Re: Row pattern recognition

2026년 3월 21일 (토) 23:16, Tatsuo Ishii <ishii(at)postgresql(dot)org>님이 작성:

> Hi Henson,
>
> >> So, if RPR is active, WinSetMarkPosition() is not called at all? That
> >> seems too strong limitation. Can't we set mark at frameheadpos even
> >> if RPR is active? It seems safe since the only allowed franme start
> >> option is ROWS BETWEEN CURRENT ROW and we never step back to the rows
> >> before frameheadpos.
> >
> >
> > You raise a valid point. Setting mark at frameheadpos would be safe
> > for the window function's own read pointer, since we only allow
> > ROWS BETWEEN CURRENT ROW.
> >
> > However, the difficulty is with the PREV/NEXT navigation. In the
> > experimental patch, PREV accepts an optional offset argument --
> > e.g. PREV(price, N) -- and this offset can be a runtime expression
> > whose value is not known until evaluation time. If the mark is
> > advanced to frameheadpos, PREV(price, N) with a large N could try
> > to access rows that the tuplestore has already truncated.
> >
> > This is why the experimental patch takes a different approach: it
> > creates a separate nav_winobj with its own mark pointer pinned at
> > position 0, so that the tuplestore never truncates rows that PREV
> > might need. The window function's own mark management is left
> > unchanged.
> >
> > The mark=0 approach is safe but conservative -- it prevents the
> > tuplestore from releasing any rows within the partition. The SQL
> > standard (ISO/IEC 19075-5, Subclause 5.6.2) requires the offset
> > argument to be a "runtime constant" -- meaning it cannot reference
> > columns or row pattern variables. So in practice, the maximum
> > offset is always known at plan time, which opens the door for a
> > future optimization: advance the mark to (currentpos - max_offset)
> > when all PREV offsets in the DEFINE clause are constant.
> >
> > Since the experimental patch redesigns how mark works for PREV/NEXT
> > navigation, I think we should drop 0004 from the current series
> > and revisit the set_mark question together with the experimental
> > patch. What do you think?
>
> Agreed. So I just want to make sure I create v46 using 0001-0003 and
> 0005-0015. Am I correct?

Yes, that’s right!​​​​​​​​​​​​​​​​

> Best regards,
> --
> Tatsuo Ishii
> SRA OSS K.K.
> English: http://www.sraoss.co.jp/index_en/
> Japanese:http://www.sraoss.co.jp
>