Postgres knows whether a given aggregate is parallel-safe, and creates
parallel aggregation plans based on that. The `partialize_agg` is a
wrapper we use to perform partial aggregation on data nodes. It is a
pure function that produces serialized aggregation state as a result.
Being pure, it doesn't influence parallel safety. This means we don't
need to mark it parallel-unsafe to artificially disable the parallel
plans for partial aggregation. They will be chosen as usual based on
the parallel-safety of the underlying aggregate function.
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