Should operators create new graph heads?

[p-f]

Many operators and transformations assign a new graph head, therefore creating a new logical graph (in the EPGM model). Creating a new graph head results in losing information like graph head labels and properties.

I think the Javadoc of those operators should clarify that a new head is created and assigned OR the existing graph heads should be reused. We should discuss this and related issues in the next meetup.

[ChrizZz110]

I think we should talk about that again ... the deletion of the existing graph head and addition of a new one is suspect ... maybe we should first list all operators that are doing that and decide then, which one to refactor

[timo95]

I looked up the handling of graph heads in our current operators. This includes 61 operators. Some are missing (e.g. DIMSpan).

legend
Y	LogicalGraph input: passes head
N	LogicalGraph input: creates new head
A	GraphCollection input: passes all heads
K	GraphCollection input: passes no heads
YN/KK/etc.	binary input, value for each input

output	category	name	passes head	modifications
LogicalGraph	Main	Aggregation	Y	SetAggregateProperty
LogicalGraph	Main	Clone	N	clone
LogicalGraph	Main	Combination	NN	default
LogicalGraph	Main	ReduceCombination	K	default
LogicalGraph	Main	Exclusion	NN	default
LogicalGraph	Main	ReduceExclusion	K	default
LogicalGraph	Main	VertexFusion	YN	none
LogicalGraph	Main	GroupingGroupCombine	N	default
LogicalGraph	Main	GroupingGroupReduce	N	default
LogicalGraph	Main	ReduceEdgeNeighbourhood	Y	none
LogicalGraph	Main	ReduceVertexNeighbourhood	Y	none
LogicalGraph	Main	Overlap	NN	default
LogicalGraph	Main	ReduceOverlap	K	default
LogicalGraph	Main	Subgraph	N	default
LogicalGraph	Main	Transformation	?	dependent on GraphHead TransformationFunction (category Transformation)
LogicalGraph	Sampling	PageRankSampling	Y	none
LogicalGraph	Sampling	RandomEdgeSampling	N	default
LogicalGraph	Sampling	RandomLimitedDegreeVertexSampling	Y	none
LogicalGraph	Sampling	RandomNonUniformVertexSampling	Y	none
LogicalGraph	Sampling	RandomVertexEdgeSampling	N	default (von anderen Samplings abhängig)
LogicalGraph	Sampling	RandomVertexNeighborhoodSampling	N	default
LogicalGraph	Sampling	RandomVertexSampling	N	default
LogicalGraph	Sampling	RandomWalkSampling	N	default
LogicalGraph	Sampling Statistics	AverageClusteringCoefficient	Y	AddAverageCCValueToGraphHeadMap
LogicalGraph	Sampling Statistics	AverageDegree	Y	AddSumDegreesToGraphHeadCrossFunction und CalculateAverageDegree
LogicalGraph	Sampling Statistics	AverageIncomingDegree	Y	AddSumDegreesToGraphHeadCrossFunction und CalculateAverageDegree
LogicalGraph	Sampling Statistics	AverageOutgoingDegree	Y	AddSumDegreesToGraphHeadCrossFunction und CalculateAverageDegree
LogicalGraph	Sampling Statistics	GraphDensity	Y	Aggregation und CalculateDensity
LogicalGraph	Gelly	GellyGlobalClusteringCoefficientDirected	Y	WritePropertyToGraphHeadMap
LogicalGraph	Gelly	GellyGlobalClusteringCoefficientUndirected	Y	WritePropertyToGraphHeadMap
LogicalGraph	Gelly	GellyLocalClusteringCoefficientDirected	Y	none
LogicalGraph	Gelly	GellyLocalClusteringCoefficientUndirected	Y	none
LogicalGraph	Gelly	AnnotateWeaklyConnectedComponents	Y	none
LogicalGraph	Gelly	HITS	N	default
LogicalGraph	Gelly	LabelPropagation	N	default
LogicalGraph	Gelly	PageRank	Y	none
LogicalGraph	Gelly	KRandomJumpGellyVCI	Y	none
LogicalGraph	Gelly	SingleSourceShortestPaths	N	default
LogicalGraph	Gelly	GellyTriangleCounting	Y	WritePropertyToGraphHeadMap
LogicalGraph	Gelly	DistinctVertexDegrees	N	default
LogicalGraph	DIMSpan	?	?	?
LogicalGraph	Transformation	ExtractPropertyFromVertex	Y	none
LogicalGraph	Transformation	ConnectNeighbors	N	default
LogicalGraph	Transformation	EdgeToVertex	N	default
LogicalGraph	Transformation	PropagatePropertyToNeighbor	N	default
LogicalGraph	Transformation	VertexToEdge	N	default
GraphCollection	Main	ApplyAggregation	A	SetAggregateProperties
GraphCollection	Main	Difference	K/1?	?
GraphCollection	Main	DifferenceBroadcast	^
GraphCollection	Main	DistinctById	A
GraphCollection	Main	DistinctByIsomorphism	A
GraphCollection	Main	GroupByIsomorphism	?	dependent on GraphHeadReduceFunction - CountGraphHeads nimmt zb nur den ersten
GraphCollection	Main	Intersection	K/1?	bei bestimmter Größe das letzte Element, sonst nichts?
GraphCollection	Main	IntersectionBroadcast	^
GraphCollection	Main	Limit	A
GraphCollection	Main	RollUp	N	default (Abhängig von Grouping)
GraphCollection	Main	Selection	?	dependent on FilterFunction
GraphCollection	Main	Split	N	generiert GraphIds aus split properties
GraphCollection	Main	ApplySubgraph	K	leer - Typo in Exception in Constructor
GraphCollection	Main	Union	A
GraphCollection	Gelly	WeaklyConnectedComponentsAsCollection	Y	none (Abhängig von AnnotateWeaklyConnectedComponents)
GraphCollection	btgs	BusinessTransactionGraphs	N	generiert das irgendwie