It is always exciting when someone encounters the internet for the first time. There so much information available it can become quite overwhelming!
1)Can someone please send me a link of all companies using PostgreSQL ?
Such a list does not exist, but as someone else has mentioned, here is a link that lists some (probably a very small percentage) of all institutions using postgres.
2)Both government and private companies using PostgreSQL
See the response to 1)
3)Any security issues found earlier and the steps taken for resolution or how it was mitigated.
