On 04/19/25 15:03, Tom Lane wrote: > So what are the odds that outside PLs do it correctly (for whatever > you think "correctly" is)? It doesn't help any that we document > none of this.
My sense of Déjà vu turns out to be because I had a bunch of proposed new documentation and example code for plsample in a commitfest a few years back[0] but had to set it aside at the time. And it's becoming clear there's even more to be nailed down than I realized at the time anyway. > If I had my druthers I would flush every one of these special rules, > and just say that an argument/result gets transformed if there's a > transform matching its declared type, full stop. You want to > transform arrays or domains, you make a transform to match. > This would be a lot simpler to understand and potentially more > flexible. But I don't know if it falls foul of the wording of > the spec, and I concede that in some cases it'd require users > to do more work creating those additional transforms. I don't see right offhand that it would fall afoul of the spec. I had also proposed in that earlier CF to change some of what our doc says about the spec. We currently say "There is a CREATE TRANSFORM command in the SQL standard, but it is for adapting data types to client languages" and I don't think it's as different as that suggests. 4.9.6 clearly says the from-sql function is invoked whenever a value "is passed to a host- language program *or an external routine*" and the to-sql function is invoked whenever a value "is supplied by a host-language program *or an external routine*" (asterisks mine). The biggest difference is our use of type internal to allow transforming to arbitrary PL data types: the ISO to- and from-sql functions have to interface with the external routine using predefined SQL types. ISO does have a notion of user-defined types being subtypes or supertypes of each other, and sections 9.31 and 9.33 on determining the applicable from-sql or to-sql function do involve examining a type's supertypes to find one that has a transform matching the wanted transform group name. I don't see anything, though, about peering into array or range or domain types. Perhaps we could regard a domain as a subtype of its base type, but I do not see for sure that the spec does. I think that a transform-must-match-declared-type rule would not be at odds with the spec. Regards, -Chap [0] https://commitfest.postgresql.org/patch/3554/
