Re: [COMMITTERS] pgsql: Teach tuplesort.c about "top N" sorting, in which only the first

"Magnus Hagander" <magnus(at)hagander(dot)net> writes:

>> What 3 columns? In-memory sorts, on-disk sorts, and on-disk size?
>> (Sum of how much spilled to disk).
>
> I was thinking in-mem sorts, on-disk sorts, limited-by-LIMIT sorts (that
> would be the new feature..)

Tom's code distinguished in-memory, top-N, on-disk with final merge postponed,
and on-disk with materialized result. Four categories. But I think the
distinction between the two types of in-memory and the two types of on-disk
sorts is only really useful when you're looking at an individual query. And
even then probably only useful to a Postgres hacker, not a DBA.

It seems like it would be more useful to just break it down into in-memory and
on-disk but for each give number of sorts, number of tuples, and space used.

What would be really handy is breaking this down by table -- probably that
would only be possible when the sort is sorting directly a table scan. I don't
even know how easy it would be to get that information.

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com