How can I batch-compress DNG files in my Lightroom library?

I shoot with my Pentax K10d in the DNG format. The camera produces the files in an uncompressed format. I would like to save some disk space by compressing all in a batch process too in Lightroom.

Can I compress all files in my lightroom library at once?

Answer

According to Attila Acs:

You can compress new DNGs:

if I import them using the Lightroom “convert to DNG” command, 
they are compressed

You can also compress DNGs already in your Lightroom database:

Select the photos in your catalog and choose
“Metadata -> Update DNG Previews & Metadata”