On Tue, May 12, 2020 at 5:25 AM Robert Haas <robertmhaas@gmail.com> wrote:
> On Mon, May 11, 2020 at 4:22 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Robert Haas <robertmhaas@gmail.com> writes:
> > > If the parent is RTI 1, and the children are RTIs 2..6, what
> > > varno/varattno will we use in RTI 1's tlist to represent a column that
> > > exists in both RTI 2 and RTI 3 but not in RTI 1, 4, 5, or 6?
> >
> > Fair question. We don't have any problem representing the column
> > as it exists in any one of those children, but we lack a notation
> > for the "union" or whatever you want to call it, except in the case
> > where the parent relation has a corresponding column. Still, this
> > doesn't seem that hard to fix. My inclination would be to invent
> > dummy parent-rel columns (possibly with negative attnums? not sure if
> > that'd be easier or harder than adding them in the positive direction)
> > to represent such "union" columns.
>
> Ah, that makes sense. If we can invent dummy columns on the parent
> rel, then most of what I was worrying about no longer seems very
> worrying.
IIUC, the idea is to have "dummy" columns in the top parent's
reltarget for every junk TLE added to the top-level targetlist by
child tables' FDWs that the top parent itself can't emit. But we allow
these FDW junk TLEs to contain any arbitrary expression, not just
plain Vars [1], so what node type are these dummy parent columns? I
can see from add_vars_to_targetlist() that we allow only Vars and
PlaceHolderVars to be present in a relation's reltarget->exprs, but
neither of those seem suitable for the task.
Once we get something in the parent's reltarget->exprs representing
these child expressions, from there they go back into child
reltargets, so it would appear that our appendrel transformation code
must somehow be taught to deal with these dummy columns.
--
Amit Langote
EnterpriseDB: http://www.enterprisedb.com
[1] https://www.postgresql.org/docs/current/fdw-callbacks.html#FDW-CALLBACKS-UPDATE
"...If the extra expressions are more complex than simple Vars, they
must be run through eval_const_expressions before adding them to the
targetlist."