Since I wrote my previous blog entry on index-only scans, quite a bit of additional work has been done. Tom Lane cleaned up the code and improved the costing model, but possibly the most interesting thing he did was to allow index-only scans to be used for queries that don't involve an indexable condition at all. The classic example is SELECT COUNT(*) FROM table. In previous versions of PostgreSQL, there's just one way to implement this: sequential scan the table and count 'em up. In PostgreSQL 9.2, that method will still, of course, be available, but now there will be another choice: pick any index you like and do a full index scan, checking whether each tuple is all-visible either using the visibility map or via a heap fetch. So, how well does it work?
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