Peter Geoghegan <pg@bowt.ie> writes:
> The terminology in this area is a mess. MySQL calls
> SELECT-DISTINCT-that-matches-an-index "loose index scans". I think
> that you're talking about skip scan when you say "loose index scan".
> Skip scan is where there is an omitted prefix of columns in the SQL
> query -- omitted columns after the first column that lack an equality
> qual.
Right, that's the case I had in mind --- apologies if my terminology
was faulty. btree can actually handle such a case now, but what it
fails to do is re-descend from the tree root instead of plowing
forward in the index to find the next matching entry.
> It might be useful for somebody to go write a "taxonomy of MDAM
> techniques", or a glossary.
+1. We at least need to be sure we all are using these terms
the same way.
regards, tom lane