Re: Any better plan for this query?..

On May 19, 2009, at 7:36 AM, Simon Riggs <simon(at)2ndQuadrant(dot)com> 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

>
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