Re: Extending PostgreSQL with a Domain-Specific Language (DSL) -Development - Mailing list pgsql-hackers

On 09/07/2019 23:22, Tomas Vondra wrote:
> On Sun, Jul 07, 2019 at 11:06:38PM +0000, Tom Mercha wrote:
>> On 06/07/2019 00:06, Tomas Vondra wrote:
>>> First of all, it's pretty difficult to follow the discussion when it's
>>> not clear what's the original message and what's the response. E-mail
>>> clients generally indent the original message with '>' or someting like
>>> that, but your client does not do that (which is pretty silly). And
>>> copying the message at the top does not really help. Please do something
>>> about that.
>>
>> I would like to apologise. I did not realize that my client was doing
>> that and now I have changed the client. I hope it's fine now.
>>
> 
> Thanks, seems fine now.
> 
>>>
>>> On Fri, Jul 05, 2019 at 09:37:03PM +0000, Tom Mercha wrote:
>>>>> I might be missing something, but it seems like you intend to replace
>>>>> the SQL grammar we have with something else. It's not clear to me what
>>>>> would be the point of doing that, and it definitely looks like a huge
>>>>> amount of work - e.g. we don't have any support for switching between
>>>>> two distinct grammars the way you envision, and just that alone seems
>>>>> like a multi-year project. And if you don't have that capability, all
>>>>> external tools kinda stop working. Good luck with running such 
>>>>> database.
>>>>
>>>> I was considering having two distinct grammars as an option - thanks
>>>> for indicating the effort involved. At the end of the day I want both
>>>> my DSL and the PostgreSQL grammars to coexist. Is extending
>>>> PostgreSQL's grammar with my own through the PostgreSQL extension
>>>> infrastructure worth consideration or is it also difficult to develop?
>>>> Could you suggest any reference material on this topic?
>>>>
>>>
>>> Well, I'm not an expert in that area, but we currently don't have any
>>> infrastructure to support that. It's a topic that was discussed in the
>>> past (perhaps you can find some references in the archives) and it
>>> generally boils down to:
>>>
>>> 1) We're using bison as parser generator.
>>> 2) Bison does not allow adding rules on the fly.
>>>
>>> So you have to modify the in-core src/backend/parser/gram.y and rebuild
>>> postgres. See for example for an example of such discussion
>>>
>>> https://www.postgresql.org/message-id/flat/CABSN6VeeEhwb0HrjOCp9kHaWm0Ljbnko5y-0NKsT_%3D5i5C2jog%40mail.gmail.com 
>>>
>>>
>>>
>>> When two of the smartest people on the list say it's a hard problem, it
>>> probably is. Particularly for someone who does not know the internals.
>> You are right. Thanks for bringing it to my attention!
>>
>> I didn't design my language for interaction with triggers and whatnot,
>> but I think that it would be very interesting to support those as well,
>> so looking at CREATE LANGUAGE functionality is actually exciting and
>> appropriate once I make some changes in design. Thanks again for this 
>> point!
>>
> 
> ;-)
> 
>> I hope this is not off topic but I was wondering if you know what are
>> the intrinsic differences between HANDLER and INLINE parameters of
>> CREATE LANGUAGE? I know that they are functions which are invoked at
>> different instances of time (e.g. one is for handling anonymous code
>> blocks), but at the end of the day they seem to have the same purpose?
>>
> 
> I've never written any PL handler, so I don't know. All I know is this
> quote from the docs, right below the simple example of PL handler:
> 
>     Only a few thousand lines of code have to be added instead of the
>     dots to complete the call handler.
> 
> I suppose the best idea to start an implementation is to copy an
> existing PL implementation, and modify that. That's usually much easier
> than starting from scratch, because you have something that works. Not
> sure if PL/pgSQL is the right choice though, perhaps pick some other
> language from https://wiki.postgresql.org/wiki/PL_Matrix
> 

I understand that you never wrote any PL handler but was just thinking 
about this functionality as a follow-up to our conversation. I was just 
wondering whether anonymous DO blocks *must* return void or not?

The docs for DO say it is a function returning void - 
https://www.postgresql.org/docs/current/sql-do.html

But the docs for CREATE LANGUAGE's INLINE HANDLER say 'typically return 
void' - https://www.postgresql.org/docs/current/sql-createlanguage.html

Is the implication that we can make the DO block return something 
somehow? I would be quite interested if there is a way of achieving this 
kind of functionality. My experiments using an SRF, which I have 
written, within an anonymous DO block just gives me an "ERROR: 
set-valued function called in context that cannot accept a set".

Anyway maybe I'm going off on a tangent here... perhaps it is better to 
open a new thread?

> 
> regards
>