Magnus Hagander wrote:
Gregory Stark wrote:
"Magnus Hagander" <[EMAIL PROTECTED]> 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.

Missed the two on-disk distinctions, yeah. But you're probably right
that on-disk vs in-memory is enough, the interesting thing is to get
indications on when you hit disk given what it does for performance.

Keep in mind that when the sort "goes to disk", it actually just means that it used up work_mem and switches to merge sort with tapes. In a typical configuration, there's plenty of RAM available to buffer the tapes, so the terms on-disk and in-memory sorts are misleading. If I've understood earlier discussion correctly, the quicksort -> tape sort point is not even that interesting because the tape sort is actually not that much slower than quicksort, as long as it fits in RAM.

  Heikki Linnakangas

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