1. Create a Heroku App.
2. Go to Resource Tab. Install Heroku Connect and Heroku Postgres add-ons.
3. Create a table in Heroku Postgres. Make sure to create a column to relate it to Salesforce object. In my example, I have used Account_Number column.
4. Create some data in Heroku Postgres table.
5. In Heroku Connect, go to External Objects tab. Select the table created in Step 3 under Data Sources.
6. Click Show Credentials to get the username and password.
7. Create External Data Source in Salesforce.
8. Click Validate and Sync.
9. Select the table so that it creates External object in Salesforce.
10. Create an External Id field in the object which is going to be the parent. In my Case, I have used Account object as parent Account Number field as External Id field.
11. Update the external object. Don’t update API Name and Table Name.
12. Make the Account Number column as Indirect Lookup field to Account object. Don’t update API Name and External Column Name.
13. Update the labels of the other fields. Don’t update the API Names.