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The attribute of the nid field in the {comments} table (and possibly others) does not contain "unsigned", therefore nodes with nid's above 2^15-1=2147483647 can't have comments. There might also be other tables not related to the comment.module that contain nid fields without the unsigned attribute.
This is an issue only when creating nodes outside of drupal, as I did when importing an sql file into phpMyAdmin and when you hand-edit files from the module Backup and Migrate.
Comments
Comment #1
s.Daniel CreditAttribution: s.Daniel commentedThis applies for D7 as well but has been fixed in D8 and will become irrelevant in D8.
type
andunsigned
are the important bits:Normaly this won't cause any harm but its hacky. However I am not sure if there could be situations where changing this might fail when the db has been populated.