Hmm, I'm trying this out now and I don't see the index_rebuild_count ever go up. I think it's because the indexes are built using parallel index build ... or maybe it was the table AM changes that moved things around, not sure. There's a period at the end when the CLUSTER command keeps working, but it's gone from pg_stat_progress_cluster.
-- Álvaro Herrera https://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
