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The numbers (postid in the database) go up with every post - question, answer or comment - so not every number in sequence refers to a question.
okay. got it now; any reason for using same variable for question, answer and comments?
Numbers might also be skipped because of deleted questions. The ID is probably auto-incrementing so will just keep going up without filling in the gaps. As your previous question showed, it doesn't matter since there is scope for billions of questions/answers/comments.

Re using the same table: I assume it's because questions and answers (and comments to an extent) share much of the same data structure, e.g. ID, text, votes and so on. So it's simpler to put them in one single table.
Thanks for the explanation!