Here is a v1 patch series that does the first part of what we've been talking about, which is to implement the new optimization rule for the case of a single RHS reference to the target variable. I'm curious to know if it helps you at all by itself. You'll definitely also need commit 534d0ea6c, so probably applying these on our git master branch would be the place to start.
I'll apply these tonight and get back to you asap. There are many functions in my API that take only one expanded RHS argument, so I'll look for some cases where your changes reduce expansions when I run my tests.
I tested these patches with my test setup and can confirm there is now one less expansion in this function:
create or replace function test_expand(graph matrix) returns matrix language plpgsql as $$ declare nvals bigint = nvals(graph); begin return graph; end; $$;
The second expansion in matrix_out happens outside the function, so inside there is only the one expansion for both matrix_nvals and the assignment. Thank you! All my tests continue to pass and the change seems to work well.
Looking forward to helping test the support function idea, let me know if there's anything else I can do to validate the idea.
