> can the previous error path in > mark_index_clustered() be reached through a different mean than ALTER > TABLE?
That was one of the things we discussed in the previous emails. The remaining callers of that function are VACUUM FULL and CLUSTER. It definitely can't be hit by VACUUM (it passes an InvalidOid for an index), but CLUSTER is a bit more difficult to follow. It seems like to me that it shouldn't be hit (CLUSTER only calls it for leaf partitions, where the check will be false), but I'm not 100% sure about my diagnosis.
