We could not reindex
customer_grid_flat nor we can open Customer menu at the left side in Magento 2 admin. It gives an error. Looking around i found this similar issue Then we tried to open DB table Customer_grid_flat but it says table is empty. Anyone an idea?
Try to execute the reindex command:
php bin/magento indexer:reindex
I had the same problem and I didn't understand why in the admin panel instead to retrieve data from
customer_entity table Magento use
When You create an account all information are registered only in
customer_entity but not in
customer_grid_flat, to force the system to register the new account data in
customer_grid_flat you must execute the reindex command or execute a cron job.
I have not yet understood why, but it is so.
I think it's a performance issue.