I just mean that, as you say, trace_sort is described in the documentation.
I don't think we'll end up with any kind of cost model here, so where that would need to happen is only an academic matter. The create index parameter would only be an option for the DBA. That's about the only case I can see working for replacement selection: where indexes can be created with very little memory quickly, by optimistically starting to write out the start of the final index representation almost immediately, before most of the underlying table has even been read in. -- Peter Geoghegan