A global salt is not a thing. Not is a pepper. The actual cryptographic construct is an HMAC and secret HMAC key.
Right but in this case you would essentially have to use every salt from every user to hash the submitted email and say "aha user 123,456 has a salt and hashed email which matches the submitted email of [email protected]" which is why I suggested method 2.
I am user "Arnor" and my email is "[email protected]". Ok, your salt is #@$%@#$%DGDFfdgdawer.
If your hashing algorithm is appropriately "expensive" the scan all user salts would not work.
Still, simply storing the create time or a randomly generated salt right on the user table is more secure than using a global salt.