Re: Row pattern recognition

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От David G. Johnston
Тема Re: Row pattern recognition
Дата
Msg-id CAKFQuwZA4m5bEi+TQZ2RAM8eiHQViN0_KH5nnNqgtU7JMTjUBA@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Row pattern recognition  (Vik Fearing <vik@postgresfriends.org>)
Ответы Re: Row pattern recognition
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On Tue, Oct 22, 2024 at 6:12 AM Vik Fearing <vik@postgresfriends.org> wrote:


On 22/10/2024 12:19, Tatsuo Ishii wrote: 
Acording to ISO/IEC 9075-2:2016 "4.21.2 Row pattern navigation operations",
  <row pattern navigation operation> evaluates a <value expression> VE  in a row NR, which may be different than current row CR.

From this I think PREV(col + 1) should be interpreted as:

1. go to the previous row.
2. evaluate "col + 1" at the current row (that was previous row).
3. return the result.

If my understanding is correct, prev(price + 1) has the same meaning
as prev(price) + 1.



This is how I read the specification also.



That makes sense.  Definitely much nicer to only have to write PREV once if the expression you are evaluating involves multiple columns.  And is also consistent with window function "value" behavior.

Thanks!

David J.

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