random k-sample of a file

[Carlos]

: You can't do a uniform random distribution without knowing the length.

Probably true for other distribution.
Most certainly not true for uniform distribution (with a raisonable k)

You can work on a subset of the file. Let say 1000 records.
The distribution being uniform, you can select (or eliminate),
a percentage of each subset and the results for the whole file
will be ok.