How to determine if the Persistent Derived Table (PDT) regenerator is hung

  • 4 November 2020
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The PDT regenerator process is responsible for checking SQL triggers and rebuilding derived tables that are persisted with a SQL trigger which is defined either a datagroup or sql_trigger_value.

The regenerator is considered “hung” when:

  1. It reports that it is executing SQL for a PDT build or trigger check, and that SQL is not executing on the database.

  2. The regenerator process has not begun after  the previous regenerator process has completed within the timeframe dictated by the PDT And Datagroup Maintenance Schedule connection setting. 

Why do hung regenerators occur?

The source of a hung regenerator can be difficult to track down, as it typically involves the network or database side of the process interrupting PDT regeneration. We have seen a number of reports of hung regenerators coinciding with Redshift database maintenance, including a mass maintenance that Redshift performed. Make sure to confirm this is not the case before moving forward with troubleshooting.

You can use the workflow outlined below to confirm whether the PDT regenerator is hung. 

Workflow for determining whether the PDT regenerator is hung 


You can use the following chart in conjunction with information from the Queries and Persistent Derived Tables pages in the Database section of Looker’s Admin panel:


Please open a support request with the Department of Customer Love if you need assistance determining whether the regenerator is hung, or to request an instance restart to resume regenerator activity. 

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