Re: [HACKERS] Block level parallel vacuum

Hello

Its possible to change order of index processing by parallel leader? In v35 patchset I see following order:
- start parallel processes
- leader and parallel workers processed index lixt and possible skip some entries
- after that parallel leader recheck index list and process the skipped indexes
- WaitForParallelWorkersToFinish

I think it would be better to:
- start parallel processes
- parallel leader goes through index list and process only indexes which are skip_parallel_index_vacuum = true
- parallel workers processes indexes with skip_parallel_index_vacuum = false
- parallel leader start participate with remainings parallel-safe index processing
- WaitForParallelWorkersToFinish

This would be less running time and better load balance across leader and workers in case of few non-parallel and few parallel indexes.
(if this is expected and required by some reason, we need a comment in code)

Also few notes to vacuumdb:
Seems we need version check at least in vacuum_one_database and prepare_vacuum_command. Similar to SKIP_LOCKED or DISABLE_PAGE_SKIPPING features.
discussion question: difference between --parallel and --jobs parameters will be confusing? We need more description for this options