On May 19, 2009, at 7:36 AM, Simon Riggs <simon@xxxxxxxxxxxxxxx> wrote:
On Tue, 2009-05-19 at 12:17 +0100, Matthew Wakeling wrote:
Yes, Postgres has been missing the boat on this one for a while. +1
on
requesting this feature.
That's an optimizer feature.
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".
This is an executor feature.
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.
I have no particular thoughts on priority (whose priority?), but I
will say I've run across queries that could benefit from this
optimization. I fairly often write queries where the first key is
mostly unique and the second is just to make things deterministic in
the event of a tie. So the partial sort would be almost no work at all.
...Robert
--
Simon Riggs www.2ndQuadrant.com
PostgreSQL Training, Services and Support
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