The returned column names are from the source. So if you have lazy operations
then the colnames here does not reflects the results of those
operations. Note: if you have expensive lazy function then this operation
might take some time.
colnames(x, ...)# S3 method for disk.frame
names(x, ...)
# S3 method for disk.frame
colnames(x, ...)
# S3 method for default
colnames(x, ...)
a disk.frame
not used