorphism, I'd much rather spend effort on improving that facility than in adding something that is only accessible to advanced C coders. (Yes, I know I've been slacking on reviewing [1].)
For my purpose critical information is type. I don't need to work with constant, but I can imagine, so some API can be nice to work with constant value.
Yes, I can solve lot of things by patch [1], but not all, and this patch shorter, and almost trivial.
Lastly, I still think that this patch doesn't begin to address all the places that would have to know about the feature. There's a lot of places that know about polymorphism --- if this is polymorphism on steroids, which it is, then why don't all of those places need to be touched?
