Re: Optimizing "top queries" ...

i basically thought a node would make more sense as it gives some
more flexibility.
making the "replacement strategy" inside the node a bit more fancy
this could actually open the door for further optimizations and for
other operations.

also, OFFSET would be quite easy as the buffer size needed is
perfectly defined by LIMIT + OFFSET.
taking work_mem into consideration we could safely fall back to the
old plan if too much data is fetched.

can a node like that be of any further use for other operations as
well? i am especially thinking of some other stuff related to analytics.

best regards,

hans

On Dec 6, 2006, at 4:34 PM, Gregory Stark wrote:

>
> "Markus Schiltknecht" <markus(at)bluegap(dot)ch> writes:
>
>> Hi,
>>
>> Hans-Juergen Schoenig wrote:
>>> in fact, the sort step is not necessary here as we could add a
>>> node which
>>> buffers the highest 10 records and replaces them whenever a
>>> higher value is
>>> returned from the underlaying node (in this case seq scan).
>>> this query is a quite common scenario when it comes to some
>>> analysis related
>>> issues.
>>> saving the sort step is an especially good idea when the table is
>>> very large.
>>
>> That sounds very much like what's known as 'partial sort', which
>> has been
>> proposed by Oleg and Theodor. AFAIK they had a trivial patch
>> sometime around
>> version 7.1, without integration into the planer and optimizer.
>> They were
>> talking about libpsort, but I can't find that currently. See
>> archives [1] and
>> [2].
>
> I actually implemented it again a few months ago during the feature
> freeze. I
> had a few questions but since it was the middle of the feature
> freeze I expect
> people had other things on their minds.
>
> It is an important form of query since it crops up any time you
> have a UI
> (read web page) with a paged result set. Currently postgres has to
> gather up
> all the records in the result set and sort them which makes it
> compare poorly
> against other databases popular with web site authors...
>
> The open question in my patch was how to communicate about the
> limit down to
> the sort node. I had implemented it by having ExecLimit peek into
> the SortNode
> and set a field there.
>
> This alternative of making a whole new plan node may have more
> promise though.
> It would make it easier to come up with reasonable cost estimates.
>
> One thing to keep in mind though is that I also wanted to cover the
> case of
> Unique(Sort(...)) and Limit(Unique(Sort(...))) which can throw away
> duplicates
> earlier. Do we want three different plan nodes? Are there other
> cases like
> these that can benefit?
>
> --
> Gregory Stark
> EnterpriseDB http://www.enterprisedb.com
>
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