Title: Understanding Graph Learning with Local Intrinsic Dimensionality. Abstract: Many real-world problems can be formulated as graphs and solved by graph learning techniques. Whilst the rise of Graph Neural Networks (GNNs) has greatly advanced graph learning, there is still a lack of understanding of the intrinsic properties of graph data and their impact on graph learning. In this paper, we narrow the gap by studying the intrinsic dimension of graphs with \emph{Local Intrinsic Dimensionality (LID)}. The LID of a graph measures the expansion rate of the graph as the local neighborhood size of the nodes grows.
With LID, we estimate and analyze the intrinsic dimensions of node features, graph structure and representations learned by GNNs. We first show that feature LID (FLID) and structure LID (SLID) are well correlated with the complexity of synthetic graphs. Following this, we conduct a comprehensive analysis of 12 popular graph datasets of diverse categories and show that 1) graphs of lower FLIDs and SLIDs are generally easier to learn; 2) GNNs learn by mapping graphs (feature and structure together) to low-dimensional manifolds that are of much lower representation LIDs (RLIDs), i.e., RLID $\ll$ FLID/SLID; and 3) when the layers go deep in message-passing based GNNs, the underlying graph will converge to a complete graph of $\operatorname{SLID}=0.5$, losing structural information and causing the over-smoothing problem.