Re: Reconstructing Insert queries with indirection

Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com> writes:
> Consider following sequence of commands

> create type complex as (r float8, i float8);
> create type quad as (c1 complex, c2 complex);
> create temp table quadtable(f1 int, q quad);

> insert into quadtable (f1, q.c1.r, q.c2.i) values(44,55,66);

> While parsing the INSERT query, we parse the query with three columns and
> three values in the target list, but during rewriting we combine q.c1.r and
> q.c2.i into a single column in the form of FieldStore structure. In
> Postgres-XC, we deparse these parse trees, to be sent to other PostgreSQL
> servers.

Well, basically you have a broken design there. We are not going to
adopt a restriction that post-rewrite trees are necessarily exactly
representable as SQL, so there are going to be corner cases where this
approach fails.

> The assertion is added by commit 858d1699. The notes for the commit have
> following paragraph related to FieldStore deparsing.

> I chose to represent an assignment ArrayRef as "array[subscripts] :=
> source",
> which is fairly reasonable and doesn't omit any information. However,
> FieldStore is problematic because the planner will fold multiple
> assignments
> to fields of the same composite column into one FieldStore, resulting
> in a
> structure that is hard to understand at all, let alone display
> comprehensibly.
> So in that case I punted and just made it print the source
> expression(s).

> So, there doesn't seem to be any serious reason behind the restriction.

If you have a proposal for some reasonable way to print the actual
meaning of the expression (and a patch to do it), we can certainly
consider changing that code. I don't think it's possible to display it
as standard SQL, though. The ArrayRef case is already not standard SQL.

regards, tom lane