Revert "test pg, we don't care about the internal state of `column#default`."

Revert "test pg, remove unused column assignments. Follow up to 254cdf47"

Related to #15492

This reverts commit 254cdf4728291277f3fbaa854f34495030e476b4.
This reverts commit 4bcf9029452e0c760af04faab6b549710401e8cf.

There are public methods that assume `Column#default` is type casted.
The return value of `Column#default` is publicly relevant and should not change.

/cc @sgrif