Adapter functions, updatable views and joins

With the introduction of REST Views it has become more interesting to support updatable views and joins, and more important to support adapter functions. The Beta version 5.7 of Pyrrho makes some important contributions in these aspects.
In the following sample, we model a situation where databases have columns that nearly match but do not quite do so. For example suppose Table A(B,C) has an integer primary key B, while table D(E,G) uses corresponding string values all prefixed by the letter H as its primary key E. It is easy to define a suitable adapter function F

create function f(x int) returns char return 'H'|| cast(x as char)

and create a view of A that makes sense in D's database:

create view v as select f(b) as fb,c from a

D, however, wishes to be able to update table A, so would like this view to be updatable. Pyrrho allows the declaration of an inverse of F:

create function f1(a char) returns int inverts f return cast(substring(a from 1) as int)

where the phrase "inverts f" is parsed as metadata for the new function f1 (otherwise everything so far is ordinary standard SQL). This makes f and f1 into mutually inverse functions, that is, declaring f1 as an inverse for f also declares f as an inverse of f1. The machinery works for multiple parameters using row types for the return values.

This makes v updatable and we can write

insert into v values('H91','This is new')

We can also use such adapter functions in referential constraints, e.g. a slight extension to SQL in Pyrrho allows us to declare the above relationship between tables A and D as follows:

create table d (e char references a using f1,g int)

The following SQL sample code demonstrating the above ideas is supported by Pyrrho 5.7 as of today.

create function f(x int) returns char return 'H'|| cast(x as char)
[create function f1(a char) returns int inverts f return cast(substring(a from 1) as int)]
select f(45) from static
select f1('B67') from static
create table a(b int primary key,c char)
insert into a values(23,'Twenty3'),(42,'Forty2')
create view v as select f(b) as fb,c from a
select * from v
insert into v values('H91','This is new')
table a
create table d (e char references a using f1,g int)
insert into d values('H23',234),('H91',567)
create view w as select c,e from a left join d on b=f1(e)
delete from w where c='Twenty3'
table a