title: Cross Database Query in PostgreSQL
body: <p>I am trying to build a query in Postgres. My background is in SQL Server so, I'm having some syntactical challenges. My query needs to hit two seperate databases on two separate servers. I need to do a join between the datasets. Essentially, I have a table with user login activity in db1. A entry gets added each time a user logs into the website. On db2, I have a table with purchases. For each day, I need to see: how many people logged in and how many of the users that logged in made a purchase. </p>
  
  <p>My tables look like this:</p>
  
  <pre><code>Logins                Purchases
  ---------             ---------
  ID                    User_ID
  User_ID               Amount
  LoginDate 
  </code></pre>
  
  <p>This would be easy if my Purchases table had a date field on it. But it doesn't. So, currently, I'm trying the following:</p>
  
  <pre><code>SELECT 
    // Somehow get the number of logins for the given day here
    // Somehow I need to get the number of purchases for the given day here      
    TO_CHAR(TO_TIMESTAMP((LoginDate/1000) - 14400), 'MM/DD/YYYY') AS the_day
  FROM 
    db1.Logins AS t1,
    db2.Purchases as t2
  GROUP BY the_day
  ORDER BY the_day;
  </code></pre>
  
  <p>How do I get the number of logins and purchases for a each day in Postgres? Thank you!</p>