Re: Very specialised query

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От Tom Lane
Тема Re: Very specialised query
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Msg-id 22529.1238082582@sss.pgh.pa.us
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Ответ на Very specialised query  (Matthew Wakeling)
Ответы Re: Very specialised query  (Matthew Wakeling)
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Matthew Wakeling <> writes:
> This query takes about two hours.

> Now, it happens that there is an algorithm for calculating overlaps which
> is really quick. It involves iterating through the table in order of the
> start variable and keeping a list of ranges which "haven't ended yet".
> When you read the next range from the table, you firstly purge all the
> ranges from the list that end before the beginning of the new range. Then,
> you output a result row for each element in the list combined with the new
> range, then you add the new range to the list.

> This algorithm just doesn't seem to fit into SQL at all.

No, it doesn't.  Have you thought about coding it in plpgsql?

I have a feeling that it might be possible to do it using SQL:2003
recursive queries, but the procedural coding is likely to be easier
to understand and better-performing.  Not to mention that you won't
have to wait for 8.4...

            regards, tom lane

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