Expr. extended stats are skipped with equality operator

I'm on a quest to help the planner (on pg14) use the best of several partial, expressional indices we have on some large tables (few TBs in size, billions of records).

As we know, stats for expressions in partial indices aren't gathered by default - so I'm tinkering with expressional extended stats to cover for those.

I've tackled two interesting points there:

1. Seems like expressional stats involving the equality operator are skipped or mismatched (fiddle)

Let's take the following naive example:

Now, of course one can just create the stat on x[1] directly in this case, but I have a more complex use case where an equality operator is beneficial; 

should the above case be supported ? feels like I'm just missing something fundamental.

2. Less important, just a minor note - feel free to ignore - although the eq. operator above seems to be skipped when matching the ext. stats, I can work around this by using a CASE expression (fiddle);

Building on the above example, we can:

What's a bit problematic here, though, is that if we mix other dependent columns to the extended stat, and specifically if we create an mcv, 

queries involving the CASE expression throw with `error: unknown clause type 130`, where clause type == T_CaseExpr.

The second point for me would be that I've found it a bit non intuitive that creating an extended statistic can fail queries at query time; it makes sense that the mcv wouldn't work for case expressions, but it might've been a bit clearer to:

a. Fail this at statistic creation time, potentially, or 

b. Convert the type numeric in the above error to its text representation, if we can extract it out at runtime somehow - 

I couldn't find a mapping of clause type numerics to their names, and as the node tags are generated at compile time, it could be build-dependent and a bit hard to track down if one doesn't control the build flags

Thanks a ton for your help - appreciate your time,

Danny