Library not loaded: /usr/local/opt/readline/lib/libreadline.6.2.dylib

The key problem is that your postgresql was trying to find the libreadline.6.2.dylib but your readline is version 7.0, so only 7's lib is linked.

I don't think upgrading postgresql is a good idea, it's not easy and may cause a lot of problem, especially when you already have database data.

What I do, is to switch the version of readline. When you check brew info readline, you will find different versions, mine is 6.3.8, so I switch to that version by executing:

$ brew switch readline 6.3.8

It will change to 6.3.8 and create the link:

Cleaning /usr/local/Cellar/readline/6.3.8
Cleaning /usr/local/Cellar/readline/7.0.1
Opt link created for /usr/local/Cellar/readline/6.3.8

In this case, all the linked lib becomes to 6 version, and command line works:

$ ls /usr/local/opt/readline/lib/
libhistory.6.2.dylib    libhistory.dylib    libreadline.a
libhistory.6.3.dylib    libreadline.6.2.dylib   libreadline.dylib
libhistory.6.dylib  libreadline.6.3.dylib
libhistory.a        libreadline.6.dylib

However, if you have different apps using the different version of readline, I think you should consider upgrading.

Update: 06/02/2019

What if the old version is not installed?

Homebrew doesn't provide a way to install old version formula. And since v2.0.0, it will run brew cleanup as default behaviours every 30 days. Therefore, you may accidentally update brew and delete the old version.

Here are the steps to install old version:

  1. Go to homebrew-core directory:
    $ cd $(brew --repository)/Library/Taps/homebrew/homebrew-core
  1. Check the hash for old Formula:
    $ git log master -- Formula/readline.rb
  1. Find the corresponding version of the hash value
    commit 1dd4221c35716d6bec363ad3425ef93ffe7d9b9f
    Author: BrewTestBot <[email protected]>
    Date:   Wed Aug 15 21:51:16 2018 +0000

        readline: update 7.0.5 bottle.
  1. Checkout to old formula:
    $ git checkout 1dd4221c35716d6bec363ad3425ef93ffe7d9b9f Formula/readline.rb
  1. Reinstall the formula:
    $ brew reinstall readline
  1. Reset the repo

Following those steps, you can reinstall the old version of formula.

I was getting the exact same error, but the above answers didn't work for me. I had to reinstall postgresql.

brew reinstall postgresql