Gregory Stark <gsstark@xxxxxxx> writes: > On the other there are some common situations where you could see > atypical increases. Consider joining a bunch of small tables to > generate a large result set. The small tables are probably all in > memory and the result set may only have a small number of distinct > values. If you throw out the duplicates early you save *all* the > I/O. If you have to do a disk sort it could be many orders slower. Right, we already have support for doing that well, in the form of hashed aggregation. What needs to happen is to get that to work for DISTINCT as well as GROUP BY. IIRC, DISTINCT is currently rather thoroughly intertwined with ORDER BY, and we'd have to figure out some way to decouple them --- without breaking DISTINCT ON, which makes it a lot harder :-( regards, tom lane
