Re: Mapping view columns to their source columns

After thinking about this one for a while, I imagined even more nightmarish
scenarios than what you've just described here, and mapping the source
columns no longer seems like a viable idea.

Fortunately, there are a few work arounds I can rely on that particular to
our database design, which means I will be able to use the system catalogs
in a round about way.

Incidentally, I have become interested in the pg_node_tree type. I can't
find much info on it. Would that be in the source? What would be the first
steps for writing an extension for this sort of thing?

Matt

0400 990 131

On Wed, Sep 25, 2019 at 2:54 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Matt Andrews <mattandrews(at)massey(dot)com(dot)au> writes:
> > I'm trying to map view columns to their source columns using the system
> > catalogs and information schema, but not having much luck. It's easy to
> > determine which columns a view *depends *on, but not how those columns
> are
> > mapped to the columns of the view. It seems like the only way to do is
> this
> > is to somehow query the pg_node_tree in pg_rewrite.ev_call.
>
> Yeah, that's what you'd have to do. The system doesn't track this
> any more finely than "does the view as a whole depend on this column",
> partly because we don't need to and partly because it's hard to define
> reasonably. What do you want to do with, say,
>
> select a, b+c from tab;
>
> It gets even more interesting when you think about aggregates and
> other advanced features:
>
> select a, max(b) as mb from tab group by a;
>
> Any reasonable semantic analysis would have to conclude that the
> mb column depends on both a and b.
>
> Once you had answers to these definitional questions, you could
> think about analyzing the view's query tree to get the answers
> you want. Unfortunately, doing that from client side is not
> supported at all; if you try you'll find yourself maintaining
> a lot of messy code that we *will* break on a regular basis.
> It would be less hard in a backend extension, but I suspect
> you don't want to go there :-(
>
> regards, tom lane
>