If I inspect the data source it has the right data however in a long-running flow the data source doesn't. As an example I open the flow, click on the data source and filter it specifically for the item I know is in the data (eg searching for a specific manufacturing job). In Prep nothing shows up after applying the filter. Not only this but the resulting data source created by running the flow is ALSO missing the key data.
If I add a duplicate of the data source and apply the filter to it then I see the correct data both in Prep and also in the output. Several times now to troubleshoot this issue I've had to swap out the SAME data source in Prep, replacing the existing data source in the flow with the same data source that has been newly added. This is a pain however because then I have to go through and reapply all the same changes (renaming fields, excluding superfluous data, applying filters, etc)
I assure you this is not a case of a forgotten filter or filters on objects that were renamed causing the observed behavior
EDIT - May be related or a separate issue. Applying filters at the data source in prep doesn't just fail as above. Let's say I just want to see Job = '555'. If I try filtering the data source for [Job] = '555' no data comes through. If I remove the filter and then go to a clean step I can see '555' listed in the Job column and filter for it. If I COPY the filter formula and go back to the data source and paste the formula into a filter again no data comes through. It isn't a data type formula error. Something is not working as expected
Does the issue occur if you manually select/click the Refresh icon then check for the updated data? The schema should update with any new data found in the data source.
It's also possible that Tableau Prep is sampling the data if the dataset if over 300K rows (I believe). If so, then by going to the Input Step then the tab for Data Sample, then select Stratified to see all possible examples, based on the field that would be key to obtaining sampled data for all values related to that specific field. See if that helps:
Best, Don Wise -
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