Plan operator
Append
Plan node that concatenates rows from multiple child plans, used for UNION ALL and partitioned tables.
An Append node executes each of its child plans in turn and emits all of their rows. It is the engine behind UNION ALL and behind queries against partitioned or inherited tables — one child per partition.
UNION (without ALL) adds a HashAggregate or Sort + Unique on top of the Append to remove duplicates, which is why UNION ALL is dramatically cheaper when you know there are no overlaps.
Partition pruning eliminates child plans before execution: if a WHERE clause restricts to a single partition, only that partition's child appears in the plan. If you see every partition listed in a partitioned-table query, your predicate isn't being pruned — check the partition key types and constraints.