View the original community article here
Last tested: Oct 30, 2020
This is pretty jank. Optimally, you should instead set the child PDTs to use the same caching policy, that way Looker handles cascading pdts internally for you, like in this article: https://help.looker.com/hc/en-us/articles/360023904433-Using-datagroups-with-Cascading-PDTs
However, this won't work if you only want the final PDT to build after all dependencies have updated, and those tables all build based on different triggers. In that case, keep reading:
HOW IT WORKS
We are going to manually create a table in their scratch schema to track the updates of each table. Create a table with a date field and X booleans, one for each dependency PDT (in this example code, X = 3). Upon creation, also add in a row with the current time and false for all the other values.
From there, change all the PDTS to use create_process. The first sql_step should create the table as normal, then the second one we are going to need to update the update_table like so:
insert into update_table (date, A, B, C)
select now(), true, update_table.b, update_table.c
from update_table
order by date desc
limit 1
This would be the sql_step for table A in this example. For table B and C you need to change which column the "true" is being inserted into.
Then, the sql_trigger for your final parent PDT will be:
select max(date) where a = true and b = true and c = true
This will only trigger once all the child PDTs have updated, and will trigger every new time all 3 are updated. As the final step, add another sql_step to the final PDT after it is built to reset the table update flags:
insert into update_table (date, A, B, C)select now(), false, false, false
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