I wondered if it wouldn't be better if CLUSTER ON was stored in pg_class as the
Oid of a clustered index, rather than a boolean in pg_index.
That likely would've avoided (or at least exposed) this issue.
And avoids the possibility of having two indices marked as "clustered".
These would be more trivial:
mark_index_clustered
/* We need to find the index that has indisclustered set. */
