
Hi, netters,

First of all, thanks a lot for all the prompt replies to my earlier question about "merging" data frames in R.
Actually that's an equivalence to the "join" clause in mysql.

Now I have another question. Suppose I have a data frame X with lots of columns/variables:
Name, Age,Group, Type, Salary.
I wanna do a subtotal of salaries:
aggregate(X$Salary, by=list(X$Group,X$Age,X$Type),Fun=mean)

When the levels of Group and Type are huge, it took R forever to finish the aggregation.
And I used gc to find that the memory usage was big too.

However, in mysql, it took seconds to finish a similar job:
select Group,Age,Type ,avg(Salary)  from X group by  Group,Age,Type

Is it because mysql is superior in doing such kind of things? Or my R command is not efficient enough? Why did R have to consume huge memories to do the aggregation?

Thanks again!

Zhihua Li

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