remove calls to sql_type on pg columns

This fixes #10613 (for me at least).

I've never dug into this code before, but what I ended up with looks very similar to @tenderlove's commit 4b4c8bdc776e2d42cd070d9cb1b3cc22f82469b1, which makes me think I'm on the right track.

If there's anything else I should be doing, please let me know. I'm really looking forward to getting this bug fixed. Thanks!