[datatable-help] best way to set keys, when you don't know in advance wich fields you will use

[Jean Jacques Dureau]

Hi,
i have a data.table (10,000k of rows) with 20 (factor) fields and i
need to filter data according some of them.
I use this data.table inside a function and i don't know "in advance"
wich fileds i'll use to filter data and to sum.

So, for example, consider a data.table (named dt_data) with 20 fileds,
named f1, f2, ... ,f20.

I use this approach: i set the key on the field i have to use, for
example f2. Then i "filter" the data and i use them to do some
computations.

Subsequently, with these computations, i discover wich fileds i have
to filter, for example f4 and f5. Now, i set the key on dt_data on
(f4,f5), and so on ...

I use this approach because i don't  know if it's possible to set the
key on all fields f1, f2, .., f20 in advance and then use only some of
them!

Is there a better way to use data.table?

thanks

jj