Review observations for partial REFRESH MATERIALIZED VIEW patc

Hello hackers,

I reviewed the REFRESH MATERIALIZED VIEW ... WHERE patch and had a few
questions around concurrency semantics.

- The original DELETE -> INSERT approach exposing a consistency gap
makes sense, especially once tuple locks disappear after DELETE. The newer
FOR UPDATE + single-CTE approach seems safer, though I wonder whether
overlapping refreshes could still encounter deadlock scenarios around
UPSERT conflicts.
- The CONCURRENTLY behavior also feels somewhat unintuitive here. With
WHERE refreshes, the non-CONCURRENT path appears more permissive for
writers than CONCURRENTLY WHERE, which seems opposite to the expectation
established by normal REFRESH MATERIALIZED VIEW semantics.
- It may also help to document the intended guarantees around
overlapping partial refreshes and concurrent DML on base tables.

Overall, the use case seems quite valuable for selective high-churn refresh
workloads.

Thanks for working on this patch.

Regards,

Vellaipandiyan