On Wed, Mar 21, 2012 at 10:57 AM, Andres Freund <andres@anarazel.de> wrote:
> On Wednesday, March 21, 2012 03:47:23 PM Alvaro Herrera wrote:
>> Excerpts from Tom Lane's message of mié mar 21 11:35:54 -0300 2012:
>> > Now that would all be fine if this were a widely-desired feature, but
>> > AFAIR the user demand for it has been about nil. So I'm leaning to
>> > the position that we don't want it.
>>
>> I disagree with there being zero interest ... the "order by random()"
>> stuff does come up occasionally.
> Yes.
>
> I wonder if could be hacked ontop of a plain seqscan node instead of building
> a completely separate infrastructure. The standards syntax would then simply
> be transformed into a select with some special ORDER BY
Well, the standard syntax apparently aims to reduce the number of
returned rows, which ORDER BY does not. Maybe you could do it with
ORDER BY .. LIMIT, but the idea here I think is that we'd like to
sample the table without reading all of it first, so that seems to
miss the point.
I have to admit I'm not very impressed by the argument that we
shouldn't do this because we'll need a new executor node. Creating a
new executor node is not really that big of a deal; and in any case I
don't think Tom will like hacking another bit of functionality into
seq-scan any better, since he refactored both the Merge Append and
Index-Only Scan patches to avoid doing exactly that, and those were
more similar than this probably would be.
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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