Hi Gregory,
Rong's analysis is correct. The UNION with duplicate elimination is
translated into a UNION ALL and a subsequent grouping operator on all
attributes without an aggregation function.
Flink assumes that all grouping operators can produce retractions (updates)
and window-grouped aggregate
The SQL UNION is the reason here that's causing (a) the table is not append
only, and (b) the inner GroupBy.
If you check out the UNION operator[1], it suggests that: "Any duplicate
records are automatically removed unless UNION ALL is used".
So: (1) it is definitely not append-only operation as y
