==== VARIATION: union ====
-- INPUTS:
pipeline: [{$unionWith: 'collB'}, {$match: {_id: 1}}]
metadata: 
	number of partitions: 1
	scan definitions: 
		collA: 
			options: 
			distribution and paths: 
				distribution type: Centralized
				distribution paths: 
			indexes: 
			collection exists: 1
			CE type: (empty)
		collB: 
			options: 
			distribution and paths: 
				distribution type: Centralized
				distribution paths: 
			indexes: 
			collection exists: 1
			CE type: (empty)

-- OUTPUT:
Root [{scan_0}]
Union [{scan_0}]
|   Evaluation [{scan_0} = Variable [scan_1]]
|   Filter []
|   |   EvalFilter []
|   |   |   Variable [scan_1]
|   |   PathGet [_id]
|   |   PathTraverse [1]
|   |   PathCompare [Eq]
|   |   Const [1]
|   Scan [collB, {scan_1}]
Filter []
|   EvalFilter []
|   |   Variable [scan_0]
|   PathGet [_id]
|   PathTraverse [1]
|   PathCompare [Eq]
|   Const [1]
Scan [collA, {scan_0}]


