Hi,
Le 26 mars 09 à 15:30, Matthew Wakeling a écrit :
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.
Maybe it's just that I didn't devote enough time to reading your
detailed explanation above, but this part sounds like it could be done
in an aggregate you'd use in a correlated subquery containing the
right ORDER BY, couldn't it?
http://www.postgresql.org/docs/8.3/interactive/sql-createaggregate.html
HTH,
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