Re: [HACKERS] [PATCH] Generic type subscripting

Hi,

Tom pointed me towards this thread. I've not followed the topic, so
I might miss a bit of context while commenting on expression eval
related bits...

On 2018-01-07 17:39:00 -0500, Tom Lane wrote:
> On the executor side of things, I suspect Andres will be unhappy that
> you are making ExprEvalStep part of the API for datatypes --- he
> objected to my exposing it to plpgsql param eval in
> https://postgr.es/m/20171220174243.n4y3hgzf7xd3mm5e@alap3.anarazel.de
> and there was a lot more reason to do so there than there is here,
> IMO.

Indeed.


> It looks like what you actually need is just the SubscriptingRefState and
> an isnull flag, so it might be better to specify that the fetch and assign
> functions have signatures like
>     Datum fetch(Datum val, SubscriptingRefState *state, bool *isnull)
> (representing both of the last two arguments as INTERNAL at SQL level).

That'd definitely be better.


> Now on the other hand, maybe the right way to go is to embrace a similar
> approach to what I did for plpgsql param eval, and let the datatype
> control what gets generated as the expression execution step.  The main
> point here would be to let the datatype provide the address of a callback
> function that gets executed for a subscripting step, rather than having it
> specify the OID of a pg_proc entry to call.  There would be two big wins
> from that:
> 
> * The callback function would have a plain C call signature, so we would
> not have to go through FunctionCallN, saving a few cycles.  This is
> attractive because it would pretty much eliminate any concern about this
> patch making array access slower at execution time.

I'll note that I'm not convinced that the goal this paragraph states and
having the datatype control the entire expression step are full
dependent on each other. It seems quite possible to have
ExecInitSubscriptingRef() call a datatype specific function that returns
C callbacks.

> The two disadvantages I can see of approaching things this way are:
> 
> * There'd be at least some connection of subscriptable types to
> expression compilation, which is what Andres was objecting to in the
> message I cited above.  Possibly we could alleviate that a bit by
> providing helper functions that mask exactly what goes into the
> expression step structs, but I'm not sure that that gets us far.

Yea, I'm not the greatest fan of that. With plpgsql it's at least
something in-core that's exposed, but I suspect the subscripotion
interface will get used outside of core, and I really want to whack some
of the expression stuff around some more.


> Actually, we could make it *exactly* like that, and have the
> registered handler give back a struct full of function pointers rather
> than doing anything much itself.  Maybe that's an even better way to
> go.

I'd definitely advocate for that.

Greetings,

Andres Freund