Thanks a lot.. That is what i searched.. In fact your query is very good for 
little changes, but i will have to use another method when updating all my rows 
because the performance is not very good alas.

My data set contains something like 40000 rows to update in 1+ million records 
and data_raw, data_sys are of type "real"... The complete update took 40 
minutes on a 256Mo, athlon 2400, kernel 2.6 and with no charge during the 
execution of the query.

Is this normal ? The number of columns of the table does it matter a lot (the 
table contains 12 reals and 4 integers) ?

I found that using an intermediate table which stock for every row the value 
before and the value after helps to gain speed... But it is not a very nice way 
i think..

Thanks again :)
Etienne 


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