I was following a blog post to install postgresql and start getting this error while updating my OS

Question

N: Ignoring file 'pgdg.listsudo' in directory '/etc/apt/sources.list.d/'
  as it has an invalid filename extension

Answer 1

That is not an error, but a Notice (N:).

Filename extension should be .list instead of .listsudo. Probably you made a copy-paste mistake.

Fix:

sudo mv /etc/apt/sources.list.d/pgdg.listsudo /etc/apt/sources.list.d/pgdg.list

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That said, unless you explicitly need a newer version, you should probably install the postgresql package that is provided by Ubuntu instead of using third-party repositories.