I haven't read the patch yet. But some thoughts based on the posted output:
1) At first I was quite skeptical about losing the progress reporting. I've often found it quite useful. But looking at the examples I'm convinced. Or rather I think a better way to look at it is that the progress output for the operator should be separated from the metrics logged. As an operator what I want to see is some progress indicator ""starting table scan", "overflow at x% of table scanned, starting index scan", "processing index 1" "index 2"... so I can have some idea of how much longer the vacuum will take and see whether I need to raise maintenance_work_mem and by how much. I don't need to see all the metrics while it's running. 2) I don't much like the format. I want to be able to parse the output with awk or mtail or even just grep for relevant lines. Things like "index scan not needed" make it hard to parse since you don't know what it will look like if they are needed. I would have expected something like "index scans: 0" which is actually already there up above. I'm not clear how this line is meant to be read. Is it just explaining *why* the index scan was skipped? It would just be missing entirely if it wasn't skipped? Fwiw, having it be parsable is why I wouldn't want it to be multiple ereports. That would mean it could get interleaved with other errors from other backends. That would be a disaster.
