duplicate variable declaration in nginx.conf using postgres_set

Question

I'm new to openResty and my question might appear trivial, but nevertheless I need some advice on this issue I'm experiencing with the "postgres_set" directive:

As soon as I define a variable whose value is set by "postgres_set" directive within two completely separate locations of my nginx.conf , I receive an error from nginx.

nginx: [emerg] the duplicate "VAR" variable .....

Opposite to postgres_set using set $var 1 doesn't cause any errors. What is the scope of a variable defined using postgres_set within a location?

Thanks in advance

nginx version: openresty/1.19.9.1 OS: cat /etc/debian_version 10.11

Here is my nginx.conf file:

worker_processes 2;

events {}
http {
    variables_hash_bucket_size 4096;

    upstream database {
        postgres_server  host.docker.internal  dbname=test user=postgres password=;
        postgres_keepalive max=200 overflow=ignore;
    }

    server {
        listen       8080;

        location  /loc1 {

                        postgres_pass database;
                        postgres_output text;
                        postgres_query
                        GET  "SELECT 1";
                        postgres_set $var 0 0 optional;
                        #set $var "1";
                        postgres_rewrite no_rows 404;
                        postgres_rewrite GET rows 200;
                       
        }

        location  /loc2 {

                        postgres_pass database;
                        postgres_output text;
                        postgres_query
                        GET  "SELECT 1";
                        postgres_set $var 0 0 optional;
                        #set $var "1";
                        postgres_rewrite no_rows 404;
                        postgres_rewrite GET rows 200;
        }

    }
}