How are null's stored?

I have a 40M row table I need to import data into, then use to create a
bunch of more normalized tables. Right now all fields are varchar, but
I'm going to change this so that fields that are less than a certain
size are just char. Question is, how much impact is there from char
being nullable vs. not nullable? src/include/access/htup.h indicates
that nulls are stored in a bitmap, so I'd suspect that I should see a
decent space savings from not having to include length information all
the time... (most of these small fields are always the same size no
matter what...)
--
Jim C. Nasby (aka Decibel!)                    jim@nasby.net
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