Index or View of all UUIDs in Postgres DB?
Disclaimer: I think the whole idea doesn't really make sense. But anyway here is one way to get a uniform "view" on all UUIDs in the database.

You can create a function that goes through all columns that are defined as UUID and then returns the values for those columns. You can optionally pass a UUID value that should be searched for:

Something like: 

create or replace function find_uuids(p_id uuid default null)
  returns table(schema_name text, table_name text, column_name text, uid uuid)
as
$$
declare
  l_row record;
  l_sql text;
begin
  for l_row in select n.nspname, tb.relname, col.attname
               from pg_attribute col
                 join pg_type ty on ty.oid = col.atttypid
                 join pg_class tb on tb.oid = col.attrelid
                 join pg_namespace n on n.oid = tb.relnamespace
               where col.attnum &gt; 0
                 and not col.attisdropped
                 and ty.typname = 'uuid' 
                 and n.nspname not in ('pg_catalog', 'information_schema')
                 and n.nspname not like 'temp%' 
  loop
    l_sql := format('select %L as schema_name, 
                            %L as table_name, 
                            %L as column_name, 
                            %I as uid 
                     from %I.%I', l_row.nspname, l_row.relname, l_row.attname, l_row.attname, l_row.nspname, l_row.relname);
    if p_id is not null then 
       l_sql := l_sql || format(' where %I = %L', l_row.attname, p_id);
    end if;
    return query execute l_sql;
  end loop;
end;
$$
language plpgsql;


Then you can use:

select *
from find_uuids('19d2efcc-45c1-4371-addc-45028a054b85');


to find the tables where that UUID value appears, or 

select *
from find_uuids();


to see everything.

The above returns something like this:

schema_name | table_name | column_name | uid                                 
------------+------------+-------------+-------------------------------------
public      | uuid_test  | some_id     | 666ce0a8-11c9-11e0-b5e2-3b49c86ffc2c
public      | uuid_test  | some_id     | 7fffda0c-11c9-11e0-967c-a300aec7eb54
public      | some_table | id          | 3aa04664-863c-43e6-9890-1087db021fd8


But this is going to be really slow