Check for existence and compatibility of columns being added in view

One check I realized we're not doing, but need to do, is ensuring that the column being added by the base table doesn't already exist in the view. If the column does already exist, ideally we can allow the addition to the base table if the type matches and the scale is null or >= existing scale and the maxLength is null or >= existing maxLength. Also, if a column is a PK column and it already exists in the view, the position in the PK must match.

The fact that we've materialized a PTable for the view should make the addition of this check easier.