Thanks for the thanks !
Generally, when grouping stuff together, it is a good idea to have two sorted lists, and to scan them simultaneously. I have already used this solution several times outside of Postgres, and it worked very well (it was with Berkeley DB and there were 3 lists to scan in order). The fact that Python can very easily virtualize these lists using generators makes it possible to do it without consuming too much memory.
Pierre-Frederic, Paul,
Thanks for your fast response (especially for the python code and performance figure) - I'll chase this up as a solution - looks most promising!
Cheers,
Damien
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