Re: PoC: Partial sort

On Sat, Dec 14, 2013 at 11:47 PM, Andreas Karlsson <andreas(at)proxel(dot)se>wrote:

> On 12/14/2013 10:59 AM, Alexander Korotkov wrote:
>
>> This patch allows to use index for order-by if order-by clause and index
>> has non-empty common prefix. So, index gives right ordering for first n
>> order-by columns. In order to provide right order for rest m columns,
>> sort node is inserted. This sort node sorts groups of tuples where
>> values of first n order-by columns are equal.
>>
>
> I recently looked at the same problem. I see that you solved the
> rescanning problem by simply forcing the sort to be redone on
> ExecReScanSort if you have done a partial sort.
>

Naturally, I'm sure I solved it at all :) I just get version of patch
working for very limited use-cases.

> My idea for a solution was to modify tuplesort to allow storing the
> already sorted keys in either memtuples or the sort result file, but
> setting a field so it does not sort thee already sorted tuples again. This
> would allow the rescan to work as it used to, but I am unsure how clean or
> ugly this code would be. Was this something you considered?

I'm not sure. I believe that best answer depends on particular parameter:
how much memory we've for sort, how expensive is underlying node and how it
performs rescan, how big are groups in partial sort.

------
With best regards,
Alexander Korotkov.