How to perform Partial Reload with Left Join on Qlik?

I have a Qlik script where I load two tables, Product and Customer. I join these two tables on a common field called Purchase_ID using a left join. I want to do a partial reload on the Customer table using the Partial Reload button to see the updates when new customers are added to the Customer table. However, this does not update my sheet when the Partial Reload option is selected as shown below:

I tried using the ADD ONLY LOAD option but this requires me to click the Reload button each time I load the dashboard, else I get an Incomplete Visualization error as it loads the Customer table only when a Partial Reload is initiated. I also tried using an if PartialReload = 0 then ... endif condition for the Product table but I get a No Table to Join error.

How can I do a Partial Reload in this situation? Here is my script for reference:

LOAD 
    Product_ID, 
    Cost_Per_Unit, 
    Purchase_ID,
    Date
FROM Product
WHERE Date > '2023-06-01'

LEFT JOIN

ADD LOAD 
    Customer_ID, 
    Purchase_ID
FROM Customer

Looks like your If ... Then needs to be tweaked, as you should be using the IsPartialReload() function to achieve this:

LOAD 
    Product_ID, 
    Cost_Per_Unit, 
    Purchase_ID,
    Date
FROM Product
WHERE Date > '2023-06-01';

If Not IsPartialReload() Then
    LEFT JOIN
    LOAD 
        Customer_ID, 
        Purchase_ID
    FROM Customer;
End If

Edit

After taking into account your comments and re-reading your original question, I'm now understanding your issue much more. I have spun up a database with the same two tables from your example (Customer and Product) and with the same fields to try and replicate the issue.

One possibility for why the partial reload is not working is that it actually is working but your use of the Left Join is not actually what you want. Based on your example script, you are wanting to fully load both the Customer and Product tables while only partially reloading the Customer table. The problem I found when testing this out was that the Left Join is only going to load any new Customer records if the [Purchase_ID] field is already in the Product table from the original, "normal" reload. If, when trying to load new records from the Customer table using a partial reload, you are also pulling in new [Purchase_ID]s, then there won't be values to successfully Left Join to the Product table on.

You can test this theory by simply removing the Left Join from your original script. Everything worked correctly for me when I did this by adding new customer records and then doing a partial reload.

Edit 2

The solution posted in this Qlik Community thread allowed me to use a Left Join in both the regular and partial reload scenarios. This is the script I used:

IF IsPartialReload() THEN
     DROP TABLE [Product];
END IF;

[Product]:
ADD SELECT
    "Product_ID",
    "Cost_Per_Unit",
    "Purchase_ID",
    "Date"
FROM "public"."Product"
  WHERE "Date" > 'Jan-08-2022';
;

LEFT JOIN ([Product])
ADD SELECT
    "Customer_ID",
    "Purchase_ID",
    "Customer_Name"
FROM "public"."Customer";

The DROP TABLE [Product] is what appears to get this working, along with using the ADD prefix on both tables.