On 5/19/09 5:01 AM, "Matthew Wakeling" <matthew@xxxxxxxxxxx> wrote: > On Tue, 19 May 2009, Simon Riggs wrote: >>> Speaking of avoiding large sorts, I'd like to push again for partial >>> sorts. This is the situation where an index provides data sorted by >>> column "a", and the query requests data sorted by "a, b". Currently, >>> Postgres sorts the entire data set, whereas it need only group each >>> set of identical "a" and sort each by "b". >> >> Partially sorted data takes much less effort to sort (OK, not zero, I >> grant) so this seems like a high complexity, lower value feature. I >> agree it should be on the TODO, just IMHO at a lower priority than some >> other features. > > Not arguing with you, however I'd like to point out that partial sorting > allows the results to be streamed, which would lower the cost to produce > the first row of results significantly, and reduce the amount of RAM used > by the query, and prevent temporary tables from being used. That has to be > a fairly major win. Queries with a LIMIT would see the most benefit. > I will second that point -- Although for smaller sorts, the partial sort doesn't help much and is just complicated -- once the sort is large, it reduces the amount of work_mem needed significantly for large performance gain, and large concurrent query scale gain. And those benefits occur without using LIMIT. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
