Re: BUG #19435: Error: "No relation entry for relid 2" Triggered by Complex Join with Self-Referencing Tables

I wrote:
> At the very least we need to add comments, but I wonder if we
> don't actually need an Assert that ChangeVarNodesWalkExpression
> is not invoked directly on a Query. It would have done the
> right thing before this patch, but now it won't. That's an
> okay tradeoff for fixing the bare-Var case, but not documenting
> what you did is not okay.

After further contemplation I've decided that an Assert would be
wrong, because it's not impossible that a callback would want
to invoke this on a sub-Query --- for instance, if it wanted to
short-circuit ChangeVarNodes's processing of a SubLink node,
it would need to do that. The key point is that if we do see a
Query node here, we will treat it as a sub-query not a top-level
query, which also justifies skipping the work that
ChangeVarNodesExtended does on a top-level Query. So we just
need a comment explaining that. I'm thinking about the attached.

(BTW, by this reasoning the previous implementation of
ChangeVarNodesWalkExpression was doubly wrong, since it would
have done the wrong thing at a Query node as well as a Var node.)

regards, tom lane