Extracting user data like usernames and emails of all the users in JIRA using external data bases like SQL Servers or Postgres can be achieved easily by running an SQL select. 

On which table are this information stored on my database ?

The table where all this information are stored is on your database is called cwd_user.

Here are a few SQL statements that I that help you with extracting user data from your desired database : 

All fields :

select * from cwd_user /* all fields */

Only username and email field:

select user_name, email_address from cwd_user /* only username and email field */

Oh which table are information on account names that were been changed stored on my database ?

The table where all this information are stored is on your database is called app_user

The SQL statement that you can run which shows  you the id,  original account name ( user_key ) and the new account name ( lower_user_name ) : 

select * from app_user

Known "invalid object" error while running queries or manually opening the table on SQL server.

This should refresh the data cached by Intellisense to provide typeahead support and pre-execution error detection.

1. Edit -> IntelliSense -> Refresh Local Cache

2. Ctrl + Shift + R refreshes IntelliSense as well.