On Tuesday, December 23, 2014, Robert Haas <robertmh...@gmail.com> wrote:
> On Mon, Dec 22, 2014 at 11:19 AM, Andrew Gierth > <and...@tao11.riddles.org.uk <javascript:;>> wrote: > > Tom> The other reason that's a bad comparison is that I've not seen > > Tom> many queries that use more than a couple of window frames, > > Tom> whereas we have to expect that the number of grouping sets in > > Tom> typical queries will be significantly more than "a couple". > > > > I would be interested in seeing more good examples of the size and > > type of grouping sets used in typical queries. > > From what I have seen, there is interest in being able to do things > like GROUP BY CUBE(a, b, c, d) and have that be efficient. That will > require 16 different groupings, and we really want to minimize the > number of times we have to re-sort to get all of those done. For > example, if we start by sorting on (a, b, c, d), we want to then make > a single pass over the data computing the aggregates with (a, b, c, > d), (a, b, c), (a, b), (a), and () as the grouping columns. > > > That is what ChainAggregate node does exactly. A set of orders that fit in a single ROLLUP list (like your example) are processed in a single go. -- Regards, Atri *l'apprenant*
